Batteries for CPAP - How to select, use, charge, etc.

[WNJ]

As use of CPAP with a battery for backup, camping, or traveling is a recurring topic here, I thought I would put together a somewhat comprehensive post on the subject.

First, there is no one perfect battery for CPAP. Your choices involve trade-offs. I will attempt to cover the high points of the factors you should consider in choosing, using, and managing a battery/system for backup or camping. Unless otherwise specified, the following pertains to deep-cycle, flooded-cell, lead-acid batteries as I believe they are the best choice for most uses.

Types of batteries

Deep cycle batteries - In most instances, I think this is the most practical type of battery for most CPAP use. Deep cycle batteries are designed to be deeply discharge over a long period of time and will withstand many hundreds of deep-discharge-recharge cycles with proper care. Deep-cycle batteries are available in old-fashioned flooded-cell lead-acid, AGM, or gel-cell varieties. (The latter two are lead-acid types also.)

True deep-cycle batteries will be rated in amp-hours only. If you see a cold-cranking-amps (CCA) rating, it is not a true deep-cycle battery, no matter the other claims made. True deep-cycle batteries will not be labeled for use in “starting” anything, even a boat motor. “Trolling” or “RV” labels are okay.

Deep-cycle batteries are made with thicker lead plates in their cells to withstand the repeated deep discharge-recharge cycles. The kind which will be most suited for CPAP use (Group 24, 27, or 31 deep-cycle batteries) should be rated from 70 to 125 amp-hours.

Starting batteries - This is the kind of battery in your car or truck. It is designed to put out a lot of power for a short time, then get recharged again quickly. Ideal for starting your car. Not good for deep discharge. It has thin lead plates. Yes, it will power your CPAP. But expect no more than about 30 (and maybe no more than about 10) deep-discharge cycles before a starting battery is ruined.

Starting batteries will be rated in cold-cranking amps and in reserve capacity. Ordinarily, they will not be rated in amp-hours.

Marine batteries - This battery is intended for use in boats, where it will typically be used to power an electric trolling motor and start the outboard motor. It is a compromise between the two types above. It has lead plates that split the difference. It will last longer in deep-cycle applications than a starting battery, but not so long as a true deep-cycle battery. They are usually also a bit more expensive than a true deep-cycle battery.

Marine batteries are usually rated in cold-cranking amps and amp-hours

Lithium-ion battery packs - These are special battery packs made for CPAP use. They are much lighter and smaller than lead-acid batteries. They are also much more expensive. If you need light and portable, and short times between recharges is not an insurmountable problem, this may be what you need.

Jump-starter outfits - I often see posts where someone is contemplating using an automotive jump-starter outfit for powering their CPAP. In my opinion, these are the worst of all choices. The are expensive, not particularly lightweight, have a small battery capacity, and not well-suited for deep-cycle use. I say FURGEDABOUDIT!

Types of deep-cycle batteries

Old-fashioned flooded-cell lead-acid batteries - These are the least expensive deep-cycle choice. I paid about $45 for my newest one at Wal-Mart. The technology has been around 100 years. Big, heavy, high capacity, durable. They contain a strong sulfuric acid, so you must be careful of spills, etc. (A battery box is a good choice to contain and transport this battery.) You need to occasionally check the level of the acid in the cells as charging will, over time, boil away some of the water. You should check the level when fully charged and top off each cell to a level slightly below the fill-ring that extends into the cell. Use only distilled water, if possible.

Gel-cell and AGM (absorbed glass mat) batteries - These types of deep-cycle batteries have the advantage that they should never spill any acid. They still contain acid, but it is in the form of a gel or is absorbed in a glass mat. These batteries are almost always sealed. You don’t need to check the level of the electrolyte. The downside is that they are much more expensive than the flooded type and have a somewhat shorter life. They are also more particular about the type of charger used and can be ruined by improper charging. They still put out hydrogen gas when charging, though less than a flooded cell battery.

How much battery do you need?

The answer to this will depend upon how much power you need to pull out of the battery and how long you will need to use the battery before recharging. The more power you’re going to draw and/or the longer you need to go between charges, the more battery you need.

In this regard, the only measure that will be useful to you is the amp-hour rating of the battery.

Do not be deceived by advertising claims of “Watt-hours” or “Cold-cranking amps.” Watt-hours will look like a comparatively big number because it is roughly amp-hours times 12 (volts). Cold-cranking amps has no relevance for battery capacity before recharging, and batteries rated this way will withstand very few deep discharge cycles before dying -- not suited for what you want.

Estimating amp-hour needs

You need to know (roughly at least) how many amps your machine draws at 12 volts DC with the pressure you’re using. (Higher pressure will draw amps at a somewhat greater rate as the CPAP is working a bit harder.)

