Cure for 20s battery drain when turned off

I now have 2 units that drain batteries within 24hrs, and one of them, if left for 48hr will not recharge. As they are 2 1/2 yr old, and one had been opened, support only advised to replace them, after doing the usual reset/fault code clear procedures. As it is a well known fault, has anyone actually found a cure? The worse unit was fine until we got caught in a bad downpour, so after Sena's reply, I opened the unit to find water ingress on the board in 2 places. I removed the board, to find a second battery underneath. I cleaned the board with contact cleaner on a number of occasions, even scrubbing it with a fibreglass cleaning brush, but with still no joy.. I have a spare battery, so tried all combinations, and again same issue. Seems a shame to have to throw away over $900NZ after such a short period of time, so hope someone somewhere knows of a solution that is more sensible than Sena's 'upgade to new units'. Thanks in advance.