This is my battery
charge enhancement project
. Maybe it will help some fellow like all the stuff on this forum helped me get to know my Mahe.
The setup of my Mahe is that the port engine
is connected to the house batteries and starboard engine
has it's own battery
So port charges the house battery and starboard only takes care of its own battery. Motor
sailing on starboard engine does not charge the house battery. Unless the emergency
start crossover switch is engaged to bridge both batteries, which I don't like as I know for sure I will forget to disengage it and be left with no emergency
start capabilities some day.
Both alternators are rated at 115A with a fixed 14,4 voltage with no temperature compensation and no compensation for voltage drops in the wires. With a relatively deep discharged battery it would begin to charge at 80-90A, way too much for the 315Ah battery capacity. On the other side a relatively full battery would not accept many Amps at 14.4v (minus losses) since the Mahe comes with Ca batteries that like higher voltages.
I did not like this setup so I changed it:
I moved the wires for the port engine to the terminals for the starboard engine so both start off and charge the single
starter battery that was intended for starboard only. Moving these wires is a nasty job, as the terminals are located on the right side of the box with the battery switches. At least the older models with all batteries on port have little space there. No worries, emergency starting is still functional.
Then I added a battery-to-battery or B2B charger
with full galvanic isolation between starter and house battery .
IF at least one of the engines is running this B2B charger
simply uses whatever amps it can get from the alternators and converts it into a temperature compensated and adjustable three stage charge scheme for the house batteries.
The alternators are not touched, both still operate at their fixed voltage. As the charger kicks in they sense the load and increase the output, theoretically up to the maximum they can deliver.
The B2B charger starts charging
only if the alternators deliver more juice than required to charge the starter battery and uses only surplus energy. It adjusts its charge current
so that there is no load on the starter battery even if the alternators produce little current
while iddling. Beside the battery wires it uses separate sense wires for both the starter and house batteries to know the real battery voltage, as well as a connection to the alternators D+ to switch it on only if an engine is running.
This setup works well and produces a fully charged battery in roughly the same time as the main battery charger.
The B2B charger I installed is made by Votronic in germany
. It is rated at 45A so just OK with the 315Ah capacity. It really delivers these 45A without any thermal issues. Cost was 250 euro, installation
took two hours to run the wires.
Bigger B2B chargers are available from Sterling.
This was done mostly for night passages and late season cruising when solar
does not produce enough juice to keep the batteries and watermaker
happy. This should not happen to often as I now have 560W of solar
(240w on davits
, 320w on bimini).