Multiply the amp draw times the number of hours you will use the machine per night, times the number of nights you will need before recharging the battery. Also, you don’t want to draw the battery down much more than half before recharging or you will shorten its life. Therefore, multiply by two.

Example: My REMstar Auto set at 10-14 cm seems to draw about 1 amp. I want to sleep 8 hours a night for 3 nights (long weekend camping trip.) I need 1 amp X 8 hours/night X 3 nights X 2 = 48 amp-hours battery capacity.

Are you going to run the flow generator, plus heated humidifier?

I know of no heated humidifiers that will run on 12 volts DC; therefore, you will need to convert the power from your battery to 120-volts AC, same as you have at home (in the USA.) For this you need an inverter. Consult your humidifier’s User Manual for the size and type of inverter you need. I believe ResMed machines all need a pure sine wave inverter (very clean power) and this is very expensive compared to a simple inverter.

In choosing the inverter option, be aware that inverters are very inefficient. Notice the cooling fins on the inverter? They are there to dissipate the heat “wasted” in the conversion process. Check the inverter’s spec sheet for input and output data to estimate the battery DC draw from the number of AC amps you need for the flow generator and heated humidifier combination. Figure at best the combination run with an inverter will cause your DC amp draw to be at least 4 times as much compared to running only the flow generator with straight DC.

Okay, so maybe you need to use the heated humidifier. Just be aware that much-increased battery draw is the cost to you in the trade-off. You will therefore need a bigger battery and/or more frequent recharging.

Are you going to run the flow generator only?

To do this you can use a DC cord for your flow generator. Most CPAP makers offer a DC cord for their machine. You could make one also, but they are mostly so inexpensive that it hardly seems worth the trouble. I paid only $25 for the Respironics DC cord from cpap.com. OTOH, ResMed’s DC cord is comparatively expensive. If you want to make your own, be careful of polarity. On most, the center pin is positive, on one (ResMed, IIRC) it is negative.

There is negligible loss in most DC cord setups (ResMed’s “filtered” DC cord is maybe an exception) but you can use the humidifier only as a passover. For camping, etc., passover will probably work far better than you expect because the air will be less dry than in your heated/air conditioned home.

The DC cord from your machine’s manufacturer will probably have a “cigarette-lighter” plug on the end. Unless you have a powered 12-volt outlet handy, you will need an adapter with a cigarette-lighter socket on one end and battery clips on the other to connect it to the battery. Rather than buy the expensive adapter from your CPAP maker, go to Radio Shack and get one of these for $7.99. (Note: This adapter is probably not heavy enough for use with a plug-in inverter. You will need one with a heavier cord/amperage rating to match your inverter’s input draw.)

How portable must your battery be?

If you need light weigh or high portability, a big deep-cycle battery won’t do. You will need one of the lithium-ion battery packs designed for CPAP use. The trade-off is that these units are more expensive and will need recharging sooner. For extended use, they will need recharging more frequently. You will also need more total charging time because small batteries must be charged at a slower rate than higher capacity batteries.

For most applications where you are traveling by car, etc., a deep-cycle battery is sufficiently portable. A Group 24 deep-cycle battery (the most common size) should weigh about 53 pounds.

How do I hook my CPAP to the battery?

If you’re in a car, camper, or boat, there may be a cigarette-lighter socket to plug in the DC cord or inverter. (Make sure the plug has sufficient amperage capacity for the inverter.) If you need a longer cord, you can get a 12-volt extension cord in the automotive section at Wal-Mart (for use with the DC cord only; no inverters.)

If no socket is available, the adapter from Radio Shack will work to attach your CPAP maker’s DC cord to the terminals of a lead-acid battery. The RED clamp from the adapter connects to the POSTIVE (+) lug on the battery. BLACK to the NEGATIVE (-) lug. Normally, the cable clamps are marked + and - as are the lugs on the battery. IT IS IMPORTANT THAT YOU NOT REVERSE THE CONNECTIONS!

How do I recharge the battery?

You should recharge the battery frequently -- daily if practical. This will extend its life. However, if you do not discharge a deep-cycle battery much more than 50% its life will not be shortened by much.

A digital multimeter (about $20 or less) is useful in gauging the battery’s state of charge. With no load on the battery, if the voltage is down to about 12.2 volts it is time to recharge. (At full charge, a 12-volt battery will put out about 12.65 volts at 70 degrees F.) A digital multimeter is not so accurate as using a battery hydrometer (even cheaper) but it is more convenient and doesn’t involve removing the battery caps and drawing acid from the battery cells.

You should recharge the battery in a place that has adequate ventilation because lead-acid batteries (including AGM and gel cell) give off hydrogen gas when charging. Be aware of, but not paranoid about, this. The amount of hydrogen gas given off is modest -- just don’t let it concentrate in a closed area until it reaches a concentration which can be explosive. I recharge my batteries in the garage or on the back porch.

