Thank you all for the responses. I ended up ordering a replacement battery, since i intend to sell the unit and don’t want to pass on a ‘pig in a poke’. Had the voltage been a little higher i would probably have attempted to recharge it.

And also: is a depleted battery more likely to short circuit and overheat?

Thank you for the elaborate response. Does this seem reasonable: Charging voltage 5 * 3,3v = 16,5v, charging current limit 100mA? And then put the whole thing in the garden? Would you take breaks?

Nice