Aux batteries in parallel

[sam_d]

Question:

You have an auxiliary battery in your car connected to the main battery via a dual battery controller.

The existing auxiliary battery is an ordinary cranking battery (ie. not a deep cycle battery). You want to connect a deep cycle battery in parallel to the existing auxiliary battery. Is this:

a) Okay. This shouldn't be a problem. :D
b) Okay but maybe a little bit iffy. :)
c) It won't work very well. :(
d) No! No! No! No! People like you should not be allowed near electricity! :o

Note: this is just a thought I had earlier and I haven’t actually planned to do anything yet.

[Disco_owner]

G'day Sam;

your main Cranking battery and the 2nd battery which is also another Cranking battery can be hooked up in paralell, no issues there. but If you are running a Fridge at a camp site it will be drawing from both batteries,so be warned !

some fridges eg Engels have a Cut off switch to stop Drawing current from your parallel battery set up when total voltage drop reaches a Certain voltage ,think this may be about 10 volts so that if you are on a camp site
running camp lights / fridge etc you will be able to start your car next morning with some power left in both batteries , however not all fridges are fitted with this device , so be careful not to Drain your batteries if you're static at a camp site.

I heard someone from our 4wd club mention this a while back that this actually a Prefered installation for most people who set off out push for weeks on end,No voltage sensitive relays / Electronics to control the charge patterns , the alternator is charging both batteries all the time.

However as an alternative option you can install a Manual Switch which allows you switch between your Main/Aux/Both.

[BradM]

Hi Sam,
The people use use only a very heavy duty solenoid as the Aux battery isolator have the deEp cycle and ordinary battery hooked up in paralell all the time.

I had this setup in a 1996 GQ patrol when I lived in the NT.

BradM

[BradM]

Forgive my spelling - rush job

BradM

[streaky]

I would have thought that when connecting two batteries in parrallel they should both be of the same type,size, cca etc so as to avoid the stronger of the two betteries dragging off from the lesser one?

I don't know for sure but I had two batteries in parallel lnked up for winching on my series III lightweight this way & a another battery for engine duties.
This worked fine untill one of the winching batteries dropped a cell...then the remaining battery went down hill fast.

Regards.

S.

[PhilipA]

Pre electronic Engels do NOT have a cutout.
the compressor will work down to 10.5 volts, but it will just keep on trying, flattening the battery further.
AFAIK The Waeco has a low voltage cutout.
Regards Philip A

[Disco_owner]

yes that is correct , the Post Electronic models have the cutout circuitry built in, haven't had much experience with Waeco Fridges.

[Bulldog]

G'day Sam;

your main Cranking battery and the 2nd battery which is also another Cranking battery can be hooked up in paralell, no issues there. but If you are running a Fridge at a camp site it will be drawing from both batteries,so be warned !

some fridges eg Engels have a Cut off switch to stop Drawing current from your parallel battery set up when total voltage drop reaches a Certain voltage ,think this may be about 10 volts so that if you are on a camp site
running camp lights / fridge etc you will be able to start your car next morning with some power left in both batteries , however not all fridges are fitted with this device , so be careful not to Drain your batteries if you're static at a camp site.

I heard someone from our 4wd club mention this a while back that this actually a Prefered installation for most people who set off out push for weeks on end,No voltage sensitive relays / Electronics to control the charge patterns , the alternator is charging both batteries all the time.

However as an alternative option you can install a Manual Switch which allows you switch between your Main/Aux/Both.


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 :o), 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 :D.

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.

[sam_d]

Thanks for all the responses everyone. I'm afraid none have actually answered my question (though streaky was close)...

What I was asking is this:

Is it okay to connect an ordinary cranking type battery (not the actual cranking battery) and a deep cell battery in parallel?

They will both be aux batteries connected to the main battery via a dual battery controller. Is it okay to do this?

[zwitter]

I would say d)
The ideal would be 2 identical batteries only should be connected in parallel. Infact often the cells are preferably from the same batch and even matched for performance.

But in the real world in a automotive environment things are far from ideal. I would still say do not mix types of batteries and in general the closer the batteries are in size / capacity / condition etc the better they would work.

The big solenoid system should just be wired to not connect the second battery until the car is running and preferable a few minutes later.

You could use the big solenoid charge method and just use 2 solenoids but would be better using a system to sequence the batteries into charge connection.

Or use voltage sensing relays and cascade them. or just ditch the existing second battery and purchase 2 deepcycles and use in parallel.

There are products advertised on this board that will do the trick either way. And discounts for batteries too. try a search fo some excellent past posts.

Any manual system is called an idiot switch as it relys on the idiot to switch them on and off and eventually you will forget.

Good luck
james

[Bulldog]

Oh yeah, the question...

My answer: (b). The aux side of the system will only be as good as the worst "link in the system" - don't think that was the right way to put it ;). The aux system should hold twice the capacity of one battery, but no advantage would be gained from the deep cycle. When the standard aux batt is drained the voltage will drop, and appliances will cut out etc...

Hmmm, thinking a bit more about it... The deep cycle batt might keep things running longer, but it will do more damage to the standard aux batt...

So that might be a (c). I think ;).

[Disco_owner]

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


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 :o), 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 :D.

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.

[drivesafe]

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.

[sam_d]

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! :)

[gazm3]

i am setting up a 2nd battery.
I originally parrallelled them and it seem to work fine but prob not ideal to get stuck with 2x flat batteries in the middle of nowhere.
Now setting up a main/aux one. I have a regular one as the main and a deep cycle as the aux with a piranah cutout switch. It all works fine but with one flaw. The aux battery doesnt charge up. Think of going to an auto elect to sort it or get rid of the switch and revert to the parrallell setup with a manual main battery cut