To recharge, you need a source of DC power in the range of 13.2 to 14.4 volts. This should be a regulated source of DC power, such as an automotive-style battery charger or a solar panel with a regulator. The regulator prevents the battery from over-charging; as the battery reaches full charge, the regulator reduces the charging power.

Charging with a battery charger -- The best chargers for lead-acid batteries is a three-stage (smart) charger. Here is an example. They will recharge a lead-acid battery quickly, efficiently, and safely. With the float mode, they can also be used to maintain a battery between uses. You should charge your battery at no more than 20% of its amp-hour capacity, preferably no more than 10%. For the types of batteries we’re talking about here, this means a charger of 10 or 15 amp capacity is appropriate.

To use the battery charger, you will need a source of 120-volt AC power. This can be a wall outlet or a 120-volt outlet on a portable generator.

First, attach the RED clamp from the charger lead to the POSITIVE (+) lug on the battery. BLACK to NEGATIVE (-). Only then do you plug the charger into the 120-volt outlet. If you plug in the charger first, you run the risk of allowing the clamps to make contact with one another. BAD! Sparks and maybe killing the charger. At the end of charging, unplug the charger from the outlet before disconnecting the clamps from the battery for the same reason.

For camping, Honda and Yamaha make small quiet portable generators of about 1000 watts which will power your charger. Kipor is a newer brand that is highly rated (and less expensive.) You can also use an ordinary construction-site generator when camping, but they are much bigger and heavier and the others camped near you will hate you because of the loud noise.

Charging with a solar panel -- A 15-watt solar panel used daily should provide enough power to keep your deep-cycle battery charged indefinitely while camping. (And it will make no noise in the process.) This will work even on cloudy days. You will also need a regulator for the solar panel. You do not need a battery charger to charge your lead-acid battery with a solar panel and regulator.

Charging directly from a generator -- Many generators have a 12-volt output that is labeled for battery charging use. I do not recommend this for the uninitiated. These are not only slow (~8 amps output) they are also unregulated. This means that they can easily overcharge and ruin your battery if you don’t know exactly what you’re doing.

Charging from a car/truck -- It is possible to charge a lead-acid battery from a car, but it is very inefficient. You will have to run your car for a long time and this uses a lot of gasoline, plus the wear and tear on your car. If you want to do this, the best way is to hook your battery to the car’s battery using heavy jumper cables. Let it sit this way for a couple of hours or more. This will equalize the charge of the batteries a bit. Then start your car and let it run at a fast idle. If you have an ammeter in your car, check it from time-to-time to see when the amperage sent to the batteries tapers off. At this point, you’ve done well enough and you can stop the car’s engine and disconnect the jumper cables. (The last 5 to 10 percent of charge takes a long time and isn’t worth attempting if you’re camping, etc. Just recharge the battery fully as soon as you get home again.)


I like to use a BatterMinder to keep my flooded-cell deep-cycle batteries in good shape between uses.


If you choose a lithium-ion battery pack, it should come with a charger. Some will require a 120-volt source such as a wall outlet or generator. Some will have the added option of plugging into a car’s cigarette-lighter socket. If you choose the latter option, remember that it will be drawing power from the car’s battery. Take care not to discharge your car’s battery too much or it may not start when the time comes.


For more information on 12-volt batteries the following are good web links:

The 12-Volt Side of Life

The Battery Tutorial

Happy camping!

Wayne

[billbolton]

Deep cycle batteries are designed to be deeply discharge over a long period of time and will withstand many hundreds of deep-discharge-recharge cycles with proper care.

Deep in this case should be no more than 50% discharge if you expect to achieve "hundreds of deep-discharge-recharge cycles". The use of deep is relative to "starting" batteries where the recommended discharge is 10% (that is not below 90% of charge) in order to achieve a reasonable service life.

Lithium-ion battery packs - These are special battery packs made for CPAP use. They are much lighter and smaller than lead-acid batteries. They are also much more expensive. If you need light and portable, and short times between recharges is not an insurmountable problem, this may be what you need.

Lithium Ion battery technology degrades over time whether you use it or not, so after 3 years or so the effective capacity is often very significantly reduced from the initial new capacity. Lithium Ion batteries can be cost effective if you have need to use them reasonably often, but can be quite cost ineffective if you use them only occasionally (where NiMH technology may be a much better choice).

Jump-starter outfits - I often see posts where someone is contemplating using an automotive jump-starter outfit for powering their CPAP. In my opinion, these are the worst of all choices. The are expensive, not particularly lightweight, have a small battery capacity, and not well-suited for deep-cycle use. I say FURGEDABOUDIT!

Simple jump start units (without inverters) can be quite effective if you understand how to use them. They mostly contain small deep cycle SLA gel cel batteries (now often based on AGM plate technology) of between 12AH and 22AH capacity. As long as they are reasonably matched to the energy needs of the CPAP system, they can be an extremely cost effective solution for "one night" power outage coverage during storms etc.

