“Dead” Li-Ion is rejected by charger

[Oleenick]

I was recently using a lightsaber (high powered torch) for about 10 minutes, when it suddenly shut off. This had happened before and I just had to charge the protected 18650 Li-Ion battery back to full.

This time however, my charger did not charge the battery. Instead, after 18 seconds the charger displayed "FAIL". My initial thought was that the cell was under its discharge voltage or the chargers minimum charge voltage, it was at 3.2V. However, after manually charging the cell to 3.5 volts, the charger still gave the same response. By manual charging I mean hooking the battery up in parallel with another 4.2V battery (at 4.0V). This got it to 3.5V yet the charger did not begin charging. That does point to the charger. When I put it in the charger, the cell momentarily charged to 3.7V. This is probably just the output of the charger.

The cell still functioned, but with limited voltage. It's voltage when charging went to a maximum of 3.7V but no further.

The product page says "Fully protected against over-charge, over-discharge, short-circuit, and over-current". It does not mention under-voltage, but 3.2V is hardly under voltage. Is this correct?

The product page also states that the recharging voltage is 3.7V. I assume that this means the voltage for the cell to begin charging. Does this also mean that it should not drop below 3.7V?

Interesting to note: Not long ago I gave myself a shock, which felt like an oscillating voltage from the negative terminal of the charger. I measured the current through me and it was 1-2mA. The chargers AC adapter doesn't have a ground pin and I was not isolated from ground.

Any thoughts and ideas on what went wrong here?

[Siwastaja]

Never ever parallel two cells with different voltages. Overcurrent protection is not current limiting. Following charging current limits is even more important than following discharging current limits.

This is equivalent to shorting the higher voltage cell; but even worse, allows unlimited charge current for the lower voltage cell. If the "protected" cell means a good, reliable eFuse circuit, with a separate, low threshold for charging overcurrent event, it may have protected the cell, but to be sure, I'd consider the cell dead after such aggressive abuse event; even if they claim to have a protection circuit.

It is possible that either the external protection, or the internal cell protection has blown, and you are reading the voltage you see through a highly resistive leakage - a properly designed charger could sense this and deny charging.

3.2V shouldn't be low enough to prevent charging. Depending on the implementation, overdischarge detection on a charger should trig between about 2.5 to 3.0V, and often they offer a very low current conditioning cycle which can accept cells down to 2.0V or even below in some cases.

[Oleenick]

Got it. Thank you, that makes complete sense. The cell apparently has over-current protection, which I feel may have triggered in the overdraw of current when it first shut off.

Since the lightsaber is using current limiting resistors for the LEDs I bet that they caused an over-current event. This event alone probably lead to the highly resistive leakage.

I may purchase a new charger either way, as I don't particularly enjoy being shocked by it when the negative pin springs up into my hand when removing a battery.

Thanks again for that clear explanation, I really appreciate it.