Thread: Wiring for battery box

Originally Posted by Disco-tastic

Hi all,

As I no longer have my D3 i no longer have a second battery to run the fridge. As I dont wish to install a 2nd battery in the territory i am looking to purchase a battery box and an AGM battery. It will be a 100Ah+ battery. I'm leaning thia way as it is more versatile than a dual battery system and I can move it to the next car or to our second car if I want to.

I dont want to run it of the rear cig plug as I'm concerned it will charge super slowly through the thin wiring (im not sure what its rated to and it might get hot) so i am looking to install some 6mm cabling to the boot with an Anderson plug. I was also thinking of installing a relay to isolate it when the ignition is off.

Has anyone done this before? Any tips or tricks? Also, how do i calculate the fuse size? The main danger is shorting out somewhere (I'm not running any heavy loads off it, just battery charging).

Any help is appreciated!

Dan

Hi Dan
Calculate your fuse size from the current that you intend to run through the cable and then size the cable to the fuse
Have you got somewhere secure for your battery box, if you have an accident you’ll have 25+kgs of battery flying through the vehicle unless it is secured
Gav

From my experience 12V fridges generally draw about 4 amps give or take a bit. if you were to run a 10A fuse that would be ok, I would use a 15 or 20 amp fuse just to be sure. Cig/power outlets are notoriously dangerous running fridges, there have been more than a few stories about fires. to my mind Anderson plug type connectors are more reliable.

regards
Geoff

Originally Posted by Gav 110

Hi Dan
Calculate your fuse size from the current that you intend to run through the cable and then size the cable to the fuse
Have you got somewhere secure for your battery box, if you have an accident you’ll have 25+kgs of battery flying through the vehicle unless it is secured
Gav

Cheers Gav,

How do I calculate the current draw when the battery is charging from empty? Is there info like that from the battery manufacturer? I'm pretty sure the charging current is going to be much higher then the fridge draw current. Once i have that number I can figure the rest out.

The max output of the alternator is 130A.

I had also thought of seeing what size fuse most dual battery systems use and adopting that. Or giving Tim from Traxide a call!

Regarding fixing I have a cargo barrier that the battery will likely be fixed to. Havent figured out exactly where it will go yet but its unlikely to enter the cabin.

Cheers

Dan

Originally Posted by OldGuy

From my experience 12V fridges generally draw about 4 amps give or take a bit. if you were to run a 10A fuse that would be ok, I would use a 15 or 20 amp fuse just to be sure. Cig/power outlets are notoriously dangerous running fridges, there have been more than a few stories about fires. to my mind Anderson plug type connectors are more reliable.

regards
Geoff

Thanks Geoff,

I had thought similar but just not sure if ill need a bigger one for charging the battery. Any idea what current a charging battery draws?

Originally Posted by Disco-tastic

Thanks Geoff,

I had thought similar but just not sure if ill need a bigger one for charging the battery. Any idea what current a charging battery draws?

I am running 16m2 cable to my second battery, 25 would be a better choice and have a 100A fuse in line with that. I think it is an ANL type fuse, allows you to run your cable without reducing size to the fuse.

Hope the helps.

Regards

Originally Posted by OldGuy

I am running 16m2 cable to my second battery, 25 would be a better choice and have a 100A fuse in line with that. I think it is an ANL type fuse, allows you to run your cable without reducing size to the fuse.

Hope the helps.

Regards

Yeah that size fuse is more what i was anticipating. Thanks for the help!

Originally Posted by Disco-tastic

Cheers Gav,

How do I calculate the current draw when the battery is charging from empty? Is there info like that from the battery manufacturer? I'm pretty sure the charging current is going to be much higher then the fridge draw current. Once i have that number I can figure the rest out.

The max output of the alternator is 130A.

I had also thought of seeing what size fuse most dual battery systems use and adopting that. Or giving Tim from Traxide a call!

Regarding fixing I have a cargo barrier that the battery will likely be fixed to. Havent figured out exactly where it will go yet but its unlikely to enter the cabin.
Cheers

Dan

Hi Disco-tastic

From my experience (not a huge amount with 12volts so maybe Tim is the right man to talk to) your alternator may be able to put out 130 amps, but your car probably draws 50 of that to keep running

As Oldguy has stated he has 16mm cable in his rig and that is probably the minimum size I’d be putting in, cable current ratings are usually very easy to look up on the internet,
https://www.rpc.com.au/pdf/Wire_Chart.pdf
looking at the link above 35mm2 cable with a 100 amp fuse would be the way to go

Gav

Hi Disco-tastic,

For an AGM battery the charging current it will accept is generally about 1/3 of its A/H range (Gel about 1/4 and trad flooded 10 to 15%) . So if you were putting in a 120 A?H AGM you should bank on 40 amps charging current.
In your case the receiving battery will limit the current, as long as this current draw (40) is below the alternator delivery, but in the initial couple of minutes your start battery would be supplying a fair bit of the current anyway.
You would need to get this exact current draw from the battery you install, but give yourself some head room over that.

Given the length of your wire run would be about 3 meters (?), you could technically get away with 4mm (Squared) in a pinch, but I would be installing 6mm and a 50 amp fuse, ( I use a breaker ) that would give you a bit of head room.

I have had this setup for a 125 A/H battery in both my cars and it gives me no trouble at all . The wires will get warm for the first 15mins if the house battery is well down, but once the battery starts to gain charge the current will be easing off.

Installing 25mm and 200 amps fuse, would be better but might double the market value of your Vehicle.

Cheers Glen