Thread: Decisions, decisions, which dual battery system??

Tim on my current car I am running 35mm2 to the back where it charges 1 optima for accessories, my second optima has 50mm2 running to it from the 1st battery via a usi 160, so 3 batteries in total. Winch runs from the first battery via 50mm2 cable as well, starter runs 70mm cable because I had it lying around, its a damn big starter😊. It all charges fine, I did no calcs when I did this, I just used what I had in the shed☺

I never said you did, I was just telling tim what I run now as apposed to what was in my old rangie.
Anyway Nat P, tims isolator will do the trick for what you need, over and out

Hello all,

Thank for the info gang, I will take it all on board, does anyone run anything else worth mentioning?


Cheers Nat

You do understand that the voltage drop is only at the specified current? Your quoted article therefore does not apply. As soon as the battery has taken a significant charge the current drops and therefore the voltage drop reduces. The rising voltage will allow full charging in much less time than with an artificially low charge voltage.

Originally Posted by Vern

Tim on my current car I am running 35mm2 to the back where it charges 1 optima for accessories, my second optima has 50mm2 running to it from the 1st battery via a usi 160, so 3 batteries in total. Winch runs from the first battery via 50mm2 cable as well, starter runs 70mm cable because I had it lying around, its a damn big starter😊. It all charges fine, I did no calcs when I did this, I just used what I had in the shed☺

Hi again Vern, and when the cable cost you nothing WHY NOT USE IT!!!

I tested an Optima Yellowtop in a similar test in the back of the D4 ( as in the post above ) and again, the Optima was discharged to 11.58v ( 20% SoC ) and I had the same 6m run of 6B&S twin.

The optima was up around 95% after a 1.5 hour drive.

With your cabling, I reckon you will get the same results in about 1 hour.

As to claims that batteries will take 47 hours to fully charge from 45% ( 11.95v ) using a charge voltage of 13.5v. There must have been something very wrong with the battery.

Somewhere on Fullriver’s web site, they have a graph showing the different charge time at different voltage levels, for one of there 100Ah batteries.

From 20% ( 11.58V ) to fully charged at the lowest voltage they used, 13.0v, it took 30 hours.

Raising the voltage to 13.5v would see the battery fully charged in about 7 or 8 hours.

and a battery at the back spend 100% of the time not at high temps. WIN!


and why is that?


nope, 2m.

0.125 volts, massive drop!!11!!1!one!

you havnt shown that at all

You still don't get it. The current will not remain at either 20 amps or even 100 amps. It will taper off rapidly as the state of charge increases. The last 10% will charge much more quickly at 14.2-14.4V than at your imagined 13.5V or 13.8V.

Illustrative graph of constant voltage charging: