Spend the money
to buy a quality DC to DC charger that has a remote
on/off switching ability. Use a BMS such as the Electrodacus SBMS 0 that has the separate charging cut and discharging cut ability and there are a number of units listed in the installation
manual that will work
with the Electrodacus unit and diagrams on how to wire them. A DC to DC charger is really no different to an MPPT solar controller so the wiring is the same.
Next, how many amps charging are you expecting from the solar and alternator and gen set powered inverter/charger? If you were to use the Victron 30 amp DC to DC for instance, you can run multiple units depending on the charging input you are expecting and only run the ones that suit the charging at the time, say a single
unit with only the alternator charging, but maybe 3 units with all the systems charging.
Simply wire all the charging systems you have now to the AGM start batteries and set the charger profiles to suit AGM, then power all the DC to DC chargers from the start battery. You should add a voltage sensing relay between the DC to DC chargers and the start battery to protect the start battery from excessively discharging it because that will shorten its life.
Reasonably simple to follow instructions for the Electrodacus unit and Dacian is always available via email
to guide you through any sticky spots you might have not quite understanding just what he is getting at in the instructions.
No, I don't sell these units or get any money
from Dacian for promoting his gear
. I am testing these units in regards to incorporating them into the BMS off grid systems I design and install to give a lot more information via the phone
or Ethernet or computer or what ever device suits your needs ..... the little screen
sends me cross eyed after a few mins, but I'm getting old and blind so ......