I paid only $25 for the Respironics DC cord from cpap.com. OTOH, ResMed’s DC cord is comparatively expensive

$25 for what is essentially a straight wire cord is what is compatively expensive!

The Resmed DC-12 adapter is much more than a "cord" and can power an S8 from any power source between about 11V and 24DC, while providing electrical isolation between the input and output and also providing a degree of protection for a battery in terms of ensuring it cannot be run down too low in terms of remaining charge!

IThere is negligible loss in most DC cord setups (ResMed’s “filtered” DC cord is maybe an exception)

The energy use of the DC-12 is typically between 1 and 2 Watts for a nominal 12V DC input.

Cheers,

Bill

[WNJ]

Bill,

I’ve not seen a “jump-start” unit that did not have an inverter or that did not cost well in excess of $100. But then, I keep my batteries in good shape and also carry a good set of jumper cables, so it’s not something I have thoroughly researched. It is not something I would ever contemplate buying myself.

If inexpensive and reliable units exist in the range of 12 to 22 amp-hours, they would be a reasonable alternative for powering the flow generator for one night with 12 VDC. However, powering a CPAP and heated humidifier running on 120 VAC with an inverter powered by a 12-to 22-amp-hour battery won’t work for long.

With the Respironics DC cord at $25, I think it is fair to call the ResMed DC-12 “comparatively expensive” at $80. Yeah, I could have saved a few bucks by making my own cord, but I figure the time I saved is worth the extra price. For $80, I might be motivated to build my own. It seems others have made cords for the ResMed CPAPs with satisfactory results.

The features offered by the ResMed DC-12 are worth nothing to me. I’ve never heard of anyone harming their Respironics machine by using a direct cord, either Respironics or home-made. I have hundreds of hours on mine with no problem.

Wayne

[rested gal]

What a great addition to the battery discussion topics. Thanks, Wayne and Bill.

LINKS to Battery operation, camping, power outage
viewtopic.php?t=9682

[billbolton]

I’ve not seen a “jump-start” unit that did not have an inverter or that did not cost well in excess of $100.

This one is typical of a type that, with minor variations in features etc, is quite common, world wide.

http://expo.live.com/ViewListing.aspx?l ... =SCH&pos=3

Cheers,

Bill

[ColinP]

Estimating amp-hour needs

You need to know (roughly at least) how many amps your machine draws at 12 volts DC with the pressure you’re using. (Higher pressure will draw amps at a somewhat greater rate as the CPAP is working a bit harder.)

Multiply the amp draw times the number of hours you will use the machine per night, times the number of nights you will need before recharging the battery. Also, you don’t want to draw the battery down much more than half before recharging or you will shorten its life. Therefore, multiply by two.

Example: My REMstar Auto set at 10-14 cm seems to draw about 1 amp. I want to sleep 8 hours a night for 3 nights (long weekend camping trip.) I need 1 amp X 8 hours/night X 3 nights X 2 = 48 amp-hours battery capacity.

Surely given the above figures and that fact that discharging below 50% is detrimental to the battery, you'd go for at least a 96 AH battery?

Colin

[ColinP]

Oops, I meant to thank you for the great post - really well written and well researched. Great information and should be kept around for reference.

Colin

[WNJ]

Surely given the above figures and that fact that discharging below 50% is detrimental to the battery, you'd go for at least a 96 AH battery?

Colin,

In the example I included, I already doubled the capacity to avoid drawing down below about 50% charge.

Multiply the amp draw times the number of hours you will use the machine per night, times the number of nights you will need before recharging the battery. Also, you don’t want to draw the battery down much more than half before recharging or you will shorten its life. Therefore, multiply by two.

Example: My REMstar Auto set at 10-14 cm seems to draw about 1 amp. I want to sleep 8 hours a night for 3 nights (long weekend camping trip.) I need 1 amp X 8 hours/night X 3 nights X 2 = 48 amp-hours battery capacity.

BTW, you can occasionally draw-down a deep-cycle battery below 50% - some sources say as little as 20% - without ruining the battery, but doing so will shorten its life, and doing so repeatedly will shorten its life considerably.

I recharge my batteries when I get a reading of down near 12.2 volts using a digital multimeter. At that point, they’re getting down to near half-discharged in warm weather. Full charge in warm weather is ~12.65 volts. (In cold weather, the voltage readings will be lower for a given state-of-charge, but factoring that adds a layer of complexity I think unnecessary for our purposes here.)

Wayne

[ColinP]

Sorry, that'll teach me to read more carefully and reply slower.

Colin

[GumbyCT]

Wayne, thanks for a very well written and detailed post.

