Auto Electricians advise required

Submitted: Friday, May 13, 2005 at 21:17
ThreadID: 22973 Views:2974 Replies:11 FollowUps:19
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Hi to all the auto electricians out there.

The question I have is , is there anything wrong with wiring two batteries together in paraleell with separation ( if required) with a manual switch on an 80 series diesel cruiser.

I understand you have to remember to switch and unswitch so you don't drain both etc. and I guess it would be wise if isolated to start the vehicle before switching over the switch but it seems that both batteries if only stopped overnight would discharge and charge more equally and possibly last longer.
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Reply By: Member - Davoe (WA) - Friday, May 13, 2005 at 21:33

Friday, May 13, 2005 at 21:33
nothing at all that is the original duel battery setup with the cole hersy switch (bat 1 bat2 both and off) countless thousands of vehicles set up like this including my 80
AnswerID: 111235

Reply By: techie - Friday, May 13, 2005 at 22:16

Friday, May 13, 2005 at 22:16
As long as the "car" battery remains connected to the vehicle wiring, especially whilst the alternator is working.
AnswerID: 111239

Reply By: Mainey (WA) - Friday, May 13, 2005 at 23:52

Friday, May 13, 2005 at 23:52
reading your post, you want the original starter battery to always remain the starter battery, you want to add another battery in parallel with the starter battery maybe to assist to run the fridge etc...
then put a 'large capacity' on/off (2 way) switch in line between the two batteries, as it is there only to connect/disconnect the second battery, a 3 way switch is not required because the (+) lead is either connected or disconnected by the 2 way switch.
Use the identical size battery lead as the existing leads, running (-) to (-) on both batteries, not (-) to earth on the second battery.
AnswerID: 111241

Reply By: Vinnie - Saturday, May 14, 2005 at 00:11

Saturday, May 14, 2005 at 00:11
I myself have a question similar to David.

I have a dual battery set-up using a rotronics isolator and I want to put a 3rd battery in my camp trailer and charge it while driving only.

Can I just run a 6mm twin sheathed wire from the main battery thru a fusable link to an anderson plug at the rear of the truck and just plug the 3rd battery from the trailer into it while driving. (Connecting parallel from main to 3rd)

Would that have any affect on the second battery or main battery?


AnswerID: 111243

Follow Up By: Chaz - Saturday, May 14, 2005 at 06:28

Saturday, May 14, 2005 at 06:28
The best way to run a third battery is to put a Redarc or "Smart Relay" between your main and auxillary battery, and wire the third one to your Auxillary battery, but you must remember to fuse the trailer battery from BOTH sides of the plug that you use to connect the trailer to the car. This way the redac will isolate the second and third batteries when your cranking and only charge them when your main battery is fully charged. This is the standard.

FollowupID: 367627

Follow Up By: Member - Duncs - Saturday, May 14, 2005 at 17:23

Saturday, May 14, 2005 at 17:23

I have been using the setup you describe on my camper for the last 5 years. The first battery I put in lasted four years and the second is now into it's second year and doing fine. Both batteries were second hand when I got them.

I use an 11 pin flat plug (the standard 7 plus 4 heavy duty pins in a seecond row)instead of the Anderson and have had no problems with that either, despite some suggesting it would not work. If I am planning a trip I put the camper battery on charge for a couple of weeks before I go to ensure it is fully charged. When I want to isolate the camper from the second car battery I simply unplug it, simple and positive.

I have a Rotronics 10S isolator between the two batteries in the car and it seems to do the job OK. Rotronics recommend a heavier duty isolator for this application but the 10S was in the car when I bought it. If and when it dies I will go to the Rotronics recommended unit. Until then i aint fixin what aint broke.

I am not an auto electrician but I have a philosophy of keeping it simple. My previous car had the most complicated electricals any one has ever seen. I cooked three different isolators and was forever having battery problems. This one works fine.

FollowupID: 367648

Follow Up By: Member - Robert (WA) - Monday, May 16, 2005 at 00:03

Monday, May 16, 2005 at 00:03
Hi Vinnie
you should use 8mm wire to the back of the car and 8mm to the battery in th camper

Good luck
FollowupID: 367733

Reply By: techie - Saturday, May 14, 2005 at 00:32

Saturday, May 14, 2005 at 00:32
Adding a flat battery across a fully charged battery does NOT flatten the fully charged battery. Therefore one can connect the trailer to the car before starting the engine without concern of flattening the car battery if the trailer battery is flat.

