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Charging 25 Li-ions in Parallel

Charging 25 Li-ions in Parallel

Charging 25 Li-ions in Parallel

Hi All!

I have a custom battery pack that has 25 18650 li-ion batteries in parallel, supplying 3.7-4.2 volts.

I am having trouble finding a charger to charge them!

The charger I have, a typical intelligent charger, can supply up to 5 amps, which should be enough to give them a charge over a day.

But it tends to cut off at 3.8 volts, charging from 3.7.

I am expecting I will have to build my own charger, but I am unsure where to find information on how that many batteries in parallel will behave.

For those interested, it is powering 470 LED in a vest, that draws 3.5 amps.

RE: Charging 25 Li-ions in Parallel

I bet that charger will work great on one cell, a job for which it was designed.
So buy 24 more.

Mike Halloran
Pembroke Pines, FL, USA

RE: Charging 25 Li-ions in Parallel

The 25 batteries are hard wired together around a belt.

I need to charge them all at once.

RE: Charging 25 Li-ions in Parallel

Then you need a dumb charger.
Don't expect good results.
Also put a quick release buckle on the belt.

Mike Halloran
Pembroke Pines, FL, USA

RE: Charging 25 Li-ions in Parallel

Good advice from Mike. I wouldn't even wear that belt. You can hurt/kill yourself and probably also destroy the neighbourhood.

That said, the problem is certainly that the voltage drop in the cables adds to the "perceived voltage" which makes the charger think that cell voltage is higher than it actually is.
Measure from charger plus to battery plus to find out the voltage drop. Then do same thing for the minus. Add those two voltage drops together and you will see why the charger turns off prematurely.

What to do about it? Make the cables a lot heavier.
There's also a possibility that the charger simply overloads and switches off because of that instead of doing it on voltage level.

Gunnar Englund
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Charging 25 Li-ions in Parallel

Thanks Skogsgurra.

I have 12AWG wires, and it is fused at 10A. And yep, quick release clips. :)

I know its dangerous, but it beats a 12AH 6V Lead Acid bettery which gives me about 4 hours runtime. I want this vest to run from sunset to sunrise.

Will upload a pick of the battery belt when I get home.

From 3.6volts,The charger starts off at 1.8amsp (even though I specify 5) and trickles down to 200ma.

It reported 1200mah uploaded, and I measured the final voltage at 3.8v off the charger.

RE: Charging 25 Li-ions in Parallel

Charger voltage may be very different from battery voltage. Please do the measurement across the battery.
Also, if the resistance between near battary and far battery differs, you will never get the more distant batteries fully charged.

Gunnar Englund
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Charging 25 Li-ions in Parallel

Have you considered more voltage and less current, like: http://www.ebay.com/itm/TURNIGY-4S1P-20C-30C-5A-50...

Based on your numbers, only two or three of these would be needed to supply 12 hr of operation

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RE: Charging 25 Li-ions in Parallel

The fuse would protect only the wiring downstream from the fuse. With 25 cells wired up on a belt, there are plenty of other locations before the fuse for short circuits to occur within the belt-of-death wiring, or even within a cell (has happened). You need to move safety considerations to the fore, and design in much more than just a fuse.

Li-ion charger ICs are likely to contain a myriad of safety features, such as timers, based on their assumptions of the battery under charge. Find the data sheet for the IC in your charger and see what's what.

RE: Charging 25 Li-ions in Parallel

This just sounds... dangerous. A real fire waiting to happen. Are you measuring the temp of each cell as you go (doesn't appear so)? Putting a bunch of Li-ion cells in parallel for charging is just... <shudder>

Dan - Owner

RE: Charging 25 Li-ions in Parallel

Are you using batteries with protection built in? If so the charger may be getting confused by the discrete steps (decreases) in current draw as each battery achieves what it considers a full charge. If not I agree that safety is a major concern here.


RE: Charging 25 Li-ions in Parallel

IRstuff > I did consider those batteries, but I would prefer not to run a regulator.

It looks like I just might have to based on the consensus here. An overspec'd 10-15 amp regulator should not get too hot at 3.5 amps.

I have short circuited two series wired fully charged of these 18650's on another project and it melted the 22awg wires!

Apologies for not posting the picture yet.

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