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schematic

simulate this circuit – Schematic created using CircuitLab

My Ebike uses a 70 pack of batteries 18650, 10 in series to make 42 volts with 7 parallel batteries per series. One parallel row registers zero volts after a winter of not being charged, I also suspect that the motor control has a small drain even with the key on the off position.

And I'll be attempting to get that row to hold a charge again.

I've seen tutorials where you can take a dead 18650 battery and make it able to take a charge by jump starting it with a good 18650 battery by parallel charging.

Since the batteries are spot welded I will disconnect the BMS(battery management system) and try to parallel charge a parallel row with good 18650 batteries.

But I seen it done with a 1 good to 1 dead set up but can I do that with multiple parallel batteries? Or is that dangerous or even pointless?

If all goes well every row will have a charge to give the BMS the input it needs to charge the pack and see if it works.

[UPDATE] I tried this setup and the wires were getting (can't hold on to them) hot. The volt meter when charging reads .7 volts still not getting a reading after 4 * 10 second charges.

Batteries stay cool on both sides tho.

If all fails I will have to tear apart the row and figure how to safely solder them back together.

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  • \$\begingroup\$ Have you tried recharging the normal way. You didn't actually say this. \$\endgroup\$ – Andy aka Jun 15 '20 at 12:33
  • \$\begingroup\$ The BMS will not let you charge the pack if it gets a zero voltage from one row. \$\endgroup\$ – Eric Huelin Jun 15 '20 at 17:44
  • \$\begingroup\$ "[UPDATE] I tried this setup and the wires were getting (can't hold on to them) hot. The volt meter when charging reads .7 volts still not getting a reading after 4 * 10 second charges." - that's what always happens. At 0V they are permanently damaged and will never recharge. You should buy a new battery pack. \$\endgroup\$ – Bruce Abbott Jun 17 '20 at 5:16
  • \$\begingroup\$ @BruceAbbott I'm gonna try to replace the defective row with new batteries and if I fail at that operation then I will have no other choice but to get a new pack. \$\endgroup\$ – Eric Huelin Jun 18 '20 at 0:33
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You really don't want to do this. Overdischarging a lithium battery generally damages the electrodes.

In addition, lithium batteries have different charging regimes depending upon cell voltage.

In your case, you need the constant current region for the discharged batteries and you are simply connecting a charged lithium battery which is nominally constant voltage.

An RC (radio-controlled) drone or helicopter charger may be able to recover these. It may also be able to tell you if the cell is sufficiently damaged that you shouldn't even try.

Please be careful, though. Those battery packs hold a lot more energy than you think and are quite dangerous.

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  • \$\begingroup\$ Thanks for your advice. I suspect that one battery was defective and all the parallel batteries were affected by that defective one and now they are all dead. \$\endgroup\$ – Eric Huelin Jun 18 '20 at 0:30

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