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I'm looking at making a battery pack for a medical device (CPAP) that would allow its use while camping. I'll tell you what I need and then what I'm planning.


What I need:

  • 24v at 3.75 amps DC output, typically at 58W, max at 104W, according to the manufacturer docs
  • approximately 50 Ah per day, which includes a 50% safety margin (ResMed Battery Guide)
  • at least 2, preferably 4-5 days capacity
  • reasonable portability. of course smaller/lighter is better, but I'm not looking to take it back packing, but if I can't lift it or it won't fit in my van...

What I'm assuming:

  • 4-5 days would mean 200-250 Ah total capacity
  • total capacity is the sum of all cell's capacity and configuration doesn't affect this.
  • amp draw is divided across all cells, and configuration doesn't affect this either.
  • Ni-MH batteries have a flat enough discharge that they will work within limits from full to nearly fully discharged, allowing me to get most of the power out of them. (I've also considered deep-cell batteries. If this is a better idea, please explain why.)

What I'm planning:


So based on this I have several questions I'm hoping you guys can help me with.

  1. What do I have wrong?
  2. Can the batteries be recharged in this configuration?
  3. What would I need to make/buy/do to recharge them? Is it as simple as reversing the leads and connecting it to a 24V DC source? Probably not.
  4. I would like to have a "smart" charger that I can plug in, leave, and have it done when I get back 8-10 hours later without worrying about overcharging. What would be required for that?
  5. Can it be charged at any point? For example, only used 1 night, but I happen to have access to a wall outlet the next day, so I top it up?

My experience with this sort of thing is limited, but I'm a Mechanical engineer and had limited electronics classes back when I was studying, so I can usually understand what is explained to me.

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    \$\begingroup\$ thanks to @Ricardo for making links for me since my rep wouldn't let me. \$\endgroup\$ – Matt Flamm Apr 28 '17 at 19:29
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    \$\begingroup\$ It's often better to do the calculations in WattHours than AH. The Watt Hours are independent of the voltage. \$\endgroup\$ – Kevin White Apr 28 '17 at 22:28
  • \$\begingroup\$ From the level of your questions, you will be better off with an off-shelf product, like this one, amazon.com/… \$\endgroup\$ – Ale..chenski Apr 28 '17 at 23:30
  • \$\begingroup\$ @ali chen - No I wouldn't as it doens't even come close to my requirements. It's only rated for 26Ah, which means it would die after about .5 days even assuming a 100% efficient inverter. I would have to purchase/transport 10 of them. I've looked at those types of products and none that I have found meet my needs. \$\endgroup\$ – Matt Flamm May 1 '17 at 15:46
  • \$\begingroup\$ There are bigger batteries with engineered-in power management, I saw a 1000 W*hr box. But you are asking for 1200 WH for 2 days minimum, or 2400+ WH capacity. The best Hi-MH AA cell is 3 WH, so you will need 800 AA batteries. It is already 24 kg (53 lbs). Professionally engineered batteries of this capacity run for $1500-$3000, not counting for charger, batteryspace.com/… \$\endgroup\$ – Ale..chenski May 1 '17 at 17:36
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For batteries in series, voltage adds, but ampere-hours are unchanged.

For batteries in parallel, Ah adds, but voltage is unchanged.

So, your 20, 1.2 Volt, 2450 mAh cells in series will give you 24 Volts at 2450 mAh. Five such packs in parallel will then give you 24 Volts at 12,250 mAh.

You would be much better off using two large 12 volt deep-cycle (also called "marine" or "RV") lead-acid batteries. A Group 27 (somewhat longer than a traditional car battery) lead-acid battery is about 100 Ah.

Even then, you may have to plan on charging the batteries from your van's alternator while driving.

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  • \$\begingroup\$ I had forgotten how to calculate the Ah. I think you're right and I need to reconsider the deep cycle batteries. \$\endgroup\$ – Matt Flamm May 1 '17 at 15:48

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