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Hey everyone!

I am currently working on a conversion and spotted these packs for sale on eBay. Now I know the dollar per kilowatt isn't that great on these, but I am trying to keep the costs low, so I plan to start out with a few of these for a proof of concept and then upgrade down the road.

My question is, how would I go about charging these? They're an odd voltage, 40v, most chargers for lithium batteries are 36 or 48. I don't want to undercharge or overcharge, as I know that is a huge risk with lithium, so what's the best method to proceed? I plan to get 3 or 4 of the modules, I just want to understand what the process is to get them charged up.

https://www.ebay.com/itm/2-PACK-40V-18-4AH-736WH-LITHIUM-LIFEPO4-96-A123-26650-POWERWALL-DIY-EV-BATTERY/173304996422

Thanks!
 

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You could charge it by connecting a bench supply set to 43.4V and turn the current down to 500mA. This could take a few hours to charge. It is charged when the current falls off to near zero.

I did a Honda CRF250R conversion with 400 of these A123 cells, and built a custom balancing charger for it as well.

It looks like what you are going to get is a 12S8P pack. 12 series packs of 8 cells in parallel. The 40 volts comes from 12 x 3.3V nominal for these cells. Full charge is 3.62V/cell or 43.4V for the pack.

The posting also says "commercial grade BMS" which is a "battery management system". This usually means each pack (a pack being 8 cells in parallel in this case) has a circuit across it that shunts current around the pack when the pack is full ie. 3.6V. This way, you put a charging current through the whole pack and each pack in series will top off. If it has this type of BMS, the pack will clamp charging voltage to around 43.4V assuming you have the charge current limited. If the voltage continues to climb past this, then it does not and turn the power supply off if it gets to 45-46 volts. Put a load on the pack to get the voltage back down to 43 volts or so.

The other possibility is that the connector merely brings out the voltage for each cell, and connects to a BMS that is part of what the pack plugs into. If this is the case you could use a LiFE charger that can do 12S packs and wire into the connector and that will balance the cells.

Be careful with this thing! 40V prob not going to kill you but you could start a pretty big fire in no time at all not to mention burning the crap out of yoursellf. My experience with the packs I made was not without incident!
 

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Word if caution, the BMS in this pack "balances" and reduces the voltage down to 40. I charged this pack up to 43v only to find that it was at 40volts again when I returned from work.

If you plan to use more than the 40 volts you'll have to remove the BMS
 

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Just guessing but 12S ?

Means charge at 41.40 - 41.45V just stop, no absorb or float.

Resting at 3.3-3.4V gives you nominal 40

Should be able to use for 36V applications, ~38.4V under load most of the way down

I would set that as a resting stop limit at the bottom, hard limit of 35.90V under load.

Yes sacrifice some capacity, but get lots more cycle lifetime, these will likely be pretty close to EOL if residual capacity is already down 20%.

But note that each cell is worth what $9 when new?

Can the fittings be reused with new cells when the time comes?
 

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Yes, this is a 12S 8P module. Not really a pack.
The module has a balancing board but not a BMS.
These were used 13 modules to a pack, in the day. The pack had a BMS.

Each cell (or group of 8 cells in parallel) can be charged up to 4.00 volts. So the module can get up to 48V, but it is safest to not go too much past 40V.
For charging you should monitor the voltage each charge, or limit the voltage of the charger.

Lots of power. With great power comes great responsibility.
 
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