dirtdad
1 kW
- Joined
- Mar 2, 2008
- Messages
- 309
I am wondering how two battery packs wired in series might "interact" with each other. Here are the details of my situation and why I ask:
I have 2 36V 20AH packs w/BMS protection. Last week I discharged them separately and completely. When the BMS kicked in, it would shut down the pack completely. The packs would not even operate a Cycle Analyst (CA) that draws on the order of millivolts. And no matter what I did, they would not turn back on until I got them on a charger. They did not come back on after any kind of rest, reset, etc. The BMS must just shut down hard until it sees some charge current. The BMSs are CPU controlled so that kind of thing seems possible.
This week I wired the two batteries in series. 2 BMSs. Hmmmm, I was not sure how that would play out. Obviously when one pack cut out I would lose power like before. The packs did seem to have slightly different capacities, so I was worried that I would get the lowest common denominator, but that is true with any multi-cell pack. By going from 12 to 24 cells it just means that there is more chance for a low performing cell to bring them all down.
But an interesting thing happened with the series configuration. The packs discharged until one of the BMS cut out one pack and the CA display shut off just like before. But after pushing the bike for a while like before, the CA came back to life, registering some voltage! Happy days! I got back on and rode for more miles.
I presume this is happening because the good pack discharges into the cut out pack and brings it back above the BMS voltage threshold so they tend to balance each other out, no? If so, is that good for the battery? It is definitely good from a vehicle operations view because it allows me to drain both packs evenly even though each has its own BMS protection.
I have 2 36V 20AH packs w/BMS protection. Last week I discharged them separately and completely. When the BMS kicked in, it would shut down the pack completely. The packs would not even operate a Cycle Analyst (CA) that draws on the order of millivolts. And no matter what I did, they would not turn back on until I got them on a charger. They did not come back on after any kind of rest, reset, etc. The BMS must just shut down hard until it sees some charge current. The BMSs are CPU controlled so that kind of thing seems possible.
This week I wired the two batteries in series. 2 BMSs. Hmmmm, I was not sure how that would play out. Obviously when one pack cut out I would lose power like before. The packs did seem to have slightly different capacities, so I was worried that I would get the lowest common denominator, but that is true with any multi-cell pack. By going from 12 to 24 cells it just means that there is more chance for a low performing cell to bring them all down.
But an interesting thing happened with the series configuration. The packs discharged until one of the BMS cut out one pack and the CA display shut off just like before. But after pushing the bike for a while like before, the CA came back to life, registering some voltage! Happy days! I got back on and rode for more miles.
I presume this is happening because the good pack discharges into the cut out pack and brings it back above the BMS voltage threshold so they tend to balance each other out, no? If so, is that good for the battery? It is definitely good from a vehicle operations view because it allows me to drain both packs evenly even though each has its own BMS protection.
