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This is my first ever posting on Stack Exchange so I’ll try to be thorough in my description.

I have some 18650 batteries that are about 18 months old. I have mainly used them in a gimbal for my GoPro and I’d say I’ve used that no more than a couple dozen times - so certainly not heavy daily usage. I charge them with a DFmicro Smart Li-ion battery charger I bought from Amazon.

I do have a bad habit of leaving the batteries in the gimbal after I’ve finished using it. On several occasions, I’ve found the gimbal to be totally dead. Last weekend we put the batteries fully charged into the gimbal. We didn’t end up using it but checked yesterday and the batteries were totally dead.

The charger refused to charge one of them at all - just said “Err” on the screen. I put my DMM across it and the battery voltage was 0.58V! To try and recover it, I set my benchtop power supply to 4.2V and held the connectors to the ends of the battery. It drew ~2.4A for about 15 seconds or so. I checked the voltage again and now the battery read 3.06V. Placing the battery back in the charger, it now started charging.

The other battery was recognized by the charger, which showed the voltage to be about 0.6V. However, even at a charging current of 0.5A the voltage rose quickly. In a few minutes it had risen to around 3V and continued to rise as it charged.

Questions:

  1. Are both these batteries irrevocably damaged and hence should I just dispose of them?
  2. If so, were these likely to have been faulty cells or did I damage them by leaving them in the gimbal for prolonged periods (or something else)?
  3. What would cause an 18650 battery to drop to 0.6V and why does it rise so quickly when charged - e.g. even chemically, what’s going on?
  4. Was it unsafe of me to “jump start” the dead battery using my benchtop power supply?

Thanks in advance!

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    \$\begingroup\$ 4.2V is far too high for a recovery charge. Be careful around the cells until you feel confident that they won't fail. \$\endgroup\$ – Ignacio Vazquez-Abrams Mar 17 '18 at 11:22
  • \$\begingroup\$ The desktop supply probably knackered them! \$\endgroup\$ – Leon Heller Mar 17 '18 at 12:30
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Seems like the cells have protection circuit built in them. When the cell voltages went too low (probably below 2.75V), the protection circuit disconnected the battery. When you connected the battery to power supply, the protection circuit reconnected and you could measure the voltage across it.

  1. Are both these batteries irrevocably damaged and hence should I just dispose of them?

First put the cells in the chargers and let them fully charge. If your charger let you adjust charging current, set it to .5Amps. While they are being charged, check if any of the cells are getting warm. If it does the cell is probably damaged.

2.If so, were these likely to have been faulty cells or did I damage them by leaving them in the gimbal for prolonged periods (or something else)?

Never let 18650s fully discharge by leaving them somewhere connected. Fully draining the batteries is a guaranteed way of damaging them.

  1. Was it unsafe of me to “jump start” the dead battery using my benchtop power supply?

Batteries that have entered sleep mode, ie the protection circuit have disconnected due to overdischarge, are often jump started this way. This is known as boosting. But you need to limit the current, or else tha battery may catch fire or explode.

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  • \$\begingroup\$ Awesome, thank you! \$\endgroup\$ – Matt Knight Apr 4 '18 at 11:40

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