Thanks for the comments so far. It's an Archer III, with a digital readout type ammeter, which measures alternator output - so in flight it's usually around 15 amps. It's a 24volt system and there is no obvious way to monitor the battery voltage from the cockpit. I intend to check the battery voltage before we fly next time, and also when we return. I suspect the regulator (in this case a separate unit on the engine side of the firewall), but I guess it could be bad connection somewhere, a badly done crimp, for eg. The check last time was done with everything off, and the engine speed set to give the highest reading on the ammeter....