Thread: Dual batteries?

Well the unit is in and I am very happy with the result.
Kicks in at idle to charge the second battery after a 10 second delay (13.7V).
When I hook up the charger to the back battery it starts charging the front.
Negligible voltage drop over the MOSFET (0.05 at most).

Step 1:
Buy a battery

Step 2:
If the battery is too big to fit, make it by:
Remove carpet covering the compartment and the main boot cover carpet.
Removing the aft vertical carpet by undoing the netting clips and tie down strap bolt.
Remove the popper clip and netting clips from the fwd carpet
Undo the plastic nuts on the plastic cover pop the popper clips on the outboard side of the panel and remove.
Cut as applicable, including channels underneath for cabling.
remove the wiring stud on the floor of the compartment and re route wire to inboard side.
Test and adjust fitment of battery and plastic panels.

Step 3:
Fit and wire the MOSFET and Terminal block in front of compartment;
I made my terminal block out of a fancy stereo battery terminals from Supercheap Auto and cut off the clamp end. I then put it in a sealed box cut holes for the wires and hot glue gunned it to the side of the compartment (photo 3).
I also combined my 8 gauge cable in to one crimp (photo 4) and hooked up my earth to the point in photo 5
To mount my MOSFET I cut part of my old number plate holder (bolt holes were the same spacing) and hot glue gunned it to the front of the compartment then bolted the MOSFET to it.

Step 4:
Fit battery.

Step 5:
Re assemble


Admittedly with the battery size I used this set up is not really revertible back to stock, as can be seen from the previous photos in this thread I had to modify the plastic trim to get it to fit. The MOSFET relay is a tidy well made unit built for the purpose, however expensive and requires you to do all the work your self.

Total Cost: $570
Battery: $300
Wire and Crimps: $30
Terminals: $40
MOSFET $200 + Postage

Total Time: 3 Hours
Fitment: 1 Hour
Cutting: 1 Hour
Wiring: 1 Hour

The MOSFET


Final Install – carpet


Terminal block.


Integrated wiring, almost never know it was there until you see the battery


Ground wire attaches to the visible point and then runs into the loom down to the MOSFET (black wire).

Hi Jonny and a very interesting set up.

Can I make one suggestion. Please secure the battery some how.

That could easily became one huge missile in an accident.

Originally Posted by drivesafe

Hi Jonny and a very interesting set up.

Can I make one suggestion. Please secure the battery some how.

That could easily became one huge missile in an accident.

Suggestion no necessary.. See attached

It is hard to tell, what is holding the battery down?

The battery is a snug fit on the sides, I actually have to remove the loom on the inboard side and the plastic cover slide it forward and lift from the front first to get the thing out. If it is not dead straight it wont even budge. Also the carpeted panel in the picture is hard up against the battery and the edge of the popper clip panel overhangs the battery in the center which is why the handle is removed. I was thinking of a bracket, even brought one with the battery, however I am willing to risk my safety and that of all of the members of my family and all my friends on the belief that the mount is not necessary and the battery a secure fit.

My safety is paramount in anything that I do and while some will insist on a system that is bolted down in a more traditional fashion to me secured is secured and this battery is secure. Further the impact required to move that battery form its mounted position.. I doubt I will be walking away from..

That being said I have not crash tested it, I doubt anyone who has made there own second battery kit has crash tested it (by choice). So how do you know any battery kit is going to last a collision? You don't, you secure it and hope its secure enough. Or you could get an engineer to write a report considering every variable they can think of.

If you want to put a decent sized battery in the back of your rover and are skeptical about the security of the fitment or you just want to test it out be my guest, bring your engine crane and a seat belt to Canberra and have a crack at removing it.

Complementary coffee on arrival and I might even offer a prize.

I came across an interesting comment about dual batteries on the FullFatRR forum by bgennette (also on this forum) in this thread (interesting in itself!): fullfatrr.com - View topic - L322 Trailer Wiring - How it should be done, with pictures !

I have a dual battery system and the main and extra batteries seem to be expiring sooner than expected.

Originally Posted by bgennette

* And now for the L322 Gotcha. The Engine Management Computer measures current load over the ground lead between the starting battery and the chassis along with voltage at the positive battery terminal in order to better manage charging of the starter battery. If a second battery is in parallel with the starting battery (even if it is behind a Voltage Controlled Switch) the EMC will become confused and will not charge either battery properly resulting in (dramatically) shortened battery life. The early model L322 (2002-2005) can avoid this by connecting all extra ground leads to the chassis (never to the starting battery's ground terminal). But later models have Euro 5 level battery management and require complete isolation of extra batteries via Direct Current to Direct Current step up/down Chargers. VCS is Ok for before 2006, but later models must use DC-DC Chargers instead.

My extra batteries are in the rear so the earth lead goes to the chassis, not to the starting battery.

Hi Ron, how often are you driving and/or charging your batteries?

Originally Posted by p38arover

I came across an interesting comment about dual batteries on the FullFatRR forum by bgennette (also on this forum) in this thread (interesting in itself!): fullfatrr.com - View topic - L322 Trailer Wiring - How it should be done, with pictures !

I have a dual battery system and the main and extra batteries seem to be expiring sooner than expected.



My extra batteries are in the rear so the earth lead goes to the chassis, not to the starting battery.

That's normal Ron, can't think of any reason why you would run aux earth to neg of main battery.