Battery Backup Design

[rosacer]

Hi John

I need to tell you (as a tech) I have rarelly seen a so well explained procedure, it's great: congratulations. And thanks to give us for free the fruit of your hard work.

[JohnBFisher]

Okay folks. At this point, I need to complete Part 2 (Battery). I need to show how to determine the amp hours needed to drive different xPAP units. Due to the ResMed Battery Guide, it is pretty easy to determine the power requirements of their units. That might take another day or so to complete.

I would also like to spend some time discussing how to determine how long a battery listed by Watt Hours (as is used with many of the rechargeable Lithium Ion batteries). This will make it a little easier for others to use this guide. I should be able to finish this within a week.

I will also add the battery post to marine terminal conversion, along with pictures to show how to protect that setup. My objective is to have a rock solid setup, which should have little potential problem from corrosion or decreased conductivity or loose cables. This may take a couple weeks before it is completed.

Finally, I will start to convert this to the Wiki. Though under construction it should be helpful enough "as is" to make it easier for the next person to setup this type of battery backup rig.

Hope this helps!

[Gerald]

My compliments, John.....quality work on your part!

I'm running a "M" series auto....and my backup rig is similar to your design (but, yours is more "elegant") and I have a little experience.

During the last hurricane I got a solid 2-nights run time out of a 35-amp hour deep cycle scooter battery from Interstate Battery Corp.

Power came on during the 3rd day....but, i believe I could have made it another night on the single (initial) charge. I was not running my humidifier. I was using 12V DC battery power....rather than running everything thru an inverter.

Hope this helps........

Gerald

[SoccerMan]

This is a great writeup... thank you! I do have a couple of questions for anyone who might know.

1) You mention S8's DC operation in this quote: "Other units, such as many of the ResMed S8 units, can use a DC to DC converter. This converter is designed to regulate the power from the battery, which can vary over time and use."

At least the S8 IIs have a DC input... why does one need a converter since the power off the battery is DC?

2) How does this solution switch between AC Power and DC power? Or does it always draw DC, and the battery is continually topped off?

3) Does anyone know of a solution for the ResMed S8 IIs that will:
a) Allow both AC and DC to be connected at the same time, but only allow current to be sent to AC when the power is on?
b) Switch between direct AC current to DC when the power goes out?

This would allow one to use their humidifier but automatically turn it off on during a power outage (since the ResMed S8 II units will automatically turn off the humidifier when using the DC connection).

Thanks,

SM

[JohnBFisher]

... 1) You mention S8's DC operation in this quote: "Other units, such as many of the ResMed S8 units, can use a DC to DC converter. This converter is designed to regulate the power from the battery, which can vary over time and use."

At least the S8 IIs have a DC input... why does one need a converter since the power off the battery is DC? ...

Apparently, ResMed moved some of the DC circuitry to the converter. Most power supplies (AC or DC) need to regulate the power. That is, they need to provide a certian output power, regardless of the input power. In this case, batteries can either be lower or higher than the 12V DC value. The converter raises or lowers it automatically.

Additionally, this converter allows you to use either 12V DC or 24V DC as the input power.

Other units move this type of circuitry into the xPAP unit, rather than use a seperate converter.

... 2) How does this solution switch between AC Power and DC power? Or does it always draw DC, and the battery is continually topped off? ...

It always draws on the DC power. As I use it, I use a modified sine wave, so I do not use my humidifier when I use this. I live in an area with a high relative humidity. And I've been using xPAP for a long time, so my nose is somewhat adjusted to the pressure.

... 3) Does anyone know of a solution for the ResMed S8 IIs that will:
a) Allow both AC and DC to be connected at the same time, but only allow current to be sent to AC when the power is on?
b) Switch between direct AC current to DC when the power goes out? ...

First of all, I don't think the ResMed units provide for power switching like that. In essence, if you cut the power on one it will just turn it off. You would then have to turn it back on again. I think there are a few xPAP models that do allow for continuous operations, but not the ResMed units.

Second, I suspect something that handled both AC and DC for an automatic cutover would be rather expensive. But who knows? Someone might have a great idea that will work.

... This would allow one to use their humidifier but automatically turn it off on during a power outage (since the ResMed S8 II units will automatically turn off the humidifier when using the DC connection). ...

If you want to use the humidifier regardless, then go with a larger battery and a pure sine wave inverter. For example, if you use an S8 AutoSet Spirit II with a humidifier at an average pressure of 16, then see page 11 of the ResMed Battery Guide, which indicate you would need a 45 amp-hour battery to last 8 hours. I would probably go a bit higher to provide some extra yardage. But then you could use the unit all the time and not worry about power outages.

