Dual battery issues

[Smaguire93 Lurker Find all posts View Profile Close Smaguire93]

Hello folk. Just moved to auz and bought a 2004 prado 3l diesel.

it came with a dual battery installed and a mean mother distribution board in the back and a redarc solar controller.

got a 50l brass monkey dual zone. Seems to be drawing around 2.4amps according to mm distribution board.

so the question is. The max voltage I was getting when the prado was off was 12.2 at times going down to 11.2. Then the battery died 9.2v (some leaking noticed, second battery in engine bay).


so I got a new battery and wrapped it in insulation, 97 amp.

when the car runs the starting battery gets to 13.8 volts. The second (new) battery fluctuates between 11.6 and 13.2 volts (depending if the fridge is cycling). All I run is the fridge when camping, driving around 5-8 hours a day. Currently with car off and fridge running battery is reading 12.1 volts. Battery is 2 days old and drove 6 hours today.

long story short where should I start problem solving? I am assuming it’s not a smart alternator so is it as simple as a second positive from the alternator to the second battery? The previous owner who paid someone to do the install explained how there’s a push down button which links the two battery’s together.

I can provide any photos or additional information as needed, just need a helping hand to point in the right direction

thanks in advance (from the girlfriend, who says this battery and the voltmeter are going to be the end of our relationship)

[carco Advanced Member Find all posts View Profile Close carco]

You might need to visit an auto sparky.
If there’s a “button to push”, sounds like there’s a VSR somewhere.
The button would be to link both batteries in a hard to start situation.
A VSR will fully charge your start battery first then start charging the second battery.
An auto electrician will test both batteries and the alternator’s output, cable sizes etc.
From his results, you’ll know where to go next.

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

Something wrong, volts at aux should be very close to those of the cranking battery, check isolator, wiring etc. You can also install a booster diode to up the charge voltage.

[amts Ninja Poster. Find all posts View Profile Close amts]

What's the wire gauge to the aux battery?

[GeeWhizz Senior Member Find all posts View Profile Close GeeWhizz]

From what I've read from your post, you are new to 12v electrics. I don't know where you're reading the Aux battery's voltage from but my guess is that it's not directly across the battery itself. If the aux battery has a half-decent charge, it's voltage (measured across the battery's terminals) should not change much when a load like a fridge switches on and off.

The symptoms suggest that either the wiring is undersized or there is a bad joint somewhere between the battery and the fridge.

[Smaguire93 Lurker Find all posts View Profile Close Smaguire93]

So just to close the post. Took into the electrical system with my limited 12v experience to try and sort it.

a few things to note which may have contributed to the issues.

-winch and compressor we’re hooked to the deep cycle battery. Changed these to the starter battery
-a set of after market daytime running lights seemed to be dropping the voltage below the desired voltage to activate the vsr so disconnected these
-found one significantly melted connection running to the compressor (scary…)

after removing a shed load of old spot light wiring and replacing some dodgy/ loose connectors with soldered connections and heat shrink. Managed to get running voltage up to 14.2 volts.

then got on marketplace. Got a second hand redarc bcdc charger for 150 bucks. Now all seems good.

I removed the red box section of the redarc isolator which seems to enable it to act as a simple isolator now (I.e press the button links the battery’s, but doesn’t click on when the car is started- so only the bcdc is charging which is what I want)

so seems alls well that ends well