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I left my 3.7 V Li-ion cells charging for a long time with a low current 5 V charger. Now when I check the voltage, it is at 4.31 V. The cells didn't get hot, not even warm, but I'm afraid of what this could mean, since they were only supposed to charge till 4.2 V.

Does this mean they are now useless? Or lousy? It's a lot of cells in parallel and losing them would be the worst, so I need to know if 4.3 V is not so bad or have I destroyed them, or maybe decreased the capacity?

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    \$\begingroup\$ I'd discharge them a bit to minimise wear. \$\endgroup\$ – user_1818839 Mar 13 at 20:25
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The open-circuit output voltage of your Li-ion cells (what you have measured and reported) depends on the specific chemistry of your cells. 4.3V is not uncommon.

The main issue here is that you are not using an inexpensive charger that is specifically designed to properly charge your expensive cells. I recommend such a charger. A simple power supply with a 5V output is not suitable.

I believe that your cells will continue to serve you well once you invest in a proper charger.

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  • \$\begingroup\$ So for now the cells are fine? And I definitely will invest in a proper charger \$\endgroup\$ – Bleeding Edge Lab Mar 13 at 20:21
  • \$\begingroup\$ They probably sustained damage due to your use of an incompatible charging technique (this last charge isn't the only culprit), but what can you do? Just use the cells for as long as they provide service. Proper charging in the future will afford you more service out of any cell. \$\endgroup\$ – HypeInst Mar 13 at 20:34
  • \$\begingroup\$ Just to reinforce what HypeInst has written here, never charge lithium ion batteries except with a circuit specifically designed to do so and a BMS(Balancer board for series cells) if necessary. Never discharge lithium ion batteries without a properly designed circuit to prevent overcurrent and over discharge. This last bit is what is meant when you see "protected" 18650 cells, but if it's not present, you need it as an external board. \$\endgroup\$ – K H Mar 14 at 3:56

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