Triple battery set up

Submitted: Tuesday, May 18, 2004 at 14:31
ThreadID: 12973 Views:8606 Replies:8 FollowUps:8
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I have a standard toyota set up with dual batteries in parrallel.
I have a third battery (AC Delco) which I want to connect up in the rear of my car for running a fridge, similar to a camper trailer set up.
I want a simple system and was wondering if I can connect the third battery directly to the outlet on the alternator. Will such a set up still drain my other batteries when the car is not running? If so, can a simple switch disconnecting the third battery when being used prevent draining the other batteries.
I do not want to spend big dollars on isolaters etc.. and am happy to use manual switches.
I have read a lot of previous posts but none seem to address this option.
Any advice or direction greatly appreciated.
Thanks in advance
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Reply By: Rosscoe - Tuesday, May 18, 2004 at 15:06

Tuesday, May 18, 2004 at 15:06
I know lots of people carry additional batteries in the cabins of their vehicles but is this a good idea? Is the AC Delco a sealed lead acid or gel filled unit?
Is it "legal"? - Even if it's sealed.
I'd hate to have a battery with all that acid flying around if I was in an accident.
If you do it, I'd make sure the battery is installed in a properly constructed and vented battery box and anchoured down well. A number of people on the forum have reported doing this and seem very happy with the results.
In most modern caravans, motorhomes and camper-trailers the batteries are mounted in enclosures separated from the living areas and vented to atmosphere or mounted externally on the draw bar or similar.
I would go for simple manual battery isolator/switch if you have the dicipline to throw the switch whenever you stop for any lenth of time. Check the 12 volt shop in Auburn Sydney or similar organisations in other states.
They use these isolator switches both internally and externally in ralley cars.
AnswerID: 59080

Follow Up By: Lachlan - Tuesday, May 18, 2004 at 15:54

Tuesday, May 18, 2004 at 15:54
Thanks for the reply. I have a battery box and was intending to vent outside. The main issue I am not sure of is the whole charging 3 batteries set up and whether the third battery will drain the others if not isloated and any other potential problems. In other words, will the third battery when in use drain from the other batteries via the output on the alternator.
FollowupID: 320724

Follow Up By: Rosscoe - Tuesday, May 18, 2004 at 16:10

Tuesday, May 18, 2004 at 16:10

My logic (which sometimes aint great) say yes, you need to isolate the battery. Effectively you're connecting it to the main battery in parallell and any load will draw on the main battery as well as the rear mounted battery. Lots of people use a simple manual switch BUT if you forget to change it over when you stop you could end up with a dead cranking battery.
The automatic smart separators (Piranha, Sure, Rotronics etc) do this when the ignition is off and or when the main battery drops below a preset limit.
The other way is to connected the rear mounted battery to the vehicles auxiliary battery assuming it's separated from the cranking battery by a smat solenoid/separater.
FollowupID: 320732

Reply By: Lachlan - Tuesday, May 18, 2004 at 16:13

Tuesday, May 18, 2004 at 16:13
Thanks Rosscoe,
I have the aux battery on a manual isloated switch but my concern is that if I connect the third battery up to the aux battery it affects the charging of the third battery when the vehicle is running, ie I am only getting a charge from the aux battery and not straight from the alternator.
AnswerID: 59097

Follow Up By: Rosscoe - Tuesday, May 18, 2004 at 17:06

Tuesday, May 18, 2004 at 17:06
With your isolator switch closed you are effectively connecting the batteries in parallel and they will both have 14.2 V (typically) applied across them from the vehicle's voltage regulator. The third battery when connected is again in parallel and will have the 14.2 Volts across its +ve and -ve terminals. Now, the only problem is that if all three batteries have a low state of charge the alternator will try and pump full current at the 14.2 volts until the charge level rises and the internal resistance of the batteries increase.
So what's current rating of your alternator?