However, how I intend to use this is essentially plan if there is going to be a power outage. If there are storms, snow, ice, wind, or the like, then I will turn off the humidifer, plug into the inverter and not worry about it. But if it is a calm night (most nights), then I will just leave it plugged into the wall.

[OldLincoln]

Great engineering John! I have a couple questions....

1. You say it's OK to use a wet cell battery. Are you sure? I would be reluctant to suggest anything other than a gel filled sealed unit because of the outgasing by wet cells during charging.

2. I don't know too much about chargers in parallel but if power goes out will the battery try to overdraw the charger? I looked up the charger you have and didn't find my answer so it may be a non-issue.

I really like the way you wrote this as a true tutorial. I had my first power outage (that I know of) a couple days ago. I have considered a backup and with your tutorial I may actually get 'er done now. Thanks

[JohnBFisher]

... You say it's OK to use a wet cell battery. Are you sure? I would be reluctant to suggest anything other than a gel filled sealed unit because of the outgasing by wet cells during charging. ...

Yes, it is okay with some clear caveats:

• Be certain it is a sealed battery - it can still vent gas, but it is somewhat less likely to do so
• Keep the battery out of direct sunlight (or away from a heater or heater vent)
• Put the battery in a vented battery box
• Put the battery in a "spill proof box" (no seams or openings around the bottom part)
• Do not cover the battery or battery box
• Do not place the battery or battery box right near power outlets or electrical devices

But shoot! I would recommend that with any battery. No matter the chemistry, there is a lot of potential power in a deep cycle battery. It is always wise to be cautious when placing a battery - inside the garage or inside the bedroom.

One reason this is a good arrangement (besides the battery box that does not need to be opened) is that it uses an intelligent charger. This slowly charges the battery. This reduces the venting. This also helps maintain the battery for a longer productive amount of time. Also, when it gets "topped off" it does not continue to charge. Instead it just keeps an eye on the charge and keeps it topped off, but never over charged.

Even Gel batteries can outvent if charged too quickly or left in heat or cold conditions. Rather than a Gel type battery, I personally was able to purchase an AGM (Absorbed Glass Mat) type battery for my setup. It had a three year warranty, so I felt a little more comfortable using it.

Additionally an AGM battery has some key advantages. Per the Battery FAQ:

AGM batteries have several advantages over both gelled and flooded, at about the same cost as gelled:

Since all the electrolyte (acid) is contained in the glass mats, they cannot spill, even if broken. ... since there is no liquid to freeze and expand, they are practically immune from freezing damage.

Nearly all AGM batteries are "recombinant" - what that means is that the Oxygen and Hydrogen recombine INSIDE the battery. These use gas phase transfer of oxygen to the negative plates to recombine them back into water while charging and prevent the loss of water through electrolysis. The recombining is typically 99+% efficient, so almost no water is lost.

The charging voltages are the same as for any standard battery - no need for any special adjustments or problems with incompatible chargers or charge controls. And, since the internal resistance is extremely low, there is almost no heating of the battery even under heavy charge and discharge currents. ...

AGM's have a very low self-discharge - from 1% to 3% per month is usual. This means that they can sit in storage for much longer periods without charging than standard batteries. ...

AGM's do not have any liquid to spill, and even under severe overcharge conditions hydrogen emission is far below the 4% max specified for aircraft and enclosed spaces. ...

... I don't know too much about chargers in parallel but if power goes out will the battery try to overdraw the charger? I looked up the charger you have and didn't find my answer so it may be a non-issue. ...

Sorry, I'm not sure I understand your question. If you wonder if you can charge the battery faster, then yes. You can use a heavier duty charger. I chose the low end for two reasons. (1) It cost less. And (2) we rarely have power outages. So I saved a few bucks and it might take longer to recharge, but we tend to have lots of time between power outages.

But if you want to charge it faster, you can always get a more powerful charger. For example the Battery Tender Jr is about $40. For about $125 (three times the cost) you can get a charger that will provides six times the amperage to charge the battery. (Do you need to charge it that quickly is the question.) Just be certain the charger provides a float charge feature. Over charging a battery can be dangerous as well as ruin a battery.

If you wonder if the Battery Tender draw current from the battery when the AC power goes out, then the Battery Tender site indicates it does not do that:

Zero to Minimal Current Draw from Batteries: When the AC power is disconnected, most Battery Tender battery chargers draw zero current from the battery. A few of the higher power models draw less than 1 milliamp from the battery.

If you wonder about ganging the chargers together, then the Battery Tender site has FAR more detail on that:

http://batterytender.com/resources/conn ... argers.htm

If you wonder about running the charger and inverter at the same time, then don't think of it as a configuration that might "pull more current" than the charger can handle. Rather think of it as a situation where the battery and charger both supply current to the inverter. At 12V DC with 0.75 amps (or 750 milliamps) the Battery Tender Jr provides 9 Watts for the Inverter. However, the Battery Tender site provides an answer to a very similar question:

Can I leave the Battery Tender Plus battery charger connected to a battery while I’m using the battery to power another appliance like a radio?

