Thread: Beware Charging Dual Battery in D4.

I have installed a dual battery system in a D4 TDV6 3.0. I also had a GME remote head UHF Radio fitted which is powered directly from the dual battery system. Luckily the GME radio has a setting that allows you to constantly see the voltage being supplied.

I soon learnt that the second battery was often not being charged even when it should have been (ie voltage of dual battery was between 11.9 and 12.7 volts). I then replaced the normal dual battery isolator with a DC to DC charger (Redarc BDC1220). This will start charging a second battery when the main vehicle battery is above 13.2v and will stop charging when the main vehicle battery drops below 12.7v. It also progressively stops charging the second battery when it becomes fully charged.

This set up also has problems with not always charging the second battery when needed and I have finally worked out why.

By monitoring the GME radio voltage I noticed that the second battery would only receive a charge when I was rolling down a long hill or rolling to a stop. When I drive on a relatively flat freeway the second battery would receive no charge.

The D4 trick is to only charge the vehicle battery when the car is slowing down. This is so fuel and power are saved when cruising or accelerating by not driving the alternator. When cruising the main battery does not receive any significant charge from the alternator and therefore the voltage does not go above 13.2V and therefore the second battery isolator (or DC to DC charger) does not charge your second battery. Only when you are rolling to a stop and the car charges the main battery does the voltage momentarily exceed 13.2V.

It is also interesting to note that the D4 will let the battery drop below 12.7V from time to time. I know this because the Redarc stops charging when the voltage drops below 12.7.

So if your second battery in your D4 is either not fully charging or taking a long time to charge then you now know why. I don’t have a solution yet so if someone does then please let me know. I also don’t know if this also affects the TDV6 2.7.

By the way the battery is an Allrounder 60AH under the bonnet and a second Allrounder 60AH in the cargo area.

Peter

Hi RoverLander and Blknight, and close Blknight.

I don’t actually use comparators but similar.

Where the operation of my isolators differs other isolators and corrects the low voltage problems the others suffer from is that all other isolators simply cut-out once the motor has stopped running or shortly there after.

My SC80 keeps the cranking battery and any number of auxiliary/house batteries connected and what ever loads are applied to the auxiliary/house batteries is shared with the cranking battery.

When the common voltage of all the batteries reaches 12.0v, the isolator cuts out and protects the cranking battery from any further discharging.

This is not aimed at you Blknight, but a correction of a commonly stated but totally misleading bit of advertising hype, used by most dual battery isolator sellers and thats the myth that an isolator whats till the cranking battery is fully charged before connecting the auxiliary/house batteries.

There is no polite way to put this because it is just total crap, with the exception of one or two very expensive DBS, isolators have absolutely no idea whether the cranking battery is fully charged or dead flat.

All isolators, including mine, monitor the voltage applied to their input terminal. When the motor is started and the alternator raises the vehicle's voltage to a set level in the isolator, the isolator cuts in and that’s all there is to it. The cut in level has nothing to do with the State of Charge of the cranking battery.

This myth is just one of a number of cons used to win over unsuspecting customers.

Now back to the SC80. Because it keeps all the batteries connected until the common voltage drops to 12.0v, unless there is something seriously wrong with your D3/D4, all your auxiliary/house batteries will have no problems charging while you drive.