adding 3rd battery

Submitted: Saturday, Jun 02, 2007 at 11:00
ThreadID: 46174 Views:1695 Replies:4 FollowUps:2
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Hello comrades

I have a Calcium SLA battery as a starter in the TD Jack. I have a 70 a/hr AGM as an auxillary under the bonnet, isolated by a Pirahna 150 amp unit. I used to have a 120a/hr spare AGM in the back which I just kept charged and chucked in the back when going away when the possibility of staying stationary for 3 or 4 days occurred. It was not charged via the vehicle in any way. I have just made a fridge slide/drawer unit and the 120a/hr no longer fits. I have purchased another 70 a/hr AGM and am wanting to link the 2 auxillary batteries together, both to be charged via the vehicle.

That was a bit of history, now for the question. Is it possible and advisable just to link the 2 auxillary batteries in parallel via 6mm wire, positive to positive, negative to negative, and have the isolator connected only to the battery under the bonnet, and to draw power only from the one under the battery? I am thinking with my limited 12v knowledge, all I am effectively doing is creating a 140 a/hr battery, but just wanting to check. Also, is there a need for some sort of fuse or circuit breaker in between the two? Should I run negative wire from battery to battery or will be earthing it to the chassis be sufficient?

Thanks in advance.

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Reply By: D-Jack - Saturday, Jun 02, 2007 at 11:01

Saturday, Jun 02, 2007 at 11:01
2nd paragraph should read 'draw power only from the battery under the bonnet'!!!
AnswerID: 244154

Reply By: Member - 120scruiser (NSW) - Saturday, Jun 02, 2007 at 11:27

Saturday, Jun 02, 2007 at 11:27
D Jack
I have been thinking the same thing for the past 3 weeks as I consider which new car I will purchase to upgrade from the Prado. I am thinking of a dual cab hilux with a Metal Form Industries tray.
Derek from Affordable Batteries, a member and advertiser on this site should be able to help you out. Here is his link.
Site Link
Hope this helps
AnswerID: 244157

Follow Up By: Rumbler - Saturday, Jun 02, 2007 at 12:46

Saturday, Jun 02, 2007 at 12:46
D Jack & Scott
I'm another with "exactly" the same thinking/issues, for a 78 Troopy. Would love to know the outcome. If you seek & get a "personal response" from Derek, could you please let us all know in a posting of the outcome.

Cheers, Rumbler
FollowupID: 505122

Follow Up By: Member - 120scruiser (NSW) - Saturday, Jun 02, 2007 at 12:58

Saturday, Jun 02, 2007 at 12:58
Hi Rumbler
I just checked out Derek's web site. Good read.
Here it is
in case you are interested.
FollowupID: 505125

Reply By: drivesafe - Saturday, Jun 02, 2007 at 17:16

Saturday, Jun 02, 2007 at 17:16
Hi D-Jack, there’s no problem linking the two batteries together but 6mm ( which is actually 4.5mm2 ) is way to small. If the batteries were side by side 6mm would be fine but even somewhere else in the engine bay and you need at least 8B&S ( 7.9mm2 ) but for a battery located at the back of the vehicle you need 10mm2 minimum or you will get to much of a voltage drop, particularly while the fridge is running.

You will need both positive and negative cables and you need to fit 30 amp circuit breakers at either end of the positive cable.

AnswerID: 244187

Reply By: obee - Saturday, Jun 02, 2007 at 18:03

Saturday, Jun 02, 2007 at 18:03
Never earth via the chassis if you can help it. Stray currents equals corrosion. Many electrical faults in old cars used to be caused by earthing lights etc thru chassis. Might be ok using the main beams on a proper chassis but have to make sure the main battey has a good earth straight to same.

My opinion anyway

AnswerID: 244200

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