CPAPtalk.com

HoseCrusher wrote:If your machine uses 6 Ah a night and you have a 45 Ah battery and the battery is actually capable of delivering all its capacity...

45/6 = 7.5 nights of use.

It's not a good idea to discharge a lead-acid battery all the way, they're not designed to be used that way and won't last very long at all if you do that. It depends on the specific battery, but a good rule of thumb is not to go beyond 50%.

Princess Tia wrote:Forgive me, another electricity "dummy" here...

You've posted a lot of good information here, but I can't parse it all. And I'm not sure how it applies to my situation, which is this: I'm going to be car camping for about a week, with no access to "the grid." I want to be able to use my CPAP every night (pressure is 13 mm, no humidifier). I will NOT be driving the car around while there, and when the car is turned off, there is no power to the cigarette lighter, so cannot use that to recharge a battery.

I had originally thought to use a li-ion battery such as this one and charge it during the day with a portable solar charger. But another constraint on this project is an extremely tight budget, and this simply costs too much.

One issue with this battery is that "40,000 milliamp hours" does not equal 40 amp-hours!. This is measured at about 3.6 volts, the size of a Li cell, and is the common measure for cell phone chargers. We need 12V, so in reality, its something like 12 Amp-hours and would certainly not last a week. Solar panels are fine when there's sun, I actually live by solar panels much of the time. But yesterday it was cloudy so I ran my engine for an hour. Solar is nice when it works, but you need a fallback.

Princess Tia wrote:

I was heartened to see your more cost-effective solutions. Now I'm looking at this 45ah battery and hoping I won't need to recharge it at all during the trip. Does that sound feasible to you?

That looks like a good battery but perhaps small. My pump (same as yours) uses 4 AH a night at pressure 10. You'll probably be 5-6 AH, or 35 to 42 for a week. Deep Cycle Lead Acid batteries, which includes traditional flooded batteries and AGM batteries, do not like to be discharged deeply. If kept above 50%, they can get 500 or more cycles. Discharged to 20% and you might only get 50 cycles. Beyond that could cause damage you'd notice immediately. (Starting batteries are much worse, and using them as deep cycle will lead to early death!) If you're using this once or twice a year, it will probably die of "old age" before you wear it out, as long as you don't push it too far. For a full week, you might look for one a size up, or get two batteries.

Princess Tia wrote: I had some questions about the other things you talk about in your posts.

Battery tender: If I understand correctly, this is a device to recharge the 12V battery and maintain its charge during storage. I assume that it plugs into a wall socket and is connected to the battery terminals.

The Battery Tender comes with almost everything including fuses (get spares), but you'll need a "cigarette lighter socket" (or maybe two)

Is this cable used to help recharge the battery from the car cigarette lighter while driving? If so, I wouldn't be able to use it. And I understand the concept of the splitter (I do have several USB devices to also charge), but how does it get used? Which end plugs into what? (I warned you I was a dummy! )
TIA

Sorry that this is a confusing part. The cigarette socket in the basic setup is to accept the plug from the DC adapter that goes into the cpap. This has to be added to the Battery Tender cable setup for about $10. If your cpap adapter comes with alligator clips to connect to a battery, I advise against using them.

You CANNOT use the cables to charge from the car's cigarette lighter socket without modification. It can be done but it has to be thought out carefully. The problem is that the little connectors are setup assuming one battery and one charger, but when you use two batteries, something has to get reversed. [There is a post somewhere that covers this, I'll try to track it down]

It is possible to charge from the car using regular jumper cables - just connect ground to ground and hot to hot like you're jump starting the car, and run the engine at a fast idle, keeping an eye on the cables. Charging takes time, so about an hour would add maybe 12-15 AH to the battery. It goes faster at first, then slows down, so don't expect to get it past 85% unless you run all night. Frankly, I would just spend a few bucks for a larger/second battery.

HoseCrusher wrote: You can do a test to see if these assumptions are close and then you will know for sure. An alternative is to purchase 2 batteries.

I like the idea of two batteries for a couple reasons.
First you can use one while charging the other. This is esp. handy when using at home for a power outage or when camping and you can't charge daily.

2nd this will reduce the weight of the larger batteries. I have a 100A Deep cycle which iirc weighs about 66lbs. I think a 45A will work for most people and be much easier to move out to the car then to a tent etc.

Even car camping it is a better choice to have two batteries but do NOT for any reason use the starting battery for your car or you may end up buying a new one after you get stuck.

