Creating a huge 12v battery pack from many 3.7 cells

I have a simple idea - to take a huge lot of 3.7v lithium-ion cells that I have, and combine them into one very large 12V battery pack that could power a 12v CPAP machine for a couple nights (for camping)

I currently use a battery pack like this … 222 watt, 8A max output. It has been great - Ive been able to use it for 2 nights of powering my cpap machine while camping (or both my and my wife’s machine for 1 night, or her machine for 3 nights) since 2008 or so. For extended camping, I just recharge it during the day from an outlet somewhere at the campsite with a big “do not touch” sign on it.

I’ve thought of taking the 100 or so 3.7v 2200 mah batteries that I have (same form factor as an AA battery) and link them all together, using dozens of 4-AA battery holders, to make another battery pack like this (never have enough power when camping)

My power-knowledge is very basic - so my thought was to just wire them all together in parallel, and run the whole thing through a 12 volt/8A regulator. For recharging I would take the batteries out and just use my standard nightcore charger, and put them back in.

two questions:

1] What else would I need to be concerned with and
2] How many 3.7v 2200mah cells would I need in this sort of setup, to reach the same level of 12v power-bank-ness as the cp222 I listed linked to above? (I’m sure there is a simple formula)

You are better off if you Invest in one or two of these:

Btw the above can be recharged with a simple 5v 2A phone charger… so no special charger is needed :slight_smile:

Oh and you can even jump start your car or RV with it if it ever comes to that :slight_smile:

@ mtylerjr -

Parallel gives you the voltage of one battery. Serial adds voltage.

Yes… I know that much. Thats why the batteries were to be in 4-packs, which puts 4 batteries in serial, giving 14.8v or so.

Then I was going to put all of those 14.8v 4-packs in parallel, giving the whole thing more juice at 14.8v, then going through a step-down regulator to 12v

Not nearly powerful enough.

“12000Mah” is surely at 3.7v

From tests in reviews, at 5v USB at 2A load it only supplied 7200 mAh.
At 12v, 2A load I imagine it would be maybe 2000mAh.

There is a reason it’s only $70, weighs only 15.2 oz, and contains only 3 lithium cells. It’s great for very short bursts of power, but it would be useless for overnight camping cpap machines (maybe a 30 minute nap?)

The cpap battery I listed is 18,500Mah at 12v… a full 222Wh, and using the same lithium-ion technology weighs 6 or 7 times as much (and would last at least 6-7 times as long)

@ mtylerjr, Why not just buy a 12v battery and spare yourself the wiring of all those batteries.


what weight aim do you have?

I would suggest that it’s likely the PAP machine already has regulation in it, and possibly may not have an issue with over 12.0v. You should measure the output voltage of the pack you’re using, over time, to see how the tolerance of it’s output is, as that may show you if it’s possible to just use the packs of multiple 3.7v’s without regulating.

Doing some reverse engineering of discharge rates and expected runtimes on one of the other batteries on the site you listed above ( [url][/url] in particular) shows .65A discharge rate of a 10.4Ah battery giving 16h (using the @ 6CM rate). That means the “222 W” battery is a 12v 15.6Ah battery. Do you have an indication of the current draw of your actual CPAP units? Looking at the stats of the 222 unit, what approx hours of running time equivalent do you get, and what CM rate equivalent is that? It obviously changes the discharge rate and may have other implications for your batteries

If you didn’t have those batteries sitting around, I’d have suggested a LiFePO4 battery pack. Even small packs are quite energy dense, take for example the ~1kg pack from [url][/url] is a 6.4Ah, or an 18Ah battery is ~3kg.

So back to your questions.
1] What else would I need to be concerned with and
2] How many 3.7v 2200mah cells would I need in this sort of setup, to reach the same level of 12v power-bank-ness as the cp222 I listed linked to above? (I’m sure there is a simple formula)

2] first.
each 4x cell pack is nominally (3.7x4)=14.8V, and hold nominally 2200mAh or 2.2Ah. Then, you add the 2.2Ah together each additional pack you add. To get to the same ballpark of capacity as the 15.6 calccuatee above, you need to have 7x sets. An eigth pack and you’re nominally over their rated capacity. But, I suspect you wont be able to get the same runtime out of those; typical Li-ion batteries (assumed based on your 3.7v) don’t have anywhere near the rated capacity, maybe it’s 1.5Ah not 2.2Ah per 4-pack. The best bet would be to experiment and see.