Yes, you can leave the Battery Tender Plus [or Battery Tender Jr.] battery charger connected to a battery even when the battery is being used. As far as the Battery Tender Plus battery charger is concerned, the appliance just makes the battery look like it’s not fully charged. The Battery Tender Plus battery charger can supply up to its full 1.25 amp [or 0.75 amp for the Battery Tender Jr.] current output even while its output voltage is at the lower, float level of 13.2 volts. It is only when the battery voltage drops below somewhere between 12.0 and 12.5 volts that the Battery Tender Plus battery charger will reset and begin the full charger cycle. All that means is that when the appliance is no longer being used by the battery, the battery voltage will rise normally and there will be an absorption period of a few hours where the Battery Tender Plus battery charger holds the battery voltage at 14.4 volts until the charger current drops to below 0.1 amp, or until 8 hours has elapsed during the absorption charge period. Then the Battery Tender Plus battery charger goes back into float mode where its output voltage is constant at only 13.2 volts.

In short: Yup! No problem. If you want to always run it with a inverter and xPAP unit, then the Battery Tender Plus would be a better choice. Of course, if you have a really heavy current pull on the battery, it might be better to use a much more powerful charger, such as those designed to charge batteries used for alternative power sources (solar, wind, etc):

http://store.solar-electric.com/ioen12vo15am.html

The cost is more, but not all that much more expensive.

... I really like the way you wrote this as a true tutorial. I had my first power outage (that I know of) a couple days ago. I have considered a backup and with your tutorial I may actually get 'er done now. ...

Same thing happened here. I finished this off last night and used it the very same night. It's great when a plan comes together! Now if I could only remember why I started this!?!?

And I'm glad this is helping everyone. Since I simply don't sleep without my ASV unit, I decided I needed to be a bit more sophisticated with my battery backup system. So, while I was at it, I took notes, which made it much easier for me to share this.

[DreamStalker]

Great battery power tutorial John. Congrats!

[JohnBFisher]

Okay, it's question time. I still have pieces to complete on this. I've also started to move this into a Wiki article. So, that's moving along. Version 0.2 is already out there and moving toward Version 1.0 pretty quickly.

But what about Version 2.0? So, I have some questions and ideas about this:

• Should I add a section about pros and cons about different battery technologies?
• Should I add a section about portable batteries? I would include information about power requirements (usually in watt hours) and options.
• Should I add a section about using determining recharge times / charger requirements?
• Should I add a section with a checklist to help people determine their needs? It could highlight different technologies and approaches based on the answers.
• Should I add a section that then shows how to calculate approximate costs of various options?
• Should I add a section that you can take to the store to help purchase all the pieces?
• Should I add a section with links to various travel related websites? (to answer the travel with a battery on a _____ )?
• Should I list specific parts for different xPAP units? Such as DC connection cables and/or converters? Such as inverters?
• Should I add a section about using UPS units?
• Should I add a section about using other types of batteries, such as golf cart batteries, such as scooter batteries, such as...?
• Should I add a section with a checklist to walk through the construction of the battery backup setup?
• Should we put together a brochure and/or presentation for A.W.A.K.E meetings?
• Should we put together a pamphlet for stores (such as Batteries Plus) and/or DMEs?
• Any RT/DME/Sleep Lab Tech want to help move this toward a periodical article?
• Should we work wiith CPAP.com to put together some videos to show how to do this?

My hope is to make this as simple, but as effective as possible. Though I don't mind answering questions, I am not always available. And frankly, I want this information to be as widely available as possible. Not that I'm an expert on this. But rather if someone is worried about power, I want the general consensus from the community to be fully and readily available.

Any other bright ideas? I took a first stab at this, but it can and should be more than my effort to build a battery backup system.

[DirtyDingus]

John, nice write up.

I've been using a 400w inverter quite a while-for use when traveling and somebody else was driving so I could lay down and nap. When packing, I've been known to leave enough room for an air mattress-and having an Avalanche, I could put the seats in back in the folded position, let the divider between the cab and the bed down, leaving plenty of room to lay down.

I moved to the country a few years back, and had the problem of not being able to rest while the power was out. So I built a backup system similar to yours. I think I may have ended up with the setup costing more than I really wanted. One thing that my system required is a longer backup time-as our power has been known to go out for a few days-living in the country ain't always convenient. And, for a long time, I didn't have a DC cord for my CPAP.

