Batteries in parallel. What'sthe go......?

Submitted: Thursday, Jun 23, 2005 at 20:42
ThreadID: 24118 Views:2681 Replies:14 FollowUps:11
This Thread has been Archived
Out in the Gibson Desert my auxilliary battery started faltering. I ascertained that my Rotronics Battery Isolator had failed.

So after a lively discussion with my travelling mate I hooked up the two batteries in parallel. My mate was of the opinion that I would now have 24volt. I disagreed.

Have done around 10,000km since then and have had no ill effects to the electronics, though I have been turning the fridge off at night(it has been cold outside anyway).

Both batteries are newish Century 17plate 4WD Overlanders 700cca.

I do intend to get another battery isolator.

The question is: Will there be any detrimental effects to the vehicle electronics when running two batteries in parallel?

Back Expand Un-Read 0 Moderator

Reply By: GOB & denny vic member - Thursday, Jun 23, 2005 at 20:49

Thursday, Jun 23, 2005 at 20:49
goodday willem
how are youy going
sounds like you had a great trip

2 batteries in paralel = 12 volts
2batteries in series =24volts
you shouldnt do any damage but if you forget to turn the fridge or other current drawers off you may end up with a flat cranking battery

AnswerID: 117122

Reply By: ColinD - Thursday, Jun 23, 2005 at 20:54

Thursday, Jun 23, 2005 at 20:54
You are correct, batteries in series-add the voltage of all the batteries however current flow will be limited to the battery with the smallest amperage.
Batteries of the same voltage in parallel, add the amperage capacity. ie you have 12V 1400cca. no probs for your electrics-ever.
AnswerID: 117125

Reply By: Brew69(SA) - Thursday, Jun 23, 2005 at 21:35

Thursday, Jun 23, 2005 at 21:35
Hi willem, I have recently done just what you have and my take on it is now i have longer fridge power when camped for a couple of days. The fridge cuts out before the battery goes flat. In the event that one day it doesn't then i have my trusty jump starter battery pack.
AnswerID: 117132

Reply By: Member - Davoe (Widgiemooltha) - Thursday, Jun 23, 2005 at 21:44

Thursday, Jun 23, 2005 at 21:44
Interesting. How old were the batterys? I just turfed my 550 cca batterys and spent $ 250 on a couple of the batterys you mention. I have a cole - hersy switch much cheaper than new fandangled contraptions and dont let you down while doing the same job
AnswerID: 117136

Reply By: Crackles - Thursday, Jun 23, 2005 at 21:46

Thursday, Jun 23, 2005 at 21:46
Have had my 2 batteries in parallel for a couple of years now & it can't hurt the cars electronics. It's really the same as having one big battery.
It doesn't seem to matter which brand of isolator people choose they all seem to have common failures when taken out in the outback heat and vibration. Why is it they always seem to let you down when you need them most?
I've seem so many go over the years that I now just have a 200 amp key switch to keep it simple & reliable. What brand will you go for next Willem?
Cheers Craig.........

PS. With 2 x 700 cca batteries in parallel you would have no problem starting your car in the morning even if you left the fridge & camping light on all night.
AnswerID: 117139

Reply By: Member - Phil G (SA) - Thursday, Jun 23, 2005 at 22:02

Thursday, Jun 23, 2005 at 22:02

I'd leave them in parallel. Just suggest you slip in a low voltage cutout for the fridge, set at an appropriate voltage (say about 12.1 volts) and you'll have the most reliable system around.

Its what I'd have except my Rotronics won't die :-)))

AnswerID: 117144

Follow Up By: Mad Dog (Australia) - Thursday, Jun 23, 2005 at 22:52

Thursday, Jun 23, 2005 at 22:52
My understanding with this configuration is that if a cell shorts then that battery will drag the other down to a condition where we may be able to listen to the radio but we wont be going very far till the vehicle is either crash started or another charged battery is brought into play. I think I'd rather avoid that possibility especially if travelling solo or just to avoid embarrasment.
FollowupID: 372617

Follow Up By: Nudenut - Friday, Jun 24, 2005 at 07:34

Friday, Jun 24, 2005 at 07:34
yep...agree, good battery will try and dump its load to crook battery.