I bought a Roadmaster Portable Jump Starter: Power Station; 17.2 Amp
on sale locally for $25 (usu. $30) w/o an inverter but who cares. If you want that YOU could always buy an inverter separately. This really surprised me b/c it has the exact battery I need 2 of for one of my UPS's - I can NOT buy the battery this cheap - then add shipping. So this was no brainer - even if my tinkering doesn't work I could use this SLA battery. I only wish I bought a couple more.

I also bought a power cord at Radio Shack for about $8. I could have made one much cheaper if I wanted to solder. My requirements were it had to have a power ON LED and a fuse. A fuse the is common & readily available.

More on the power cord later cuz yea people say oh I got it at Radio Shack, yea go ahead, tell the little geek there what you're looking for when you walk in, go ahead and good luck to you.

For more on Batteries, read Battery University at http://www.batteryuniversity.com/


Wayne, looks like you have written your very own reality check my friend.

ps. Freezing rain here now - may give me the opportunity to see if my car starter will help in anyway? Just another use for something just laying around the house & not being used.

Bill,

I’ve not seen a “jump-start” unit that did not have an inverter or that did not cost well in excess of $100. But then, I keep my batteries in good shape and also carry a good set of jumper cables, so it’s not something I have thoroughly researched. It is not something I would ever contemplate buying myself.

If inexpensive and reliable units exist in the range of 12 to 22 amp-hours, they would be a reasonable alternative for powering the flow generator for one night with 12 VDC. However, powering a CPAP and heated humidifier running on 120 VAC with an inverter powered by a 12-to 22-amp-hour battery won’t work for long.

With the Respironics DC cord at $25, I think it is fair to call the ResMed DC-12 “comparatively expensive” at $80. Yeah, I could have saved a few bucks by making my own cord, but I figure the time I saved is worth the extra price. For $80, I might be motivated to build my own. It seems others have made cords for the ResMed CPAPs with satisfactory results.

The features offered by the ResMed DC-12 are worth nothing to me. I’ve never heard of anyone harming their Respironics machine by using a direct cord, either Respironics or home-made. I have hundreds of hours on mine with no problem.

Wayne
--------------------------------------

As use of CPAP with a battery for backup, camping, or traveling is a recurring topic here, I thought I would put together a somewhat comprehensive post on the subject.

First, there is no one perfect battery for CPAP. Your choices involve trade-offs. I will attempt to cover the high points of the factors you should consider in choosing, using, and managing a battery/system for backup or camping. Unless otherwise specified, the following pertains to deep-cycle, flooded-cell, lead-acid batteries as I believe they are the best choice for most uses.

Types of batteries

Deep cycle batteries - In most instances, I think this is the most practical type of battery for most CPAP use. Deep cycle batteries are designed to be deeply discharge over a long period of time and will withstand many hundreds of deep-discharge-recharge cycles with proper care. Deep-cycle batteries are available in old-fashioned flooded-cell lead-acid, AGM, or gel-cell varieties. (The latter two are lead-acid types also.)

True deep-cycle batteries will be rated in amp-hours only. If you see a cold-cranking-amps (CCA) rating, it is not a true deep-cycle battery, no matter the other claims made. True deep-cycle batteries will not be labeled for use in “starting” anything, even a boat motor. “Trolling” or “RV” labels are okay.

Deep-cycle batteries are made with thicker lead plates in their cells to withstand the repeated deep discharge-recharge cycles. The kind which will be most suited for CPAP use (Group 24, 27, or 31 deep-cycle batteries) should be rated from 70 to 125 amp-hours.

Starting batteries - This is the kind of battery in your car or truck. It is designed to put out a lot of power for a short time, then get recharged again quickly. Ideal for starting your car. Not good for deep discharge. It has thin lead plates. Yes, it will power your CPAP. But expect no more than about 30 (and maybe no more than about 10) deep-discharge cycles before a starting battery is ruined.

Starting batteries will be rated in cold-cranking amps and in reserve capacity. Ordinarily, they will not be rated in amp-hours.

Marine batteries - This battery is intended for use in boats, where it will typically be used to power an electric trolling motor and start the outboard motor. It is a compromise between the two types above. It has lead plates that split the difference. It will last longer in deep-cycle applications than a starting battery, but not so long as a true deep-cycle battery. They are usually also a bit more expensive than a true deep-cycle battery.

Marine batteries are usually rated in cold-cranking amps and amp-hours

Lithium-ion battery packs - These are special battery packs made for CPAP use. They are much lighter and smaller than lead-acid batteries. They are also much more expensive. If you need light and portable, and short times between recharges is not an insurmountable problem, this may be what you need.

Jump-starter outfits - I often see posts where someone is contemplating using an automotive jump-starter outfit for powering their CPAP. In my opinion, these are the worst of all choices. The are expensive, not particularly lightweight, have a small battery capacity, and not well-suited for deep-cycle use. I say FURGEDABOUDIT!

