Aha! Well despite the fact that this is going to make me look pretty stupid, I'll tell you what the problem was...having stated that my meter was correct and that the batteries were down around 200ah and should therefore be demanding more from the alternator
, it would appear that I was wrong! We turned off all power sources (solar and wind), turned the fridge up, left a few lights on and went to bed
. In the morning the battery
voltage was down to around 12.4V, and on starting one engine
was producing 65A, the highest yet seen. The other engine
on it's own produced about the same, although with both running together there was no real difference.
Further, having done a voltage drop test across both the negative and positive sides of each alternator/battery bank, it seems I have a drop of around 1V on the negative side - very bad according to Mr Calder. Cable sizes are fine so must be poor connections somewhere, that's tomorrow's job...
We're hauling the boat in a few days so will have mains power and can therefore fully charge the battery bank and recalibrate the Mastervolt meter.
Many thanks for everybody's help and opinions, and a very Happy New Year to you all!