FollowupID: 372637

Follow Up By: Member - Phil G (SA) - Friday, Jun 24, 2005 at 09:05

Friday, Jun 24, 2005 at 09:05
Hi Ray,

Yep, you've pointed out the downsides of that setup. Especially the bit about travelling alone outback. The other downside is that you want a low voltage cutout set a bit higher than the ready made ones - but thats OK if you're handy with electronics and can adjust up a Jaycar kit. And naturally the batteries need to be identical in every respect.

There's some upsides. Simplicity, reliability and cost. You've potentially doubled your CCA. And running the fridge for one or two nights won't draw the battery down as far. So they tend to last longer. They should also charge up quicker than an isolated system.

The 100series TD LandCruisers and later TD Jackaroos come standard with this setup to aid in cold weather starting.

Many friends at our club run their batteries this way.

FollowupID: 372648

Follow Up By: Member - Davoe (Widgiemooltha) - Saturday, Jun 25, 2005 at 22:00

Saturday, Jun 25, 2005 at 22:00
I have seen 3 vehicles set up this way and every one of them has had problems as you cant isolate the batterys to check them and then isolate a crook battery and the crook one theives the charge off the good one so you think everythings good until it doesnt go
FollowupID: 372877

Follow Up By: Member - Roachie (SA) - Sunday, Jun 26, 2005 at 19:34

Sunday, Jun 26, 2005 at 19:34
G'day Willie and others,
Just going thru this trouble myself on camper trailer. Have 2 Exide Orbital spiral gel cell is 33months old the other is about 8 months old.
I had to go away for about 10 days recently, leaving the trailer in the garage. Whereas I usually leave the 3 stage charger on 24/7 to keep the batteries fully topped up, I have been having some reservations about the state of the electrics in the camper, so I turned the charger off before I went. I made sure that everything was isolated, so there was NO drain on the batteries what so ever. Checked the batteries when i returned and 7.98 volts!!!!!!!!!!!!!
Just an hour or 2 ago, i pulled them both out of their compartment and checked them independantly (ie: once I'd separated them). the newer one showed 12.38v and the older one 7.98v.
So now i know why I'nve been having troubles!!!!
I've now decided to put the "good" one in the back of the Patrol to make a 3rd battery in it (I have 2 x Exide Extremes under the bonnet) and I will now get a 120 a/h Calcium or AGM battery for the camper. That way I will have about the same (or more) a/h than the 2 Orbitals had anyway, and they will be in one package so I won't have the problem of a good battery "masking" a weak/dead battery.
I'm still happy with my dual battery set-up on the Patrol which has a manually operated isolator. I will simply connect the Orbital in the back of the Patrol to the 2nd battery.
FollowupID: 372937

Reply By: Vinnie - Thursday, Jun 23, 2005 at 22:05

Thursday, Jun 23, 2005 at 22:05
G`day Willem

I have my auxillary managed by a Rotronics and all is still going good (cross fingers)

I wired a connection to the back of the truck to store & charge a battery on my camp trailer - this works as a parrallel connection with a fusable link on the positive wire only. It is charging while on the move and if stopping for any length of time I just disconnect the plug.

I am using 50amp anderson plugs and 6mm wire, so far this is working all good too.

I can get three days using the Engel on the camp trailer battery. To top it up I plug it into the Genny at night while running lights etc and get three more days...... and so on.

