Sharp eyes, @Manul! I had not noticed the electrolytic capacitor behind the plastic divider. I think that "corrosion" is actually glue that was poured over that part of the board. Just to be safe I cleared the glue around the pads and scrubbed the area with water and IPA.
Thanks for the great tips. Here's what I've tried so far:
- Charged batteries and disconnected from the circuit to make sure they are not self-discharging. --> Ok, voltage (~2.7x V) was maintained overnight.
- Scrubbed the PCB with demineralized water and IPA with a brush, let dry overnight. Did not get rid of the parasitic drain.
- Measured the drain at ~25 mA with the DMM in series with the batteries.
- Heated with a hairdryer and noticed an increase in the current to ~32 mA within seconds. Heating the end of the board nearer to the charging coil (right, in pictures) makes no difference, but heating the opposite (left, in pictures) end of the board quickly sees a jump in current. No effect was noticed based on which side (top or bottom) of the board was heated, just which end.
Does this point towards a cause?
I don't have a bench power supply, so I could not test putting a voltage across the capacitor. I had previously measured a capacitance in-circuit with the flat batteries connected (I don't remember the value, maybe 8.8 uF or mF ?) but recently, and with the batteries disconnected, no capacitance is read (OL). Maybe that's something...
I have not yet removed the board. Aside from the 8 wires from coils etc, the board is secured by two plastic "rivets" which appear to have been melted or expanded after passing through holes in the board. In the first two pictures, they are whitish circles next to the Coil- and Coil+ solder points. Can the board be removed in a way that it can be reinstalled later? I could just squeeze the melted heads with pliers to remove the board, but I don't know how it could be secured later.
Anything else to try, especially without/before removing the board? If I can, I might de-solder the capacitor to measure it out of circuit. I don't have an ESR meter (yet).