Types of deep-cycle batteries

Old-fashioned flooded-cell lead-acid batteries - These are the least expensive deep-cycle choice. I paid about $45 for my newest one at Wal-Mart. The technology has been around 100 years. Big, heavy, high capacity, durable. They contain a strong sulfuric acid, so you must be careful of spills, etc. (A battery box is a good choice to contain and transport this battery.) You need to occasionally check the level of the acid in the cells as charging will, over time, boil away some of the water. You should check the level when fully charged and top off each cell to a level slightly below the fill-ring that extends into the cell. Use only distilled water, if possible.

Gel-cell and AGM (absorbed glass mat) batteries - These types of deep-cycle batteries have the advantage that they should never spill any acid. They still contain acid, but it is in the form of a gel or is absorbed in a glass mat. These batteries are almost always sealed. You don’t need to check the level of the electrolyte. The downside is that they are much more expensive than the flooded type and have a somewhat shorter life. They are also more particular about the type of charger used and can be ruined by improper charging. They still put out hydrogen gas when charging, though less than a flooded cell battery.

How much battery do you need?

The answer to this will depend upon how much power you need to pull out of the battery and how long you will need to use the battery before recharging. The more power you’re going to draw and/or the longer you need to go between charges, the more battery you need.

In this regard, the only measure that will be useful to you is the amp-hour rating of the battery.

Do not be deceived by advertising claims of “Watt-hours” or “Cold-cranking amps.” Watt-hours will look like a comparatively big number because it is roughly amp-hours times 12 (volts). Cold-cranking amps has no relevance for battery capacity before recharging, and batteries rated this way will withstand very few deep discharge cycles before dying -- not suited for what you want.

Estimating amp-hour needs

You need to know (roughly at least) how many amps your machine draws at 12 volts DC with the pressure you’re using. (Higher pressure will draw amps at a somewhat greater rate as the CPAP is working a bit harder.)

Multiply the amp draw times the number of hours you will use the machine per night, times the number of nights you will need before recharging the battery. Also, you don’t want to draw the battery down much more than half before recharging or you will shorten its life. Therefore, multiply by two.

Example: My REMstar Auto set at 10-14 cm seems to draw about 1 amp. I want to sleep 8 hours a night for 3 nights (long weekend camping trip.) I need 1 amp X 8 hours/night X 3 nights X 2 = 48 amp-hours battery capacity.

Are you going to run the flow generator, plus heated humidifier?

I know of no heated humidifiers that will run on 12 volts DC; therefore, you will need to convert the power from your battery to 120-volts AC, same as you have at home (in the USA.) For this you need an inverter. Consult your humidifier’s User Manual for the size and type of inverter you need. I believe ResMed machines all need a pure sine wave inverter (very clean power) and this is very expensive compared to a simple inverter.

In choosing the inverter option, be aware that inverters are very inefficient. Notice the cooling fins on the inverter? They are there to dissipate the heat “wasted” in the conversion process. Check the inverter’s spec sheet for input and output data to estimate the battery DC draw from the number of AC amps you need for the flow generator and heated humidifier combination. Figure at best the combination run with an inverter will cause your DC amp draw to be at least 4 times as much compared to running only the flow generator with straight DC.

Okay, so maybe you need to use the heated humidifier. Just be aware that much-increased battery draw is the cost to you in the trade-off. You will therefore need a bigger battery and/or more frequent recharging.

Are you going to run the flow generator only?

To do this you can use a DC cord for your flow generator. Most CPAP makers offer a DC cord for their machine. You could make one also, but they are mostly so inexpensive that it hardly seems worth the trouble. I paid only $25 for the Respironics DC cord from cpap.com. OTOH, ResMed’s DC cord is comparatively expensive. If you want to make your own, be careful of polarity. On most, the center pin is positive, on one (ResMed, IIRC) it is negative.

There is negligible loss in most DC cord setups (ResMed’s “filtered” DC cord is maybe an exception) but you can use the humidifier only as a passover. For camping, etc., passover will probably work far better than you expect because the air will be less dry than in your heated/air conditioned home.

The DC cord from your machine’s manufacturer will probably have a “cigarette-lighter” plug on the end. Unless you have a powered 12-volt outlet handy, you will need an adapter with a cigarette-lighter socket on one end and battery clips on the other to connect it to the battery. Rather than buy the expensive adapter from your CPAP maker, go to Radio Shack and get one of these for $7.99. (Note: This adapter is probably not heavy enough for use with a plug-in inverter. You will need one with a heavier cord/amperage rating to match your inverter’s input draw.)

How portable must your battery be?

If you need light weigh or high portability, a big deep-cycle battery won’t do. You will need one of the lithium-ion battery packs designed for CPAP use. The trade-off is that these units are more expensive and will need recharging sooner. For extended use, they will need recharging more frequently. You will also need more total charging time because small batteries must be charged at a slower rate than higher capacity batteries.

