You need not worry about overcharging by a big alternator. The alternator gives out a certain regulated voltage, e.g. 14.4 V.
The battery will accept, at that voltage, a certain current
. This current depends on the state of charge of the battery and things like plate surface area and internal resistance (which are coupled to battery Ah size). It will not get damaged by a big alternator.
So you can connect a 115A alternator to an 8 Ah motorcycle battery without causing damage.
The advantage of a 115 Ah alternator is that you can charge even a very big battery bank, e.g. our standard 315 house battery bank. Or even bigger.
I have seen a maximum charge of 80 A, even at modest RPM's of the port engine. However, this is when the batteries are half empty; when they get fuller the current decreases.
One problem of the "stupid" 115 A alternator is that it keeps giving out 14.4 V even when the batteries are already full, e.g. you motor
out when you were previously on shore power
. In that case the batteries will start gassing by overcharging, and this is not good for them.
To prevent this you can connect a smart charger
, e.g. one of the Sterlings, to your alternator. These have 3 charging phases.
This improvement has already been discussed on this thread. It gives the even more important advantage that your batteries charge a lot faster, without harming them. Also the voltage can be adjusted to the battery type (gel, AGM
, open acid, maintenance
free closed type). And you can connect a temperature sensor which reduces cgaring voltage at higher temperatures; again to protect against overcharging.
Something else: we often go on one engine, at 1700-2000 RPM. STW is then 4.5 or 5 knots. We stop sailing when the boat speed goes below 2-3 knots. Then the 4.5 - 5 knots under one engine is fast enough, taking 1 liter of diesel
In case we used the starboard engine (nice for hot water), we would connect the house battery with the emergency switch. The charging current will be lower than by using the port engine. The reason is the higher resistance caused by the longer wires from starboard to port.
Now that we have our 470 Watt solar panels
charging by the engine is not necessary anymore. This is also the reason we don't have a smart alternator regulator