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:
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
So based on this I have several questions I'm hoping you guys can help me with.
- What do I have wrong?
- Can the batteries be recharged in this configuration?
- 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.
- 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?
- 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.