For most applications where you are traveling by car, etc., a deep-cycle battery is sufficiently portable. A Group 24 deep-cycle battery (the most common size) should weigh about 53 pounds.

How do I hook my CPAP to the battery?

If you’re in a car, camper, or boat, there may be a cigarette-lighter socket to plug in the DC cord or inverter. (Make sure the plug has sufficient amperage capacity for the inverter.) If you need a longer cord, you can get a 12-volt extension cord in the automotive section at Wal-Mart (for use with the DC cord only; no inverters.)

If no socket is available, the adapter from Radio Shack will work to attach your CPAP maker’s DC cord to the terminals of a lead-acid battery. The RED clamp from the adapter connects to the POSTIVE (+) lug on the battery. BLACK to the NEGATIVE (-) lug. Normally, the cable clamps are marked + and - as are the lugs on the battery. IT IS IMPORTANT THAT YOU NOT REVERSE THE CONNECTIONS!

How do I recharge the battery?

You should recharge the battery frequently -- daily if practical. This will extend its life. However, if you do not discharge a deep-cycle battery much more than 50% its life will not be shortened by much.

A digital multimeter (about $20 or less) is useful in gauging the battery’s state of charge. With no load on the battery, if the voltage is down to about 12.2 volts it is time to recharge. (At full charge, a 12-volt battery will put out about 12.65 volts at 70 degrees F.) A digital multimeter is not so accurate as using a battery hydrometer (even cheaper) but it is more convenient and doesn’t involve removing the battery caps and drawing acid from the battery cells.

You should recharge the battery in a place that has adequate ventilation because lead-acid batteries (including AGM and gel cell) give off hydrogen gas when charging. Be aware of, but not paranoid about, this. The amount of hydrogen gas given off is modest -- just don’t let it concentrate in a closed area until it reaches a concentration which can be explosive. I recharge my batteries in the garage or on the back porch.

To recharge, you need a source of DC power in the range of 13.2 to 14.4 volts. This should be a regulated source of DC power, such as an automotive-style battery charger or a solar panel with a regulator. The regulator prevents the battery from over-charging; as the battery reaches full charge, the regulator reduces the charging power.

Charging with a battery charger -- The best chargers for lead-acid batteries is a three-stage (smart) charger. Here is an example. They will recharge a lead-acid battery quickly, efficiently, and safely. With the float mode, they can also be used to maintain a battery between uses. You should charge your battery at no more than 20% of its amp-hour capacity, preferably no more than 10%. For the types of batteries we’re talking about here, this means a charger of 10 or 15 amp capacity is appropriate.

To use the battery charger, you will need a source of 120-volt AC power. This can be a wall outlet or a 120-volt outlet on a portable generator.

First, attach the RED clamp from the charger lead to the POSITIVE (+) lug on the battery. BLACK to NEGATIVE (-). Only then do you plug the charger into the 120-volt outlet. If you plug in the charger first, you run the risk of allowing the clamps to make contact with one another. BAD! Sparks and maybe killing the charger. At the end of charging, unplug the charger from the outlet before disconnecting the clamps from the battery for the same reason.

For camping, Honda and Yamaha make small quiet portable generators of about 1000 watts which will power your charger. Kipor is a newer brand that is highly rated (and less expensive.) You can also use an ordinary construction-site generator when camping, but they are much bigger and heavier and the others camped near you will hate you because of the loud noise.

Charging with a solar panel -- A 15-watt solar panel used daily should provide enough power to keep your deep-cycle battery charged indefinitely while camping. (And it will make no noise in the process.) This will work even on cloudy days. You will also need a regulator for the solar panel. You do not need a battery charger to charge your lead-acid battery with a solar panel and regulator.

Charging directly from a generator -- Many generators have a 12-volt output that is labeled for battery charging use. I do not recommend this for the uninitiated. These are not only slow (~8 amps output) they are also unregulated. This means that they can easily overcharge and ruin your battery if you don’t know exactly what you’re doing.

Charging from a car/truck -- It is possible to charge a lead-acid battery from a car, but it is very inefficient. You will have to run your car for a long time and this uses a lot of gasoline, plus the wear and tear on your car. If you want to do this, the best way is to hook your battery to the car’s battery using heavy jumper cables. Let it sit this way for a couple of hours or more. This will equalize the charge of the batteries a bit. Then start your car and let it run at a fast idle. If you have an ammeter in your car, check it from time-to-time to see when the amperage sent to the batteries tapers off. At this point, you’ve done well enough and you can stop the car’s engine and disconnect the jumper cables. (The last 5 to 10 percent of charge takes a long time and isn’t worth attempting if you’re camping, etc. Just recharge the battery fully as soon as you get home again.)


I like to use a BatterMinder to keep my flooded-cell deep-cycle batteries in good shape between uses.


