I need help!

[chef]

I've read instructions by one of the manufacturers that doesn't recommend paralleling batteries. In the case of LFPs it wouldn't work unless they're at a knee of the voltage curve, in which case they're already balanced. Same reason as above: LFPs have a long flat part of the voltage curve. With no potential difference, no current would flow between cells at 30% and 90% SOC for example.

Edit: Sorry I misread the post and thought it was about placing them in parallel without a charger and letting them self-balance. Charging in a massive parallel configuration should work.

[podolefsky]

I've read instructions by one of the manufacturers that doesn't recommend paralleling batteries. In the case of LFPs it wouldn't work unless they're at a knee of the voltage curve, in which case they're already balanced. Same reason as above: LFPs have a long flat part of the voltage curve. With no potential difference, no current would flow between cells at 30% and 90% SOC for example.

Is this specifically in regard to charging, or just not paralleling in general? I think that is due to concerns about parallel cells if one of them fails to short. But it's done all the time (discussed in another thread, consensus was that it's OK since failures are very rare).

[edit] I removed what I just said - I'm actually not sure what will happen if the cells have very different capacities, but I think it's like this: Normally, with cells in series, they all see the same current and so the low capacity ones fill up first. But if they're in parallel, they'll see different currents as they charge, so even if the low capacity ones fill up, the large capacity ones might keep going. Then when you put them back in series, you'll kill the low capacity ones too early.

It might also work kinda funny because the charger is expecting to see the load of a single cell, but it will instead see the load of several cells. Not sure if this will mess up when the charger knows to stop charging.

[jpanichella]

Requoting from Frodus at DIYElectricCar

"get a single cell charger, parallel them all, and fully charge them all. Use copper wire to connect all of the + terminals together and connect to the + of a single cell charger for lifepo4. Then connect the - terminals together and connect that to the - of a single cell charger.

This will slowly charge them to the same level and insure they're equalized. Then you can take them out of parallel, and wire as your project requires (I'm assuming series). This will make sure they're all the same voltage and on the top end of the charge, they'll be balanced."

[Richard230]

Requoting from Frodus at DIYElectricCar

"get a single cell charger, parallel them all, and fully charge them all. Use copper wire to connect all of the + terminals together and connect to the + of a single cell charger for lifepo4. Then connect the - terminals together and connect that to the - of a single cell charger.

This will slowly charge them to the same level and insure they're equalized. Then you can take them out of parallel, and wire as your project requires (I'm assuming series). This will make sure they're all the same voltage and on the top end of the charge, they'll be balanced."

Is this a procedure that could be performed with a group of random-condition cells like I have, or is this type of balancing/conditioning more useful with a set of new batteries? And if it can be used for batteries of different ages and capacities, would you discharge them to a certain level first?

Chef, thanks for that tip on the charger than can be adjusted for different battery chemistries. That looks like a very useful device. I will have to get one.

[frodus]

That sounds like a good idea. Where can I buy a single-cell charger? I have a couple of 12V motorcycle smart chargers and I thought that maybe they could charge up four cells at once, but I have never tried it.

get a single cell from here:
http://www.batteryspace.com/Smart-Ch...240VAC-CE.aspx

[frodus]

Could all the cells be top balanced at once by paralleling the whole pack and using a single cell charger?

Sure, that's the best, to parallel and also charged fully. But I was trying to keep him from having to rip all the batteries out of the bike.

[frodus]

It might also work kinda funny because the charger is expecting to see the load of a single cell, but it will instead see the load of several cells. Not sure if this will mess up when the charger knows to stop charging.

They'll just current limit to the limit of the charger.... i.e. if its a 6A charger, it'l put out 6A, and thats what the batteries get, no matter how much they want. The charger I lined to is smart enough to go CC/CV, where CC is up to and including 6A.

[Richard230]

Thanks for the tip, Frodus. However, I had just ordered the variable-voltage charger that chef recommended. It seemed that it would be useful for use when charging other cell types that might come along in the future. I got the last one in stock, it appears.

[frodus]

Is this a procedure that could be performed with a group of random-condition cells like I have, or is this type of balancing/conditioning more useful with a set of new batteries? And if it can be used for batteries of different ages and capacities, would you discharge them to a certain level first?

Chef, thanks for that tip on the charger than can be adjusted for different battery chemistries. That looks like a very useful device. I will have to get one.

Doesn't really matter. They'll all be fully charged to the same level, and since they're parallel, the current goes to the ones that have "lower" charge. I'd still single cell charge them first, then parallel them, because if one is really low, and one is fully charged, you will get a spark and things may get hot. There is nothing to "control" current when you wire batteries in parallel. Its best to start as close to the same charge as possible.

Now, you could just charge each one fully, one at a time, and it'l do pretty much the same thing. I was trying to keep you from tearing the batteries out. If its fairly easy to do, then its not a bad idea.



Thanks for the requote jpanichella!

[Richard230]

I learned that lesson about attaching parallel batteries that are not all at the same voltage yesteday. I was planning to hook up four of my old batteries in series and try charging them with a 12v charger just to see what would happen. I got the polarity mixed up and connected the first two in parallel. I got a big spark, quickly disconnected them and decided not to play with the batteries any more.