Outer Limits 4x4 Board

I have a GQ TD 42 Patrol running a piranha Duel battery system.
Its been in since about June 2011. Deep cycle 2nd at the same time, new main about feb last year.
I run a 50ltr fridge in the back of the ute and have run a 40ltr at the same time for 6week trips. The pissy Patrol alt. has coped with that so far and the system has worked ok till recently. The fridge ran both batteries down to sweet F.A.
I basically get the same voltage reading on the monitor in the cab for both batteries. So I guess the management is not isolating the batteries from each other?

i dont have a piranha set up so i'm not exactly sure how they operate, i run the sidewinder auto unit and the way they say to test it is working is this:

with a multimeter check both batteries while the car is off

start the car, the main battery should start charging immediately, once the main battery voltage hits the 'link voltage' (in my case 13.7v) it should start charging the aux battery (my unit has an indictator light to show they are linked). -- if the aux battery is low from running a fridge it won't jump straight up to around the 14.4v but the voltage obviouly be should increasing if they are linked.

to test the battery separate once the engine is off, turn the headlight (and spotties if you have them) til the main battery drops to the cut off voltage (12.4v in mine) and with my unit the indicator light goes out, or check with the multimeter.

have you checked the voltage coming off the back of "the pissy patrol alt."?? that can rule out one thing if it's charging correctly.

do both your batteries have good earths to the block?

these are obviously simple things to check, but i would reckon 9 out of 10 times or better it won't be the control unit, unless you've unlucky enough to get the lemon of the bunch

hope you have a win. lepa

Cheers.
Will investigate over the weekend.

Patrols, more so GU's, are known to have crap earthing and related issues.
I'm with Lepa on this. Make sure each battery is earthed back to the body, AND the chassis and/or block. Then run a decent earth cable from the block to the body for good measure, making sure all contacts are good and clean.

If still issues, ring AlAn at Pirhanha, he's a nice guy and should be willing to assist.

Just a question, are they both wet cell, or one wet cell ie exide and one gel, ie optima? Generally, they should be the same so they are both able to receive charge at the same rate.