Well, I PM'd you, and you never got back to me.
I'm not located in LA (rural Idaho), but I'm set up for hazmat shipping, so I'd be able to handle the shipping aspects. I regularly rebuild 18650 based packs for BionX systems, and have worked with other packs as well. I'm quite happy to build custom testing gear, if you have a need for it, and I'm fully equipped with spot welders, power supplies, capacity testers, and the like for pack building/repair. Fair warning, I only will build with brand new, OEM 18650s, and I greatly prefer to source them myself.
> Low mileage before running out of juice.
Often, this is just bad cells. Sometimes the BMS won't let the pack fully charge, but the usual reason is that one parallel group is just crappy and hits low voltage cutout early. The BMS, correctly, cuts the pack off entirely at this point.
> Wont take a charge.
Won't charge at all, or won't charge fully? Won't charge at all is probably the BMS being fried and not allowing current through. If it won't charge fully, it's possible one group is hitting high voltage cutoff early and the pack is only able to charge at the rate of the bleeding resistors. It's usually the fault of, again, a bad bank of cells, though it could be the BMS getting weird. A voltmeter should help you determine the actual state of the pack, and if the pack looks reasonable, suspect the BMS.
> More often then not it is the BMS, which we CAN get from the Chinese supplier.
That's good. It's a whole lot easier when you can source replacement parts.
> But how do we test the BMS?
Examining the behavior of the pack when things are suspect is a good start. Stick a voltmeter on each parallel group and see what they're doing. If something is really off with one group, it's probably a bad parallel group, but if the groups look good, suspect the BMS.
An alternative is to build a test rig for a specific BMS that allows the BMS to be exercised across a range of situations, and verify that it behaves properly. A large Arduino could easily drive this type of test equipment for comprehensive BMS tests.
> How do we replace it?
Ideally, the balance leads are socketed, so that's easy, and the main power leads are soldered, which is easy enough to deal with. How are they attached that this is a question?
> How do we test for bad cells?
If you're using quality 18650s (which is anything but a given with packs from China), "bad cells" are rare. You should only be seeing long term degradation of the whole pack capacity over time instead of individual cells going bad. Testing for individual cells going bad in a pack is hard - you have to narrow it down to a parallel group, then disassemble that group and hunt the cell. Given the cost of cells today, if a pack is bad, it's usually going to be cheaper to rebuild it than to do the labor to tear it apart. If you have one bad cell, there are likely to be more in the future.
If you're having regular problems with your battery packs, might I suggest sourcing better packs made of higher quality components?
