When I bought this product, it didn't work out of the box and after disassembling it, the internal NiMH battries were leaking acid so they needed to be replaced.
I'd like to replace those cells by a single 18650 battery but the problem is for some reason the provided charger is rated at 6 V and 0.25 A and when I tried inserting the 18650 into the circuit and charging it, it would keep climbing above 4.2 V. I had to stop at 4.35 V since it would potentially explode and/or catch fire.
I tried inserting diodes to drop the voltage down but it seems to limit the current to ridiculously low levels so it would probably take a week to charge the device that way.
What else can I try to do to stop charging at 4-4.2V without limiting the current much?