You will do no damage to electrics running parrallel batteries, but you will drain both if left running stuff. Be sure you can disconnect when not required

AnswerID: 117145

Reply By: Member - Pesty (SA) - Thursday, Jun 23, 2005 at 22:12

Thursday, Jun 23, 2005 at 22:12
Hi willem
You dont need to turn fridge off over night, 2 bat would be enough for 24 hrs in hot weather so you will be ok this time of year and your fridge may have cutout anyway
mine goes for days this time of year on 2 battery's.
AnswerID: 117147

Reply By: Niko - Friday, Jun 24, 2005 at 01:00

Friday, Jun 24, 2005 at 01:00
Mad Dog is right if you have a short in a cell then it will affect the good one, plus the good one tries to charge the bad one unless you have the right circuitry to stop that from happening. I do believe ARB have have something to assist you in that area on display in Osborne Park, WA...... if you're not in WA then take a little drive over :)
AnswerID: 117171

Reply By: Outnabout David (SA) - Friday, Jun 24, 2005 at 09:18

Friday, Jun 24, 2005 at 09:18

See post 22973. Asked the same question after reading about your problem on your trip and have wired the 80 series up in paralell. No problems in going through the night and into the next day without switching the very simple high amp switch over.

Plenty of people will knock the system but after originally having one a few years ago and then every brand of "smart" type solenoids I will stick with the Kiss principle of the parralell wiring as I have less problems with that. My batteries are both new and of the same type so I am starting off with everything matched fore my own piece of mind.

To all the knockers who bring up the crook cell battery scenario I ask. What happens when the fridge battery has run down and the start battery is crook? This happened to me once and the answer is you need a jump start because the way those systems work is that the aux battery is isolated until the engine is running.

So you see there CAN be problems with both systems but I like the paralell system as when stopped you are just taking a little bit off the top of both batteries rather than a lot out of one.

AnswerID: 117207

Follow Up By: Mad Dog (Australia) - Friday, Jun 24, 2005 at 12:33

Friday, Jun 24, 2005 at 12:33
David, yes correct but it's important to present Willem with the facts so he can make an informed decision on what suits HIS circumstances armed with the facts. It's no getting caught short in the bush and then saying "nobody told me that could happen"

Personally I would never operate without an isolator because I make extensive use of a HF radio not only for voice but data that is controlled by a computer and transmits periodically unattended drawing over 20 amps....just too easy to flatten batteries if not very careful. My point here being we have different circumstances and expectations.

By the way, with the redarc it is simply a matter of grounding one little wire to parallel the batts, can be done from inside the cab with a switch. With some simple electronics both batts can be left paralled until the main drops to a preset level and then isolate automatically.

No matter what the system there are positives and negatives (pun intended) if we are aware of the negatives we are forearmed
FollowupID: 372670

Follow Up By: Outnabout David (SA) - Friday, Jun 24, 2005 at 13:04

Friday, Jun 24, 2005 at 13:04
Mad Dog,

You are correct. Go for what suits the owner. Yeh I know about the bridging on the redac etc but the installer forgot to tell me that and I had to wait ages for a jump start when all the time I had a good battery.

I just assumed that they would both come in to play for sarting.
FollowupID: 372672

Reply By: Member - Duncs - Friday, Jun 24, 2005 at 10:53

Friday, Jun 24, 2005 at 10:53

My GQ ran like that for years with no ill effects.]

For about $20.00 I bought a terminal to attach to the second battery which had a screw on top of it I attached the standard car stuff to the OE battery and the aftermnarket gear to the aux. When i screwed up the terminal it seperated the batteries when I screwed it down it connected them.

Screw it up when you make camp and screw it down when you break camp. Make it part of the routine and it is never a problem.

AnswerID: 117228

Reply By: Member - Jeff M (WA) - Friday, Jun 24, 2005 at 13:59

Friday, Jun 24, 2005 at 13:59
Mate you'll be fine, no harm will be done at all. In fact if the rotronics is out of warranty I'd bebleepit off and getting an Arrid Smart solinoid or Redarc or similar. About $130 bucks and you'll be away again and never look back.

