Thread: Aux batteries in parallel

my appologies sam;

I misunderstood the question from the beginning , I actually read it again and my thinking is what you are trying to do is connect a deep cycle battery to an existing Auxillary battery which is isolated from the main battery using an isolation switch ,so another words having a triple battery set up ? yes / no

Originally Posted by Bulldog

Umm, have i missed something here?

If you're running camp lights and a fridge you WON'T be able to start the car the next morning. The fridge will cut out nicely at X voltage, and the camp lights will drain the remaining charge until (a) the lights go dim and you walk home, or (b) you don't notice the lights go dim, and you walk home. Correct me if i'm wrong.

Don't know about anyone else, but i'd prefer my fridge AND camp lights to cut out when the aux batt is flat/over flat/dead/etc and still be able to start the car the next morning, keeping in mind (being a LR ), it might need to be cranked for 30 seconds before it starts.

Alternator DOES charge both batteries all the time, except for the first minute or so of running... big deal on a X hour drive... Also, if the aux batt is overdrained and stuffed, the main is still as good as new. OK, the beer's a bit warm, but i can still DRIVE to the pub for a cold one .

The waeco fridges have a 3 position cut off selector: low, med, high. Doesn't stop your camp lights stuffing things up!

IMHO, the manual switch is useless (had one). As soon as you turn the ignition off, your main batt starts charging the flat one until you remember 3 hours later. Been there, done that.

Hi sam_d, the short answer to your question is yes, you can parallel the two auxiliary batteries.

Now for the long winded version.

You need to be aware of a few potential problems but these problems would exist whether you had two totally different batteries or two identical batteries.

Even having two identical batteries does not mean they will charge or discharge identically but they will behave in a very similar manor.

Having a worst case scenario of say a Flooded Wet Cell Deep Cycle ( WDC ) and an AGM battery paralleled will have some differences in charging but in the situation you are planning to use them in, it won’t really matter when it comes to discharging them together.

With two different types of batteries in a low state of charge, if you go for a drive, while driving the AGM will charge quicker than the WDC but if you drive long enough, and the amount of driving time needed depends on how low the batteries were when you started out, both batteries will be fully charge or near enough to make little difference.

In this situation where the batteries are fully charged, when you stop the motor, as some AGM can have a slightly higher voltage when fully charged than does a WDC, so with the motor off, the AGM will discharge the difference into the WDC. This won’t occur instantly but more likely over a period of several hours but other than waste a very small portion of the AGM’s stored capacity, there is not going to be any major problems.

BTW, this discharge from the AGM to the WDC will NOT charge the WDC, it will just use up ( waste ) the difference in the two batteries

On the other hand, if you don’t drive long enough to fully charge both batteries, when you stop the motor, because the AGM is likely to be at a high state of charge than the WDC, again, over a period of a few hour to even days, depending on how great a deference there is in voltage levels between the two batteries, the AGM will again discharge down to the WDC battery’s voltage level. Doing this sort of thing MAY eventually have an effect on the AGM battery.

Now to discharging. With the batteries paralleled and fully charged, when a load is applied, both batteries will supply the current needed to operate the appliance(s) at a current rate proportional to the size and state of charge each battery. In other words with two fully charged batteries, if one is a 50 A/H and the other is a 100 A/H and the is a 6 amp load, the 50 A/H battery will provide 2 amps and the 100 A/H battery will provide 4 amps. They will both discharge at the same voltage level whether you have 120 A/H AGM and a 55 A/H WDC or vice versa makes no difference.

If on the other hand, one of the batteries is in a higher state of charge ( again it makes no difference as to which one has the higher charge ), depending on how great a difference there is, the higher charged battery will provide most, if not all the current until it’s voltage level is equal to the lower charged battery, at which point both batteries will then supply the required current,

The equalising of the batteries voltage caused by a load is going to occur MUCH quicker then what happens with the two batteries just sitting paralleled with no load applied and again, there shouldn’t really be any problems.

Ideally though, once your trip is over, it would be best to make sure both batteries are fully charged, even if this is simply achieved by a long drive home.

This is more to give both batteries a chance at a long life rather than having two different types of batteries paralleled.

Cheers.

Thanks for the new answers

I only asked as I have had a snorkel fitted recently and as a result I no longer have that trumpet thingy sticking out of the air filter box. I noticed the new gap created in the engine bay an wondered what I could do with that if I increased it's size by also removing the bottle jack.

I already have a aux battery set up in the rear of my car (my old cranking battery) and was just wondering if I could do anything with my new gap!