My setup is this:

One cooler with wheels and a pull up handle in the back (I thought I might need to move it-and sure enough, I've had to move it twice. I had an emergency power supply...used to jump start a car if necessary-it also has a light, an air compressor, an inverter, and 2 ea 12v outlets and 115v outlets. If I hadn't had this already, the cost might have been prohibitive.

I built a top for the cooler (cut off the hinges and threw the cooler lid away) with a vertical "wall" out of the same plywood to mount the items and route wiring. I put a 105 ah sealed marine deep cycle (which negates having to put the marine terminals on it) with the battery placed inside the cooler, then wired it to the Emergency Power Supply (which had a 23 ah battery in it). Positive to positive and negative to negative gives me 128 ah to use. I then put a marine charger on top of the plywood top, plugging it in to a switched outlet. The charger was not cheap-but it does condition the batteries, does a fast charge first, then a slow charge, then goes to a float as the batteries get fully charged. If I need to charge the batteries, all I have to do is flip the wall switch, and when the lights tell me the batteries are on "float", I switch the charger off.

I haven't figured the total cost, I'm somewhat afraid to. The battery (top of the line) cost about $200. The charger cost about $180....but that should be a one time purchase since it's of good quality. The only thing, that really helped is the fact that I had the oak plywood, enough 4 gauge wire to do the job along with the fittings needed, and had the tools so that I could do what I wanted.

So, if the battery in the EPS fails, there's enough room in the cooler for a second marine battery. Now that that problem is solved, I have only one problem with my CPAP setup. If someone could tell me where I could find headgear for a nasal mask that adjusted vertically in the rear, I'd be set. I'm tired of having to cut and sew the backs of the headgear every time I get a new one.

An 18 year CPAP user,
DirtyDingus

[5aces]

Here is the video link to the Best of Revision3 UPS Battery Hack:

http://revision3.com/systm/hackedups

Similar to what you describe and a very thorough video to go along with the instructions.

Even the PowerBox portable unit that I use has one of those small 12V acid batteries sealed inside the unit.

It's just personal choice how elaborate and expensive one feels is necessary to obtain alternate backup power.

[JohnBFisher]

5aces, thanks for the pointer to the video. It gives me some good ideas:

• As OldLincoln also raised the concern, it would be good to add a safety section.
• The safety section should include some clear warnings.

Though the UPS hack will work, I worry there is too much variability in the UPS units. I've seen some REALLY bad UPS units that can overcharge their batteries. That's fine (though expensive) with a small battery, it's not good with larger batteries.

But it is a good source with some good ideas. Thanks for the pointer.

DirtyDingus, thanks for describing your setup. With 128AH you should have a good bit of time available. I know from private messages I've received others find it helpful to find what others have done. So, thank you!

[reitzell]

So, as I go along I am learning...

The video is basically what I did. It works for me but I live in San Diego and only visit snow. I'm not planning on having even an entire day outage. This is an important point because this solution is horribly inefficient. I know manufactures and and others will quote really high efficiency but efficiency is a curve based on load and the inverter startup costs at the types of load we are talking is really bad. I measured it - 20 percent. That is 5 amps in to one amp out for our application. This is really bad.

Take someone who says, "If I am going to go through all of the trouble of doing this, I want to plan for two nights of backup power."

OK, let's say we guess they draw is two amps @ 12V DC. That is (2 * =16AH -This is the amp hours they will need for one night and you are not supposed to completely drain down your battery. A very safe calculation is 50 percent so on night with that extra at that margin is 32AH per night, right? Ok, so for two nights we are talking 64AH. Now, pay for the DC --> AC -->DC conversion 320AH. (Sure, maybe I have a cruddy inverter but the basic startup cost of the inverter is going to be there no matter what. The manufactures don't even publish the low end of their efficiency curve and this is why.)

Also, the problem that John stated... I don't know how the UPS is tending my very expensive battery. (I got a blue top Optima AGM battery which I think is a good idea...)

but I am doing a CPAP backup power project version1.2 that float charges and is just directly DC. (Basically, John's design)

Thanks for sharing John!
Rob

[billbolton]

I know manufactures and and others will quote really high efficiency but efficiency is a curve based on load and the inverter startup costs at the types of load we are talking is really bad.

This is why commercial UPSs are mostly not good choices, as they are almost always designed for purposes (typically, high loads for shortish times) which have quite different usage profiles to xPAP support needs. Running small loads off inverters meant to handle large loads is certainly inefficent.

The whole point of going to the trouble of putting together the readily available building blocks to make a battery power system (as distinct from a UPS) is that you can appropriately size the inverter to the anticipated load, so it runs at a point of good efficiency!

Cheers,

Bill

[OldLincoln]

John, "Can I leave the Battery Tender Plus battery charger connected to a battery while I’m using the battery to power another appliance like a radio?" is what I was getting at. Answered very well, thanks.