Thread: Discovery 3 - additional dual battery addition - unexpected!

Originally Posted by theelms66

I see the box has a 50 amp charge plug that says VSR BUILT IN . Does this mean this battery wont receive charge until the other 2 batteries have a reasonable SOC?

Again because the primary charging source is the alternator, the battery in the box will start receiving a charge shortly after the motor starts.

Where any potential problem can arise is if the battery in the box has been used independently of the other two batteries.

In situations like where a fridge is powered from the battery box, and the BLACK socket is being used to charge the battery box, the other batteries will not provide any power to the fridge one the motor is turned off.

The battery in the box will continue to power the fridge and will become discharged at a lower level than the other two batteries.

When the motor is again started, because the cranking battery is in a near fully charged state, the D4's BMS will see this and run the alternator at a lower operating voltage.

This will in turn, both cause the battery in the box to charge at a slower rate, if at all, and if the D4 drop the alternator voltage below 12.7v, the battery in the box will not charge at all and as it is likely to still be powering a fridge, it will just continue to be discharged further.

The best operation is to use one of the battery box's GREY Andersons and this will mean all batteries will be used to power the fridge while the motor is off.

When the motor is started, the D4's BMS will see the cranking battery is not in a fully charged state and will run the alternator at a higher voltage for a longer time, and this will result in all three batteries being rapidly charged, in a short drive time.

I am still a little concerned with the visible size of the cables running from the GREY Anderson plugs to the circuit breaker and the size of the circuit breaker.

Hi Jeepster, and while a D4 BMS is a lot smarter, the D3 also has a smart alternator and you get some interesting voltage reading when the batteries are down a little and you start your motor.