By all means folks always always try your setup at home BEFORE leaving and when possible do not wait until the week before to start asking questions.

All it takes is one sustained power outage at home to realize you just can't sleep without your cpap.
Just ask Goofproof who never had a power outage until this year.

Did I forget to mention to buy and pack spare fuses with your cpap? No sense having all this stuff and not being able to use it now is there?

Pap-Daddy wrote:do not wait until the week before to start asking questions.

Too late!

I really had no idea this would be so complicated, so started off trying to figure it out on my own, which often works. But it finally became clear that I need help with this.

CapnLoki wrote:It is possible to charge from the car using regular jumper cables....Frankly, I would just spend a few bucks for a larger/second battery.

I'll go with a second battery (and I'll need a second box, too, I guess). I will NOT be able to charge a 12V battery by any method while on the trip.

Help me picture how this is all connected:

1. The 12V battery connects via
2. [what cord/connector?] to
3. the Battery Tender, which connects via
4. the "cigarette lighter socket" to
5. the DC cord, which plugs into
6. the CPAP.

When the first battery gets depleted [how will I know?], I switch it out for the second battery.

Is all of this right?

PT

OK... let's go through this step by step.

You pick up a battery (or 2). It needs to be charged.

Attach the Battery Tender to the battery and plug the Battery Tender into 120V AC household power. The Battery Tender will charge the battery. When one battery is charged switch the charger to the other charger to charge it up.

After charging remove the Battery Tender and figure out how to attach your cigarette lighter socket to the battery safely.

Now you are ready to go camping. Bring your batteries that are already fully charged and bring your cigarette lighter socket to attach to the battery. Leave your Battery Tender charger at home.

When you get to your camp site, hook up the cigarette lighter socket to the battery, then hook up your DC cord to your xPAP machine. Turn the machine on and use it.

If you plan for 5 nights of use, I would switch the battery after 3 nights.

After your trip you then connect the Battery Tender charger to the battery and then once again plug it into 120V household power and charge it up. When one battery is fully charged, charge the second one up. Now you are ready for the next trip.

Princess Tia wrote: Help me picture how this is all connected:

1. The 12V battery connects via
2. [what cord/connector?] to
3. the Battery Tender, which connects via
4. the "cigarette lighter socket" to
5. the DC cord, which plugs into
6. the CPAP.

When the first battery gets depleted [how will I know?], I switch it out for the second battery.

Is all of this right?

PT

The BatteryTender should come with a cord that has color coded rings on one end and a "quick connector" on the other. Make sure the battery has tabs with about a 1/4 inch hole, it may come with bolts or you'll need a pair of stainless bolts with nuts to attach the rings, red to "pos" or "+", black to "neg" or "-". The quick connector can then be plugged into the charger or the socket. The "1 to 5 splitter" can be used to connect the battery to both the charger and the pump for home backup purposes.

Knowing when to switch batteries is the downside of using two batteries. Reading voltages is an inexact science - you'll get lots of claims like 11.8 volts is down to 30%, but each situation is different and you really need some experience. I've been doing this for 25 years and have a $300 Amp-hour meter on my system - still I find myself guessing. The only time I'm sure is when its charged up.

My advice is if you have two batteries for a week, use one for 4 nights, the other for three. If you get one large battery, you only have to have faith you went big enough. At some point the voltage goes too low and the pump will stop, hopefully before damaging the battery. This is why I always say "go bigger." Although using two batteries rather than one big one has many advantages, having one big one simplifies this situation, and removes the possibility of having two batteries with half a night each left in them! All that said, two 35 AH batteries should work for you.

@HoseCrusher and @CapnLoki, thank you so much for your clarifications! Now I have a clear idea of how to go about this project.

Off to order what I need now...

PT

If you want to drain the two batteries equally, why not swap them every day?

Princess Tia wrote:@HoseCrusher and @CapnLoki, thank you so much for your clarifications! Now I have a clear idea of how to go about this project.

Off to order what I need now...

PT

One more thing - if you have a second battery, get a second cable to plug in -
https://www.amazon.com/Battery-Tender-0 ... 000NCOKZQ/

EDIT: It looks like the 45AH battery you selected has a "button" terminal. Hopefully, this comes with the bolt, if not, take it to HomeDepot and find one that works. Probably metric, 6mm but some are 5mm.

CapnLoki wrote:...
You CANNOT use the cables to charge from the car's cigarette lighter socket without modification. It can be done but it has to be thought out carefully. The problem is that the little connectors are setup assuming one battery and one charger, but when you use two batteries, something has to get reversed. [There is a post somewhere that covers this, I'll try to track it down]
...