If you choose a lithium-ion battery pack, it should come with a charger. Some will require a 120-volt source such as a wall outlet or generator. Some will have the added option of plugging into a car’s cigarette-lighter socket. If you choose the latter option, remember that it will be drawing power from the car’s battery. Take care not to discharge your car’s battery too much or it may not start when the time comes.


For more information on 12-volt batteries the following are good web links:

The 12-Volt Side of Life

The Battery Tutorial

Happy camping!

Wayne

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CPAPopedia Keywords Contained In This Post (Click For Definition): respironics, resmed, humidifier, cpap.com, CPAP, Power, auto, clean

[RipVW]

Does anyone know anything about these? They look pretty cool! Pricey though!
http://www.batterygeek.net/v/vspfiles/C ... PAod9i9Adg

[JeffH]

Does anyone know anything about these? They look pretty cool! Pricey though!
http://www.batterygeek.net/v/vspfiles/C ... PAod9i9Adg

Click on the link included in your URL...see where you end up.

http://tinyurl.com/3y5prw

[sgtbrown]

I'm on a Respironics BiPAP Pro but I just recently purchased a Goodnight 420G to use for travel/camping and power outages at home. I forgot you have to order the 12v power cord separately and was not thrilled at the $34 price tag for a simple cord. I did some research and found virtually the same thing for $4.95 plus (ouch!) $5.10 shipping. Still, that is 1/3 the going price.

I have the 12v cord for the Respironics but, naturally, it doesn't fit the Goodnight. Ah well. The one time I tried the BiPAP with my jumpstart box, it only lasted about 1.5 hours. That booger draws a ton of juice! When the new cord for the Goodnight arrives, it will be interesting to see how it does using the jumpstart box for a power supply. My main goal here in the house is to just get 3-4 hours worth of power should the electricity go out. Machine only - I do fine with a (real) passover humidifier.

When on the road, the power for the Goodnight will come from my motorcycle. Before someone says, "Motorcycle battery! Are you kidding?", my newest cycle is another sidecar rig with the electrical system converted to use an automobile battery. Many sidecarists make the conversion for more reserve electrical power plus putting the weight of the battery in the sidecar, where it is needed for ballast.

Surprisingly, a very large contingent in the sidecar world are also CPAP users. Maybe its because sidecars and grey hair seem to go together! Anyway - and contrary to some of the wisdom I've seen here - sidecar CPAP users are getting as many as four nights sleep out of an el-cheapo standard car battery before a recharge is necessary. Plenty long enough for a weekend rally. Come this spring and the start of rally/camping season, I am going to start out using the car battery I have now. Sleep at night, ride/recharge during the day. I am hoping to be independent from battery chargers and the need for 110volts. We'll see what happens.

Sarge

[GumbyCT]

Does anyone know anything about these? They look pretty cool! Pricey though!
Visit http://www.batterygeek.net/

Click on the link included in your URL...see where you end up.
http://tinyurl.com/3y5prw

That is exactly what got me looking for a cheaper way.

I did try my RoadMaster Jump start out the other night using my Repsironics auto 500M - I shut it off after 7hrs. The battery good LED was still lit. I'm sure it could have gone longer but I wasn't trying to test the limit - just make sure I could get a good nights sleep. It seems I could have easily gotten another couple hours out of that battery.

I wonder how many folks never start out with with a fully charged battery?

These type jump starters should stay on charge right up until the time they are going to be used. At least that is how I do mine.

[WNJ]

Gumby -- $25 for a 17 amp-hour battery is a good find. It should last you one night with no problems, and maybe two nights. Because this meets your needs, I say you did well.

A 17 amp-hour battery won’t run your CPAP as many nights as an 80-amp-hour deep-cycle battery, but you can’t buy a deep-cycle lead-acid battery for that money.

But, check the user manual for your battery. It may or may not be designed to stand continuous charging. Continuous charging can ruin some kinds of batteries. You may be okay, but do check the manual.

Also, good link you posted! Thanks!

Anyway - and contrary to some of the wisdom I've seen here - sidecar CPAP users are getting as many as four nights sleep out of an el-cheapo standard car battery before a recharge is necessary. Plenty long enough for a weekend rally.

I, for one, did not say this could not be done. Just don’t count on more than a couple of dozen deep discharges of a starting battery before it is ruined (shorted out.) This is an expensive solution in the long run.

Come this spring and the start of rally/camping season, I am going to start out using the car battery I have now. Sleep at night, ride/recharge during the day. I am hoping to be independent from battery chargers and the need for 110volts. We'll see what happens.

Recharging daily improves your odds, but discharging more than about 5% is still hard on starter batteries.

When your starter battery dies, you would do better to replace it with a marine battery for the use you describe. Marine batteries are designed to withstand repeated discharges of 30 to 40 percent without undue harm. They will also work well for starting your engine.

If you have a newer bike, you probably can’t bump-start it without power to the electronic ignition. Be good to your battery.

Wayne