A number of Vans I have worked on use a relay which connects as soon as the ignition is turned on. Even if the secondary battery was flat from overnight use, the car still starts as it _Affordable_Storage_Drawers.aspx all current from the charged battery.
AnswerID: 111244

Follow Up By: Member - Davoe (WA) - Saturday, May 14, 2005 at 02:06

Saturday, May 14, 2005 at 02:06
yes but a stuffed battery will theive all the charge with that system so although only one battery is stuffed you end up with one stuffed battery and one flat battery. My camper had the same set up as did the bosses 100 series both sufffered like that it is a dud system. Also with the system you talk of if the starter battery is dead flat where does the charge come from to kick in the solonoid and bring the second battery into play? you have to add an aditional override switch for power from the aux battery to kick in the solonoide. These obsevations come from several vehicles both at work and my own
FollowupID: 367626

Reply By: Mike Harding - Saturday, May 14, 2005 at 07:08

Saturday, May 14, 2005 at 07:08
You can do as you suggest but for best results you should ensure both batteries are of the same type eg. cranking batteries or deep discharge etc. to ensure they both receive full charge. If you mix batteries (which you can do) then one of them probably won't reach full charge - but you may never notice anyway :)

You can make you own cheap and very simple semi-auto dual battery system if you prefer: I gave details of it in post #16020

Mike Harding
AnswerID: 111251

Follow Up By: bundyman - Monday, May 16, 2005 at 08:28

Monday, May 16, 2005 at 08:28
I've got the setup described above (with the big round switch in the cab) and if I'm on a trip where I'm using the fridge and driving during the day then I charge the starter (bat 1) for 30 minutes after starting and then switch to just the deep cycle (bat 2) for most of the rest of the trip usually switching back to starter before arrival. I don't use the bat 1 + bat 2 switch unless I'd drained both batteries and needed the extra grunt (hasn't happened yet) The system requires you to think/remember but I have full control over what the batteries are doing and aren't relying on some computer black box to do the thought processes for me.

FollowupID: 367746

Follow Up By: Member - Jeff M (WA) - Monday, May 16, 2005 at 17:59

Monday, May 16, 2005 at 17:59
If it were me, I'd preffer the computer to have control... Computers don't forget.
You know the ol' saying. Computers can't make mistakes, only the people who made/programmed them do. ;-)
FollowupID: 367817

Follow Up By: Mike Harding - Monday, May 16, 2005 at 18:26

Monday, May 16, 2005 at 18:26

The circuit I suggested does have the advantage that when the ignition is switched off it will automatically disconnect the two batteries and prevent any possibility of discharging the starter battery. But I do agree with Jeff that I would rather have full microprocessor control and _eventually_! :) I'll get around to designing one.

I have noticed there is (what may be) an error in my description in post #16020. I suggested using a starter motor solenoid for the charging relay between the two batteries mainly because such a relay is very cheap ($5? from a wreckers) and very heavy duty. However it occurs to me that some starter motor solenoids may not be designed for continuous operation and their coils may overheat and go open circuit if they are used in that mode. It's impossible to say which would and which wouldn't but it's something to keep in mind.

Mike Harding
FollowupID: 367832

Reply By: drivesafe - Saturday, May 14, 2005 at 08:49

Saturday, May 14, 2005 at 08:49
There is no reason why you can’t parallel as many batteries and different types of batteries as you need and depending on how big you alternator is, you are using large enough cable between the batteries and providing you drive long enough, there is no reason why you can’t FULLY charge all the batteries.

As long as your vehicle can supply 13.8 volts or higher then you will be able to fully charge any and all your batteries.

The only thing to watch, for starting the vehicle, make sure you have a cranking battery with 12 volts or more in it and you won’t have any problems starting.

If you know you have auxiliary batteries at different levels of charge just make sure the motor is running before you connect them to the vehicles power supply. EG If you have a caravan or trailer with a partially discharged battery or two in it, leave the motor running while you couple up to the vehicle. This way the alternator is the thing supplying the power to the discharged battery and not your charged cranking battery.

The only draw back with a manually controlled battery connect, is that you will eventually forget and end up with two flat batteries and usually at a very inopportune time.

