24v supply from battery pack. Need direction

[mrnight01]

Wow. Glad I found this site.

I have been out of electronics for 20 years or so. Things have changed a bit. I have forgotten a LOT. I am trying to get back into it as a hobby.

I am looking to make a battery pack that will vary between 21 and 29 volts, lithium. I would like to have a 24v output at 4a max.

Can I get some suggestions on what to start reading on? Or if it is even possible.

I am wanting to make a battery backup for a CPAP machine. I do not know what the voltage tolerance is on the machine.

Thanks..

 

[KJ6EAD]

24V LiFePO batteries are available off the shelf; no need to build. Is that 4A from a label, a manual or measured? How long do you need it to run?

Here's an example of a 20Ah kitted with charger and fuel gauge.

https://www.tracerpower.com/tracer-24v-20ah-lifepo4-peli-case-kit.html

 

[mrnight01]

(I reread my post and see where the confusion is. I am looking to regulate the voltage at a constant 24V dc.)

That is basically what I am putting together. I am using 18650 2000mah cells in a 7S8p configuration.

Model: LGEAMF11865 (ICR18650MF1)
Capacity: 2150mAh Rated
Voltage: 3.65V Nominal
Charging: 4.20V Maximum
1075mA Standard
2150mA Maximum
Discharging: 2.75V Cutoff
430mA Standard
10000mA Maximum

The load is labeled 24V 3.75A. The goal is 6-8 hours.

For some reason in my notes, I have a 7s2p pack as 4.4Ah and 111Wh.? The load is 76.5W ( (24*3.75) *.85). That would give 4.9 hrs. I think my figuring is wrong.

I may be over thinking things a bit and am worried that the CPAP can not run in 21-29V range.

 

[rjenkinsgb]

How about making the pack four cell series, so around 12 - 16V working range, then using an off-the-shelf car laptop PSU to give the stabilised 24V output?

eg. one of these, or similar:
**broken link removed**

That's rated 120W so should be good for up to 5A as 24V, and it doubles as a mains power unit.

Car batteries are rated 11V when near flat to 15V on charge so a very similar range.

A version for 24V, or 12 / 24V universal input for cars & trucks would be even better but I've not found one so far.
(Such things do exist, I have a Lind 12 - 24V input one with 16V out for my Toughbooks).

 

[ronsimpson]

The load is labeled 24V 3.75A. The goal is 6-8 hours.

Remember to get 24V at 4A you will need from a 12V battery 8A. 24 X 4 = 12 X 8 = 96 watts.

 

[rjenkinsgb]

The original pack was going to be 7S8p = 56 cells. That could equally be arranged as 4S14P with the same overall capacity.

 

[JonSea]

Are you sure about the CPAP voltage? Mine and every one I've see are 12 volts.

 

[mrnight01]

I just found out on an apnea board that my machines use a reference voltage on their power plug. A straight 24v will not work. They are working on what I was wanting to do.

I was just hoping to get around using a wall wart to save some power.

Thanks.

 

[mrnight01]

The new Resmed machines switched to 24v. My old one used 12v. That made life easier but there is no money in it I guess....lol

 

[JonSea]

What model is your CPAP?

 

[Externet]

I would suggest to buy a couple of 18V lithium packs meant for power tools, recharge them one by one in the charger meant for that tool, Find a couple of carrier connectors from junky tools at the pawn shop, and connect them in series at the CPAP.
In other words, use commercially available assemblies/packs.