Thread: Dual Battery Size

Hi David and the BMS is only for the battery it is attached to and does not work as an isolator.

The problem is as I posted earlier, because lithium batteries have a settled voltage of 13.2v constant and lead acid batteries have a maximum settled voltage of 12.7v, you MUST separate lithium batteries from any lead acid batteries as soon as the motor stops.

This in itself creates an additional problem in that conventional isolators are set to cut-in at 13.2v and cut-out at 12.5 to 12.7v. So they keep all the batteries connected connected

This again means your lithium battery will be continually charging your lead acid battery(s) until the lithium battery goes flat.

At this point, the lithium battery’s BMS will take over and protect the lithium battery from being over discharged.

But you still continually end up with a flat lithium battery, which makes it a total waste of time.

There is a lot of work and development needed before lithium batteries become truly RV friendly.


Hi Babs and if you are considering buying two of those battery packs, try this math.

One of my dual battery kits with the USI-160 isolator will give you a dual battery set up with emergency jump start built in.

Plus the Defender version of this kit is $195, including postage.

Next, if you get an Optima D34 - a 55Ah automotive grade AGM, from Michael at T/4, an another sponsor on AULRO, for around $275 delivered to your door.

You would have a fully automated dual battery set up including a 45Ah of auxiliary power for $470.

Add in to this Ah the additional power gain because of the way my isolators operate and you have around 90Ah of useable accessories power for that $470.

Your two battery pack set up will give you just 28Ah of auxiliary battery capacity and still needs manual connection for emergency jump starts, all for $550.

Just some food for thought!

Originally Posted by Witchdoctor

From what i have been reading, use a DC battery charger that is powered by the starting battery to charge the lithium duel.
All this set up is getting rather expensive .

Yep, so far the best set up I have tested is an inverter/battery charger configuration.

To accomplish this is an expensive retro fit set up, but the cost of the batteries pretty well demands a safe system to protect them.

The big advantage of using the inverter/battery charger set up is that while on the road, the system charges off the alternator but unlike DC/DC device, where once the motor is off they become a waste of space, whereas with the inverter/battery charger set up, as soon as you are near mains power, unplug the charger from the inverter, connect the charger to mains power and you can truly fully charge the batteries properly.

The other advantage is that with Lithium batteries being able to safely supply high currents for long periods, you can effectively use the inverter while camping away from mains power, giving you a 3 way use for this type of set up.

There is only one battery charger available that can charge any form of lead acid batteries but also is designed to properly charge lithium batteries.

Using the same battery charger, this same set up is also idea for camper trailer or caravan use, allowing the charging the house batteries and powering a 3 way fridge at the same time while on the road and because these chargers keep the house batteries and the 3 way fridge separated at all times, when you stop and turn the motor off, the house batteries and the 3 way fridge are still isolated from one another.

Again while this type of set up is initially expensive to retro fit, but if you are starting from scratch, it is actually much cheap to set up than it is to use a DC/DC device and offers much greater usage.

I have been developing a system that will enable up to 60 amp chargers to be used while on the road, which means it will easily cater for even the largest bank of house batteries ( lead acid or lithium ) yet it will operate from supply voltages as low as 11.0v at the camper trailer or caravan, which in turn, completely negates the problems caused by low operating voltage vehicles, like new Toyotas and Nissans, and can still improve the charging ability of new Land rovers as well.