1]. INSULATION. You have 7x packs, 14x wire junctions as a minimum, and a reasonably high discharge capacity battery, insulation is one of the biggest concerns. Insulate between packs as well. Don’t skimp, you don’t want to burn down the tent!
The standard wires will probably not be rated to .65A, you may need an upgrade (I’d suggest some high quality automotive wire with good insulation). Remember that you’ll heat the wires a bit, hence the reason for good insulation there too.
Charging will be a right pain. You have to charge ~30-odd batteries at a time. Just tracking which ones are charged and which aren’t is a fun task :). But if you charge all 100 and take them all, you get 3x the duration before needing to worry, so that’s a positive, right?
Single failed cells will screw you over. Make sure you check voltages once you discharge them and check they’re all within spec, and repeat after charge.
Structural integrity of the “unit” also becomes a factor since you don’t want to have to diagnose broken / loose wires at the campsite.

I’d spend the weekend testing a 7xpack out to see what runtime you get. At home, where you have a backup power source :slight_smile:

@ mtylerjr,

Concerning the batteries you shown. I do not know if your aware of this or not but these types of batteries comes in 2 flavors.
Protected and non protected.
The protected ones have a small circuit board spot welded to the tip of the battery, then the button top placed on top of it. The battery still looks the same as a regular one but just a tiny bit longer. This protection circuit prevents the battery from being drained to low.
The unprotected cells obviously do not have this circuit board, and rely on the device they are to be used with to have this protection circuit.
If this protection is not in place and you drain them to low you will ruin the battery. Just an FYI if you do go this route to make sure you get the protected version of this cell.

How about using a RC battery, they are lighter and is much less complicated.

Put 4 of these in parallel to get your 20A.

Or, you can try 8 of these. 2 packs(4 in series) then both in parallel.

You will need to add this to each cell to protect it.

Is anyone but me very confused about the stated specifications on the linked page? Charging current in mAh? Pack capacity in Watts? I think the most important question here is, what is the target capacity of your battery pack?

Also, if weight (and size) is the primary consideration, then I’m willing to bet that a giant pile of AA cells is going to have a poor showing. If weight (and size) is not the primary consideration, then my suggestion is a car battery (or motorcycle battery as was suggested previously).

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@ godefroi, Not sure if you are saying AA batteries as more of making a small joke, or that is what you saw in his OP.

Just in case for everyone, what he is probably showing in his OP, is the 18650 battery.

AA-size cells are what mtylerjr had originally suggested. I would guess that if you’re going for high energy density, you’re probably better off going with the biggest cells you can find.

The main problems I see here are first, we don’t know what capacity he needs (as the linked commercial pack doesn’t give that information), and second, what are the primary design considerations (weight, size, ease of recharging). We can’t really answer his questions until we know that…

I hear ya. I guess he should start with one of 2 things. The first, check in his manual for the CPAP machine if it shows what the current usage is when using the 12v adapter. Second plug in a 12 connector and check the current its using. From there knowing the amp draw he can figure out what he needs.

yeah, that’s the reason I used the vague marketing numbers (as dirty as that made me feel) to give actual stats that mean stuff.

cpap draws 3A at 12V

And how much runtime are you looking for?

Would you be better off with one of those small silent running generators? The guys at my camp site in Oct used a small unit and it was very quiet. Something you could easily leave running all night. Keep a smaller battery as a backup.

[quote=“mtylerjr”]cpap draws 3A at 12V[/quote] I doubt that’s real. Can you measure it running? Using the runtime estimation (yes, I know they’re marketing numbers :slight_smile: ) its under 1A. 3x is crazy.

The power supply for it is 12v 4A, and my 12v 2A power supply doesn’t give enough juice (shuts off immediately) so I think it is accurate. This is why batteries that can power it for a couple nights cost $400. Car batteries dont typically last a full night though some large deep cell marine batteries will. We tried a car battery last year and it lasted about 3 or 4 hours with both our machines connected

$260 shipped.