Bulk Charge - there must be a better way

Please help me improve my charging system.



I just finished charging my first set of 50 A123 cells, 18650 size. These are all from old Dewalt power tools. There has to be a faster way to do this. The charge took almost a whole day! !

View attachment 1

Equipment:
- 20a lab power supply, selectable for CC or CV
- spring clips
- aluminum door trim molding to hold the clips
- wire to holders - 16 ga
- wire to power supply - 10 ga



Rationale

I decided not to charge the cells in series. Charging in series may cause one cell to overcharge or undercharge. Since these are all used cells with uncertain history, I thought it best to avoid series charging.

The charge goes quick at the beginning, as you would expect using a constant voltage supply, and slows down as the charge nears completion.

I realize now I probably need a different charger. The problem there is, nothing I've seen has the juice to charge 50 cells to 3.65v. Everything I've found would require me to charge them in series as well as parallel.

Katou

Hmmm..... to 3.65v? And that's how you know they are "fully charged"? I have A123 AMP20 cells, albeit on the newer side, and use a smart charger from batteryspace.com. Your take-away is that I've deferred my decision about my pack being "fully charged" to a computer algorithm, one that seems very adequate to the task. In other words, its not voltage that is the measure of being fully charged.

I expect that now you may be doing some significant damage to those cells, because for old cells, 3.65V is almost certainly too much. Packing energy into lithium cells as they approach 100% takes exponentially increasing time. Don't. I typically aim for 80% charge, bypassing the final stage trickle charge which that smart charger is designed to do. So my charge times are very short, in the order of 45 minutes maximum for a mostly discharged 20ah 48V pack. As cells age through their charge/discharge life-cycle, they degrade. Over-charging is the fastest way to cell death. Anyway, yes, your entire approach needs to be re-evaluated.

There is nothing wrong with 3.65v on LiFePo4 cells... as long as the power supply goes into CV move and drops the current without exceeding the 3.65v mark.

If there are any bad cells in the group, they could be burning off power and getting hot and causing the CV cycle to take an eternity.. once you get to the 1~2 amp ( given the 50 cells in the group ) pull the plug and reduce the quantity of cells in the bank to see if the current drops to near zero when charge is re-applied.

I bulk charged 40x 18950 (li-ion) with a 30amp 5v psu I turned down to 4.2v

look on ebay for a big 4ish volt psu and tune it to 3.65v

do you add all the cells to the holder first, then turn the psu on ?

it's better to have the psu on, with an amp meter and volt meter and then add the cells one at a time

you can watch the volts/amps and you'll know straight away if you've added a shorted cell

with 50cells in parallel, a shorted cell will get very hot very quickly (as all the other cells, and the psu pump power into it)


I could have probably charged 40 cells every 8 hours or so... but I used to swap cells once a day so they all got about 24hours... was just easier for me to charge that way when I was busy doing other things

Hmm, it sounds like I'm doing as well as can be expected. I looked into some RC chargers, big suckers, to get the rapid charge, but they're expensive, and I couldn't verify that they could do 1s anyway.

A buddy of mine said that he could add the CC/CV behavior to my power supply via a cutoff relay or something, but that's probably 6 months away.

I overcharged today's batch to 3.70v even though the display SAID 3.6v!! I am now watching the multimeter instead of the display. I hope that those overcharged cells are still good. If not, discharge testing them should still make an interesting experiment.

I like that idea of watching the ammeter as the cells are added to catch shorts Knighty! I've been just laying my hand on the cells to find the hot ones. Very scientific. I'll get an IR gun sometime.

So far, I've only put ONE in backwards! I've tested 100 cell pairs so far, and considering how hard it is to figure out which end is positive, I don't consider 1% excessive. It was totally anticlimactic. One end was all crispy and burnt, the spring clips deformed, and there was some dark fluid on the wood underneath. I seriously thought there would be some fire. How depressingly safe.

I've got solder braid and I'm not afraid to use it! Of course, maybe I should get the other ebike stuff out and work on that instead. Now, where did that CBA get to...

Katou

5 minute charging jig:


Piece of scrap used for pallet support at Home Depot.

Bottom view shows 3/4 holes (larger than the 18650's) and flexible wire copper stapled to base.

Six cells now but there is no limit.

The weight of the batteries makes contact to the wire.



Neodymium magnets press wire to positive terminals



Lab power supply set to a constant 4.1 Volts.



Easy Peasy!

Oh, you want me to check those itty bitty marks now? !

Geez AW!

I'll do better next batch!

Thanks,

Katou