So I was surfing around and spotted this:
https://www.amazon.com/SOLAR-ESA1-Male- ... B000GTOSTE
and realized that it could be used to connect the socket on the battery cable to a socket on a car. If the car is running it will charge the battery. If the car battery is dead, you can charge it (somewhat) from the battery.

There are a few caveats - Car alternators will put out between 20 and 60 Amps if the battery is low. The car socket is generally rated for 10 Amps and often has a 10A fuse, though newer cars with 2 or more sockets may have a larger fuse. In addition, the cable supplied by BatteryTender that attaches to the battery has a fuse built in, I think it comes with a 10A fuse so that is a limit. Batteries will only accept a limited current, roughly a quarter of its amp-hour capacity, so a 35AH battery should be OK. But if you try to charge a large 100 AH battery, it could pop a fuse.

I'm a bit skeptical of of using this for jump starting a dead battery because starters tend to use 50 amps or more that will pop a fuse, but helping a weak battery may work. Jumper cables would certainly be preferable for jump starting, and a well charged small battery, 18 amp-hours or bigger is big enough to jump most cars. However jumper cables are a bad choice for powering a cpap overnight because of the risk of shorting.

Some new cars have a variable voltage alternator that vary the charge rate depending on a variety of conditions to save fuel. If you have this, check the car forums for details and workarounds - one thing I heard was that running lights forces full charging. YMMV

CapnLoki wrote:Car alternators will put out between 20 and 60 Amps

It's quite a lot more than that these days, many alternators can put out 200A or more, although the actual charging current is more dependent on the state of charge on the battery than the capacity of the alternator.

ColinP wrote:

CapnLoki wrote:Car alternators will put out between 20 and 60 Amps

It's quite a lot more than that these days, many alternators can put out 200A or more, although the actual charging current is more dependent on the state of charge on the battery than the capacity of the alternator.

My experience is with marine alternators on small diesels, and I have no doubt that you're right, especially for larger SUVs and trucks. My wife's new Subaru comes with a 110 Amp alternator, and the big engine is 130. However, these numbers are at high RPM that you're unlikely to hit in normal usage, and certainly not when charging a battery at idle. At moderate speeds 50 amps is more typical, and if you really want to get into car charging you might want to take measurements at idle to make sure where the regulator actually turns on. My real point is that modern alternators can generally charge a battery at whatever rate the battery will accept, and that will be a function of battery size and charge state (and voltage, but that is generally constant for traditional car regulators). If your charging circuit has a 10 Amp fuse, and the battery is big that could be an issue.

A more significant take-away from battery charging discussions might be that the time it takes to charge a battery depleted to 35% capacity up to 85% will be two to three hours, regardless of size. One of the advantages of large batteries is they can accept more amps in the same time. BTW, I use the acceptance to determine the charge state - I figure when its accepting 10% of it capacity an hour, its about 85-90% charged and I'm at the diminishing returns point.

Getting ready to set up my battery backup. (Resmed Airsense 10.) I bought the ResMed DC Converter For AirSense™ 10
and will buy a 35a AGM bettery so am now looking at this trolling motor box. https://tinyurl.com/y733vxnc
Not sure I'll ever buy a 2nd battery for more capacity but the box would hold two or I could pop in a 100a RV AGM battery for $160 if I need even greater power.
Does anyone see a gotcha here before I spring for this box or have a neater solution?
Thanks, Cleve

cleve wrote:Getting ready to set up my battery backup. (Resmed Airsense 10.) I bought the ResMed DC Converter For AirSense™ 10
and will buy a 35a AGM bettery so am now looking at this trolling motor box. https://tinyurl.com/y733vxnc
Not sure I'll ever buy a 2nd battery for more capacity but the box would hold two or I could pop in a 100a RV AGM battery for $160 if I need even greater power.
Does anyone see a gotcha here before I spring for this box or have a neater solution?
Thanks, Cleve

The MinnKota is a fine solution, especially if you're getting a battery that fits it. Personally I think its a bit of overkill, but I don't mind a few loose wires. It is a neat package that could also hold a battery charger.

Thanks CapnLoki,

Question,
is your opinion on a bit of overkill related to the size of the box or the features? I do think the box is overkill but would sacrifice size for what I think are very complete features. Esp not having to open the box to check voltage.
Thanks, Cleve