# Questions on rechargeable battery pack configuration and calculations

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.

• thanks to @Ricardo for making links for me since my rep wouldn't let me. – Matt Flamm Apr 28 '17 at 19:29
• It's often better to do the calculations in WattHours than AH. The Watt Hours are independent of the voltage. – Kevin White Apr 28 '17 at 22:28
• From the level of your questions, you will be better off with an off-shelf product, like this one, amazon.com/… – Ale..chenski Apr 28 '17 at 23:30
• @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. – Matt Flamm May 1 '17 at 15:46
• 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/… – Ale..chenski May 1 '17 at 17:36