Thread: Cable size for connecting main and auxiliary batteries

Now a battery question. Is it true that all AGMs can be mounted on their sides?
I was talking to a salesman who was sure that Odysseys could go on their sides, but wasn't sure about other AGMs. I had thought they could all go on their sides as they are sealed.
Reason I ask is because I want to fit an AGM in the back of the Offroad Systems drawers, but the height is a bit marginal, so I'd like to mount the battery on its side. There is about a $100 difference between the Odyssey and similar AGMs.
Will this cause any problems?

Hi Mick and if you don't mind drilling some holes, if this means you halve the cable run, then your whole setup will benefit from the shorter cable length.

This will reduced cost and voltage drop which will allow for better operation of the winch.

Next, to be able to use your auxiliary battery while you winch, you MUST use a battery that is specifically designed for high current applications, like winching.

The Odyssey is and the Optima Yellowtop is, but most other AGMs are NOT. So check before buying.

Back to your cable setup.

Because of the distance from one battery to the other, you need to run at least 3B&S but thicker is a benefit.

Also, because the batteries are located in different parts of the vehicle, you need to fit 150 amp circuit breakers at or near the positive ( + ) terminals of each battery.

Because of the high currents used while winching, I would still recommend running a negative cable between each battery's ( - ) terminal, and run earth straps or leads from each battery's negative ( - ) terminal to the chassis. Setting up this way you have a guarantied KNOWN set current path between two batteries.

Thanks for that. Good advice. So 150 amp circuit breakers - pretty high. Better to overspecify I guess.

Hi Mick, 150 amps is a fraction higher than would be the norm for that size cable.

3B&S cable has a constant MAXIMUM current rating of around 200 amps and the normal maximum circuit breaker size should be 120 amps, but when the cable is part of a winch power supply circuit from an auxiliary battery, a 150 amp circuit breaker is still safe but you can winch off both batteries and not trip the 150 amp circuit breakers.

Couple of questions, if you don't mind.


The Optima website shows the largest Yellowtop as 75 amps - is that big enough for running a fridge? I've heard it said Optimas are underrated - do you agree?
While the D31A seems to be a dual purpose battery the D31T seems to be a commercial version and mainly deep cycle.
Also, the website says Optimas need 14.4 to charge correctly, but my alternator appears to make a consistent 14 - will that matter?
They offer a extra alternator unit on the website below - is that really necessary?


https://www.optima-batteries.com.au/...batteries.html


Also, how would you compare the Odysseys and Optimas? Odysseys seem a lot dearer - are they worth the extra cost? I see Odysseys can also be laid on their side, as I may need to do?

Where did you get the 75A rating from?

My guess is that you were looking at the Amp Hour rating of the battery, which is a rating of power delivery over time to discharge. There is some info for you here.
Most batteries are upwards of 600 CCA (Cold Cranking Amps) which is measured at -18 Celsius.
more useful for the real world is Hot Cranking amps, HCA, measured at 26.7 Celsius, which is a higher value than CCA.

There is a lot of useful info in the battery tech sheets which you can view on their websites. Most info is relative to standards, so you can get a good idea of what each battery's strengths are, and be fairly confident in comparing each with these specs in mind.

Originally Posted by DiscoMick

75 amp(hours) - is that big enough for running a fridge?

it depends how long you'll be running the fridge for between recharging the battery.

On the link I gave the 31T is shown as 75ah and 900 CCA. Its the highest rating given for any of their batteries and matches the 31A.
But I recently bought a 110ah AGM of a different brand for our camper trailer, so I have to decide if a 75ah Optima will run a fridge as well as the 110ah AGM does.
For height reasons I can't fit an AGM upright in the rear compartment of the drawers in my 110, but I can lay an Optima on its side, which I'm not sure about doing with the other AGM.
Hope that makes sense.

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