Our sailboat came with a Mastervolt system on board - common to European boats-I understand they have not made much headway with the American public. They seem a good product - German - though somewhat costly when compared to the many alternatives.
We run 550 amp hours of Ample Power gels, and an Optima gel starting battery battery with 1000 CCA. The charger is the Mastervolt IVO "Smart" 50 amp charger - it runs the expected three stage charging process, and has worked flawlessly for four years powering the boat - it is on full time since we live aboard, and is designed to provide an alternative power system from the batteries - it would run the boat even if there were no batteries.
Much to our surprise on a couple of very cold starts in out of the way places, the start battery was discharged. How is this to be - the battery switch was off, and the start battery was suposedly isolated from the house bank. The house bank had been taken down to the 35 pecent floor - about 12.3 volts.
We got to Canada and ran a few tests...
The house bank and starter bank were conected via the charger - even if the charger was off, with no AC, there was a connection between the two banks (a very smart fellow, who often posts on this site, had told me this, but I had not taken the time to figure it out.
So - the two banks are connected through the Mastervolt - the house bank is discharged, the electricity flows down hill from the highly charged start into the discharging house....
At start, during a capacity test, the start battery read 8.5 volts under sustained load - not enough to turn the engine over. Because it was still connected to the house bank - it had read 12.7 volts - a fully charged state, prior to the test. Voltage, however, is not a true reading of battery capacity.
We recharged the house and start batteries, disconnected the charger, then discharged the house bank. This time, the connection between the two banks was severed.
At 12.3 volts in the house bank, we started the engine again and tested the capacity of the start battery - under load it dropped to 10.5 volts, turning the engine over quite rapidly - the start battery had remained fully charged.
We had never noticed this before because of the way we used the boat - three week vactions always on the move - lots of motoring in the fickle NW winds - and not a lot of time in winter conditions running the heater under anchor.
It's an odd design arrangement to connect the banks through the charger - I have been told by the above learned fellow that Mastervolt designed it this way to avoid discharging the start battery - when in reality, in a cruising situation, when you take your house bank down a bit too deep, it has the oposite effect.
We had installed an Ample Power "Eliminator" that connects the house and start bank and charges the start battery in a three step process when it senses a higher load at the house bank from any charging source. This will now be our primary start battery charging source. Alternative solutions would be to use an isolation diode, or a battery combiner.
This was an odd issue - caught me by surprise because I had not heard of it before - perhaps someone else may have the same problem.