We’ve recently installed a LiFePO4 house bank in addition to an AGM starter battery to our boat.
Thanks to chrigu , our wiring issue has been resolved, but a separate concern has cropped up.
Basic system is a Lucas A127 70A alternator —> AGM starter battery ---- > 60A fuse — > Orion XS set to 40 Amps input/output —> 50A fuse -----> Victron Argofet Battery Isolator ---- > LiFePO4 680AH house bank. There is a Sterling external regulator fitted.
All works well when the starter battery is fully charged and in good condition, and requires as little as a couple of amps to give it a top-up.
But what happens to the alternator when a heavily discharged AGM starter battery needs to be recharged by the alternator in addition to the load demanded by the Orion charger?
Today we experimented and drained the starter battery from 105Ah to around 75Ah. We then disconnected the Orion charger, so the system resembled a simple car, with an alternator and a single AGM battery.
The engine started on the button and gave the following readings.
Time 00 minutes 62Amps
02 minutes 55Amps Alternator case temperature 41deg C
04 minutes 54Amps 50deg C
06 minutes 54Amps 58deg C
08 minutes 54Amps 60deg C
10 minutes 54Amps 62deg C
All of the above seems fine in isolation, and the 70Amp alternator would happily cope, especially with the charge dropping over time.
How would the 70 Amp alternator cope with the additional load of, say 40Amps demanded by the Orion DC to DC charger for the LiFePO4 bank?
Another consideration is that when the alternator voltage sags due to loading, the input current to the Orion (or any DC/DC) increases, further reducing input voltage. That negative input impedance can get you.
I have a similar situation on an RV. We had a dumb DC/DC with a smartshunt in the alternator negative lead to monitor total current. Used Node red to disable the DC/DC when the alternator was heavily loaded. I’ve recently switched to the Orion XS, need to rethink my strategy.
The Orion XS1400 has input lockout limits that could help. Set the enable limit close to the charged voltage of the starter battery, current should be lower by then.
The Orion XS has settings to restrict both input and output currents. For the AGM starter battery, this could be as little as 5 Amps, freeing up safely available current for the existing Orion XS to supply the LiFePO4 house bank.
Would need to be able to protect the alternator if it was suddenly faced with no load… The existing setup uses the permanent connection to the AGM battery as a “cushion” to prevent this.
Upgrade the alternator to a higher capacity. This is not difficult or particularly expensive. You may find that a higher capacity stator can be fitted to your existing casing. This avoids the need for different brackets and drivebelts but pay attention to cooling.
Changing the 70A alternator out for a 120A will probably be sufficient. Keep the revs up when charging both sets of batteries.
You can also fit a manual switch to the XS so that it can be de-activated when the AGM battery is depleted and needs priority. This does not need to be automated because presumably you will always be present when the motor is running.
The lead acid battery should always be connected to the alternator. The alternator and the XS need the buffering effect of a battery and should never be configured to run directly from the alternator to the XS with no start battery in the system.
