Questions on rechargeable battery pack configuration and calculations

Question

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.

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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...

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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.)

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What I'm planning:

• 100 Ladda Ni-MH batteries from Ikea which look to be fairly true to their nominal 2450mAh rating (according to Test/Review of Ikea Ladda AA 2450mAh (White) 703.038.76) for a total nominal capacity of 245Ah, or almost 5 days with more than 2 days safety margin. Also, quite cheap for the capacity.

• 5 banks of 20 batteries in series for a voltage of 1.2*20 or 24V

• the 5 banks connected in parallel so all capacity is accessible

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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.

Answer 1

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.