Originally Posted by Polux
Volvo Penta has a a dedicated bank charge distributor for that set up (two alternators) as an option that does exactly that and it is not expensive.
Volvo re-labels a Sure Power Diode Isolator. Diode isolators are a very antiquated manner in which to charge batteries that have a number of issues when compared to a VSR/ACR/Combiner, Echo Charger
, Duo Charger or a DC battery to battery charger.
Unless Volvo can provide a method of externally sensing the banks then a diode isolator will result in a voltage drop across the diode of -0.6V to -1.0V depending upon rate of charge and this can result in chronic undercharging and lead rather rapidly to unhealthy batteries..
If you run one alt to the start battery, and have no parallel ability, then the vast majority of the time that alternator is simply dead weight
providing a couple amps at best to the already fully charged starting battery. This would be wasteful...
By using a combing/paralleling relay (VSR/ACR/Combiner), one that is activated & deactivated by voltage, you can wire one alt to the house and one to the start and the current
from both alternators will flow where it is needed.
This will decrease bulk charging times for the house battery bank but there will be slight voltage variances when approaching absorption and one alt will shut down before the other. This is not a big deal as it will just kick back in if voltage drops a bit and they will ping-pong their way along until one alt is all that is necessary to maintain the absorption voltage. The big gains will come during bulk charging with both alts providing all the current they can muster.
Alternatively both alts can be fed to house bank then a smaller combing relay or Echo or Duo or battery to battery charger could be used to charge the starting bank. This would give the most even voltage sensing performance but the gains would be small. If going this way an Echo, Duo or B2B can be used but if going the other way, one alt to each bank, a bi-directional VSR/ACR/combiner should be used.
I would not suggest just letting that alt to sit there and deliver 2-3A as that is a lot of wasted potential energy.......