AnswerID: 111259

Reply By: Crackles - Saturday, May 14, 2005 at 11:19

Saturday, May 14, 2005 at 11:19
Gaday Dave. That's the system I've used on the past 3 cars & works pretty well. Ideally you would fit 2 batteries of the same size & construction so they accept charge at the same rate but is not compulsory. With a standard Toyota batt & an 85 amp hour deep cycle connected together all the time I was getting 3+ years out of them. If only parked for 2 nights in one spot I would not bother Isolating the switch but could still start the diesel without trouble. I estimate each batt after 2 nights of fridge & lights was about half full. Alternativley if you had an automatic isolator system the aux battery would be dead flat over the same time & we all know thats what shortens batt life. To avoid the possibility of 2 flat batts & to maximise the usable power available you could concider fitting a variable voltage disconnect switch so that the fridge & lights would be turned off around 11.7 volts. This would give the maximum run time with just enough power to start your car, certainly a longer run time than just one aux batt isolated on it's own.
Run 35mm cable between them for improved starting & if you are going to fit a winch the switch should be rated at 200+ amps. (never use the red key type)
My little experiment of removing the starting batt & replacing with two 115 amp hour deep cycles is now 2 years old & they said it wouldn't work ;-))
Cheers Craig.........
AnswerID: 111261

Follow Up By: drivesafe - Saturday, May 14, 2005 at 12:49

Saturday, May 14, 2005 at 12:49
Hi Crackles, if you do some snooping on the net, you will find that there are some battery charge controllers ( isolators ) that actually exploit the surplus power stored in the cranking battery to do just what you have proposed and then once the common voltage drops to a set level, the cranking battery is isolated and the auxiliary battery can then be taken down to 10.5 volts and this way, you end up with an even longer operating before you need to recharge everything.

Two points I would raise, first there are some diesel powered vehicles that have a hard time starting if the battery voltage is taken below 12 volts.

Second, if you use a deep cycle or agm type battery, you can take them down to 15.5 with out damaging the battery and if you check, most 12 volt fridges have a cut-out level of 10.5 volts.

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Follow Up By: Outnabout David (SA) - Saturday, May 14, 2005 at 14:32

Saturday, May 14, 2005 at 14:32

I was thinking along those lines. Why run an auxillary battery down all the time when you can just take the top charge off both.Did you wire the second battery earth to the originals earth or just to the body?

I had a vehicle done in paralell once befor ebut can't remember if the earth had a long run as well.
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Follow Up By: Crackles - Saturday, May 14, 2005 at 16:01

Saturday, May 14, 2005 at 16:01
Dave, I have run earth leads from the winch direct to both batteries. I never rely on the body or chassis for any high load electrical connections.

Drivesafe, that battery charge controller sounds interesting but after seeing so many automatic battery isolating systems fail from heat & vibration I've grown to like my KISS setup. To do as you suggest I would need to manually isolate & have done so to get 3 nights in one location. You are right that it's often hard to start some cars under 12 volts but thats usually with only 1 batt. With 2 batteries at around 11.7 volts I found my cars started ok.
Cheers Craig...........
FollowupID: 367644

Follow Up By: drivesafe - Saturday, May 14, 2005 at 19:31

Saturday, May 14, 2005 at 19:31
Hi Outnabout David, you should never use the vehicle’s body or chassis as a high current earth return.

At best you will most likely end up with some form of resistance caused by paint and/or rust and you will dramatically increase the time required to recharge the battery and even then you may not be able to fully recharge it.

In a worst case scenario you can end up with a fire being started because of a resistive point in the earth returning to the battery via the body.

FollowupID: 367657

Follow Up By: Outnabout David (SA) - Saturday, May 14, 2005 at 21:45

Saturday, May 14, 2005 at 21:45
I thoughtb that was probaly the case but isn't the original crank battery earthed to the body or engine anyway?
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Follow Up By: Crackles - Saturday, May 14, 2005 at 22:24

Saturday, May 14, 2005 at 22:24
Yes Dave there is a thinner earth wire to the body although the main one normally goes to the engine block. Lower power items like internal lights, CBs & GPS are fine earthed off the body but the winch, HF radio, fridge & any auxillary power outlets ideally should have an earth return wire. 99% of the time earthing through the body will be fine but for long term reliability you'll get a better connection direct to the battery.
Cheers Craig.......
FollowupID: 367669

Follow Up By: drivesafe - Saturday, May 14, 2005 at 23:50

Saturday, May 14, 2005 at 23:50
Just continuing on with what Crackles posted, the main one ALWAYS goes to the engine, this is your earth return for the starter motor.

