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I recently received a cheap (and probably non CE-certified product) that is capable of charging a few 18650 cells. I use it to charge recycled cells before decharging them to measure capacity. However, I found that the charger charges my batteries to 4.27 volts, a meaningful bit higher than the 4.2 volts that is usually regarded as a common lithium-ion chemistry maximum. I never leave the charger unattended, especially now that I have seen this interesting no-load voltages. I know there exist special more expensive cells that are specified to go up to 4.35 volts, but these can not be assumed to be.

So, would there be any danger because - or any damage done to - charging normal 18650 lithium-ion cells to be around 4.27 volts at no load? The batteries seem to even keep such a voltage over the few days that they've been sitting around.

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  • \$\begingroup\$ 4.27V is in the danger zone. You may have no disasters, but it would be of no surprise if you do. The danger is painting out metallic lithium. As Ryan says, cycle lifetime will be noticeably reduced. || Does the charger terminate charging ever? If not, discard it immediately. If it does it's still extremely bad. The voltage may be able to be adjusted - possibly by altering a voltage sense divider. But maybe not. \$\endgroup\$
    – ATCSVOL
    Jan 20, 2022 at 22:21

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Charging a Li-Ion battery to 4.27V probably won't cause a fire, but it would make me uncomfortable. Your batteries will likely suffer from a reduced lifetime however. Fully charging and discharging the battery puts stress on it. That stress will reduce the lifetime of the battery, and the capacity will continue to reduce as time goes on.

To limit the stress put on the battery and increase its lifetime, it's usually a good idea to keep the SoC between 20-80%. This will increase the lifetime of the battery, but obviously you don't get to utilize the entire capacity of the battery if you do this, so it's up to you to decide what's important.

Once the battery voltage goes above 4.3V, you start to get into the danger zone. I would probably buy a new charger if I were in your shoes. Even if the only reason was to help save the life of your batteries, it's probably a good investment.

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A more nuanced answer than you received so far is that 4.27 V is not a significant problem as long as the charger is turned off (disconnected) after the cell is full. Then, the cell voltage will relax down to a safer value.

Compared to charging to 4.2 V, the advantage of a higher charging voltage is that it stores a tiny bit more energy in the cell. The disadvantage is that the life of the cell is reduced.

There is no "wall" at 4.2 V, below which everything is fine, and above which bad things suddenly happen. No. Instead, it's a gradual increase in the rate of degradation. 4.2 V is more of a convention than a hard physical reality. 4.1 V is better, 4.3 V is worse. 4.2 V is a good compromise.

The real damage only occurs if you keep the cell at 4.27 V continuously: the cell degradation is proportional to the length of time it is kept at that voltage. If the cell is not kept at that voltage, the degradation is small.

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You are using charger with a bit too high voltage for capacity measurements. Each cell may read be 2%-3% or even 5% higher capacity on the discharge test after above mentioned charge. Your discharge test are quite a bit off and not as consistent as could be. Cell damage from one-off charging to 4.27V followed by discharge - I would say that is not a concern, but you'r measuremets are a bit off.

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