Member message me if you need any info, I've done HEAPS of resarch into this over the last 6 months and have tried various setups, I'm actually doing an experiment this weekend with some batteries from work and will be interested to prove a few common mis conceptions (that even I had previously believed) wrong. (it's really just an excuse to tinker).
AnswerID: 117249

Follow Up By: Mainey (WA) - Friday, Jun 24, 2005 at 19:47

Friday, Jun 24, 2005 at 19:47
Jeff, can you setup an independent battery and attach a solenoid in the usual manner, then add an aux battery to the solenoid, forming a sort of ‘fully independent’ dual battery system that will never get charged or have any electronic leaks, because they are both removed from a vehicle and not grounded independently.

Obviously both batteries initially should be fully charged 24 hours previously and the ‘isolator’ has to be ‘activated’ by the current from the starter battery as is the usual practice in the conventional dual battery system.

Test the voltage at both batteries independently, and also each side of the solenoid, with the solenoid in both the open and also the closed situation, and then do the same with a Piranha (or similar) electronic system for a comparison!

Now you have another excuse for some tinkering....
FollowupID: 372742

Follow Up By: Member - Jeff M (WA) - Monday, Jun 27, 2005 at 14:30

Monday, Jun 27, 2005 at 14:30
Except I don't have a Piranha... I had an electronic one but it pooped itself, hence going back to relay tecnology. So far so good with this latest experiment, but I'll wait for my camping trip in two weeks down south the see if it actually works the way I think it will. Then I'll post what I did and the resutls. If it does'nt work, I slink off into the distance and won't say anything! LOL
FollowupID: 373021

Reply By: Willem - Friday, Jun 24, 2005 at 16:59

Friday, Jun 24, 2005 at 16:59
Thanks for all the comments fellas. I have taken it on board. I tend to want to work on a switch priciple from now on. I actually have a battery terminal switch but havent figured out how I am going to attach it as it does not fit in with my battery set up. Out of habit I have turned the fridge off at night as it usually freezes even at a half setting on the dial. Normally ony carry one cold beer for the evening repose. We carry cryovaced meat and marg and other things which need to be cooled. It is not a big issue. I have heaps of wires running off the auxilliary battery including the HF, UHF, Inverter, CD Player, GPS and spare socket. In fact I had an excess supply of red wire so my earth wires are red as well(with a bit of black tape at the end) :o)
AnswerID: 117279

Follow Up By: Michael ( Moss Vale NSW) - Saturday, Jun 25, 2005 at 20:40

Saturday, Jun 25, 2005 at 20:40
Hi Willem, I have my batteries in parallel and have one of those $10 manual isolators with the green knob on the auxiliary battery. i have a Liemack fridge so you need both batteries to get you through the night, they suck power like a jet ... The point is you can just pull the bonnet and undo the knob to isolate the auxiliary battery with a normal fridge,it takes a few secs only and you always have heaps of cranking power with both batteries connected. It's better for the batteries, they are allways both fully charged. Michael
Patrol 4.2TDi 2003

Retired 2016 and now Out and About!

Somewhere you want to explore ? There is no time like the present.

My Profile  My Blog  Send Message

FollowupID: 372861

Reply By: Alloy c/t - Sunday, Jun 26, 2005 at 20:49

Sunday, Jun 26, 2005 at 20:49
Willem , would be very interesting to find out how/why your rotronics 'failed' ,,have you contacted the makers? , my cheapy rotronics , [classed as a w/ender] is fully charging my start...aux ... and 3x c/trailer batts ,,all batts same age + type = 5x fullriver 80amp AGM ,,
AnswerID: 117593

Follow Up By: Willem - Sunday, Jun 26, 2005 at 21:08

Sunday, Jun 26, 2005 at 21:08

My Rotronics MH10 was bought mid 2002 and fitted to the G60. Then it found a home in the GQ. So it worked for about three years.

I am still on the road and will send the unit back to Rotronics so that they may have a look at it. I only discovered that the Isolator wasnt working as my new battery started failing and output dropped below 10 volt. The battery came good when I hooked it up in parallel to the other one.

Will post result of finding sometime in the future.
FollowupID: 372947

Sponsored Links

Popular Products (11)