Where to buy new cells?

You cannot buy individual new traction battery modules.

 

I can sell you modules, reconditioned to the voltage you need them. I live in California, los Angeles. $60 reconditioned.

 

Makes me want to take apart my 28 cells and sell them individually

SM-N900P ?

 

I've experienced similar results with some of my mods that I've reconditioned. I don't know for sure why some of them increase to a point and then start decreasing after more cycles. I've found that if I leave them alone for 2 or 3 weeks and then do only 2 cycles on them, most will increase more than they did originally. I've also raised my charging cutoff to 7500mAhr. Just keep in mind that the 1st discharge value is going to be lower because after it sits a while after being charged, the voltage is going to drop some and will not take quite as long to discharge compared to one that just charged 5 minutes ago. I also have several modules for sale that I can sale for $45 apiece plus actual shipping. I can give you the capacity of each module because I record each one. Most are around 6000 mAh. I didn't try another round to get them any higher. I have several in the 5400-5800 mAh range. I'll sell them for $40 each. I'm getting ready to post some on ebay next week hopefully. Work has kept me pretty busy here lately. Let me know if I can help in any way.

 

Could charging a at 5A (close to 1C) be bad for the modules given that they may be initially unbalanced. I thought the idea behind top end balancing was to charge at a very slow rate like 0.1C so that you could safely allow the more highly charged cells to overcharge while the lower charged ones were accepting their full charge.

Higher charge rates like 1C are ok if you're using the negative delta V approach to terminate the charge cycle, but if the cells in the used module are initially well out of balance then the negative delta V method mightn't work so well.

 

Charging at 5 amps to 100% SOC is a bad idea. No reputable builder would use that for cycling used packs.

You are about 15 times the amperage what most consumer grade high voltage battery chargers use.
The charger I sell to consumers top out at 1amp, and usually use less than 1/2 an amp when in the upper SOC range.

This isn't a race and it is not good for the cells.

 

I wouldn't go against anything Eric said above since he certainly has a wealth of knowledge we could all learn from. That being said I've had very good luck reviving hybrid modules using capacity set to 7000 mAh and charge rate set to 2A (the higher the ambient temps the lower the charge rate should be, min discharge voltage on the Prius modules should be 6.0V). Using good quality chargers is key since it is pretty easy to damage them, especially if you don't know what you're doing. Modules should get a final cycle of 4500 mAh before balancing the entire pack overnight. I highly suggest getting a practice module to test your charger settings on first before starting on the actual battery pack.

 

Alternately you could do a multi-stepped approach, dropping down when you reach the higher SOC levels.
Say above 70% SOC or so.

It is possible to charge them at any rate you like. It is just much harder on them than you really need/want to be. Unless you are in an extreme hurry, I'd encourage slow and easy.

 

I totally agree.......having tried 5A in testing it was a bit harder on the modules at higher SOC's than I wanted so I dropped it down to 2A and got much better results (ie modules continued to improve their capacity when cycled and produced very little heat). Since overcharging produces heat and heat damages batteries it's much better to take your time and do it right (and if you want to speed up the process, buy more chargers).