Unfortunately this is where many people come to the conclusion that you can earth to the body because the vehicles earth seems to be via the engine block but as stated above, the vehicle’s largest current drawing device is the starter motor and as such needs the heavy cable to allow it to operate at maximum efficiency.

This, if you think about it should tell most people that if the tight a**ed car makers reckon that the only way to get the starter motor to work properly is to supply the starter motor’s power via both a thick positive and negative cable, then anything else requiring high current should also have the power supplied through heavy cables.

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Reply By: Barnray - Saturday, May 14, 2005 at 13:38

Saturday, May 14, 2005 at 13:38
With a lot of late model vehicles the radio is turned off during cranking, so to prevent the rear batteries from trying to start the engine, take a power feed from the radio power (check that it is being switched off when you crank) feed to a constant duty solenoid and use it to isolate the batteries when the engine is shut down or when cranking. If needed, use a double throw double pole switch and attach main battery feed to one side and radio feed to the other and solenoid to the centre which gives the ability to bring extra power in when winching or starting. I also hook the winch into the aux batteries so the main is engine only until needed. Barnray

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AnswerID: 111268

Reply By: F4Phantom - Saturday, May 14, 2005 at 17:39

Saturday, May 14, 2005 at 17:39
i recon use a deep cycle second battery instead of a normal car batt. Car batts hate being discharged and are not designed for it. Use a deep cycle and fully isolate the batts with solenoids they are not running together as this will eventually wear one battery out. When you are camping you can be rest assured you can use the battery right up without any problems shortening it's life. If you discharge a car batt right down 5 to 10 times it will be stuffed, they should not be discharged more than 2 - 5%. Slightly off topic but I hope it helps.
AnswerID: 111277

Follow Up By: Member - Jeff M (WA) - Monday, May 16, 2005 at 18:02

Monday, May 16, 2005 at 18:02
You shouldn't go below 50% even with a Deep Cycle. A Deep Cycle will take longer to charge but will handle tough off road conditions better as the plates are thicker/stronger.

If you keep draining a starter you will kill it.

That's why I mix bats and run a starter and a deep cycle.
Not ideal, but better than the alternative for me as my cooler does flattern the battery after about 18 hours and a starter type bat would die quickly.
FollowupID: 367818

Reply By: Member - Phil G (SA) - Tuesday, May 17, 2005 at 14:44

Tuesday, May 17, 2005 at 14:44
Hi Dave,
Our mate Neil has this setup in his farm truck. He's got two identical crank batteries (mine and his :-)) We used a marine switch for isolation should that ever be needed - got a nice one on special at Whitworths for $35. But you never need to isolate them for overnight. Give me a call if you want to check it out. Very important to have the pos cables thoroughly insulated with a huge degree of paranoia as a short circuit can be a disaster with those thick cables.

AnswerID: 111627

Follow Up By: Outnabout David (SA) - Tuesday, May 17, 2005 at 22:27

Tuesday, May 17, 2005 at 22:27

Did you run both positive and negative cables to each battery?
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Follow Up By: Member - Phil G (SA) - Tuesday, May 17, 2005 at 22:37

Tuesday, May 17, 2005 at 22:37

Ran pos to pos, with compact marine switch in between.
Second battery had two negs - 2Ga to motor, 8Ga to body.

But, yes most people would run neg to neg, but I figure each battery having a separate neg to the motor may be better.

FollowupID: 368071

Follow Up By: drivesafe - Tuesday, May 17, 2005 at 22:58

Tuesday, May 17, 2005 at 22:58
Hi Phil G, the reason the cranking battery has a cable connected to the engine is to allow for the large current the starter motor pulls.

For the best results with a dual battery set up, run the negative between the cranking battery negative terminal and the auxiliary battery terminal.

FollowupID: 368078

Follow Up By: Member - Phil G (SA) - Tuesday, May 17, 2005 at 23:08

Tuesday, May 17, 2005 at 23:08
Sorry it may not have been clear from my post, but both batteries have a big fat neg to the motor for the reason you stated.

FollowupID: 368081

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