The issue is when you stop. Because, unless you indivdually isolate the batteries you will draw current from each in the same way you applied current during the charging cycle. Worst case - you flatten the cranking battery. Not a good idea! So you need discipline to make sure you isolate.
When I am in holiday mode I rank very low in that department so I go automatic when I can afford it and because I work in an industry where automation is the norm, I have a lot of confidence in brand name electrical and electronic hardware.
FollowupID: 320750

Follow Up By: Lachlan - Tuesday, May 18, 2004 at 18:09

Tuesday, May 18, 2004 at 18:09
Not that I am doubting you, but reading an earlier post it saud about joining the batteries in parrallell and then trying to charge:
"you are not getting the full CHARGE into the BATTERY from the alternator. You are only getting the potential difference between the two BATTERY voltages. The alternator only senses the BATTERY that is most fully CHARGEd. This could result in requiring quite a time to fully CHARGE" this is why I was thinking of hooking straght up to alternator.
FollowupID: 320762

Follow Up By: Rosscoe - Wednesday, May 19, 2004 at 08:38

Wednesday, May 19, 2004 at 08:38

This is probably correct to some degree, but under normal circumstances the voltage regulator looks at the average voltage of the batteries and typically maintains this voltage at about 14.2 V. At this level, as you point out you never achieve a fully charged battery.
What I can't tell you (coz I don't know) is what "raw" voltage is generated at the output terminals of the alternator. If its around 15V or more this could overcharge and damage the battery or at least "boil" off the electrolyte.
I think there is a response later on this thread where someone has done exactly what you want to do.
You will still need to isolate your auxiliary batterioes when you stop.
Let's know what you decide to do and how it goes.
Good luck with your project and have fun.
FollowupID: 320847

Reply By: -OzyGuy- - Tuesday, May 18, 2004 at 17:32

Tuesday, May 18, 2004 at 17:32

connect your AC DELCO battery Positive lead direct to the positive connection on the FIRST battery lead from your altinator.

Have a decent "smart" solenoid eg Pirahna etc, in-line to the AC Delco battery.

Earth your AC Delco battery to an effective common earth that is used by the other batteries, eg maybe chasis rail.

This way the altinator power is going to BOTH systems inderpendantly and the AC Delco battery is isolated when the ignition is off. that way you don't drain the original batteries.

AnswerID: 59113

Follow Up By: Eric Experience. - Thursday, May 20, 2004 at 21:37

Thursday, May 20, 2004 at 21:37
The modern charging systems use a diferent set up to the older units. in the older machines the alternator was connected directly to the battery, but in the vehicles with fusable links the alternator output and the referance for the regulator go to a distibution piont usualy where the fusable links are and then a feed is taken from there to the battery. The result of this is that when the alternator is working hard the output can be more than 1 volt higher than the mian battery this is why it is best to connect to the alternator, the deap cyle battery needs the extra voltage. Eric.
FollowupID: 321130

Reply By: Member - The ghost - Tuesday, May 18, 2004 at 20:37

Tuesday, May 18, 2004 at 20:37
Hi Lachlan
I have the same set up as yourself being 2 in parrell and 1 in the back of the ute purely for the fridge. I have a 5mm wire direct from the main battery thru a hella headlight relay with an inline fuse connected to the ignition, charges whilst driving it charges and disconnects when ignition is off. If the fridge battery should go flat which it never has you can swap terminals on the relay and direct feed from the front. This has been in operation for 7 years and has not failed once. I can't take the credit though a auto elec friend suggested it. I have recently fitted a volt meter in the cab connected to the back battery. the total outlay would be $20 so far.
Good luck with your project.
AnswerID: 59147

Reply By: ianmc - Tuesday, May 18, 2004 at 22:18

Tuesday, May 18, 2004 at 22:18
When I fitted a battery in the tray of my ute for fridge etc I ran a fairly heavy cable from starter battery + terminal to the rear via a 50a fuse at each end for safety as recommended by elecy. In the cab I fitted a 50a switch on/off so I can isolate it manually when I stop.
For extended use I intend to carry a $200 genny but a third battery stored in the tray could also be used for back up with jumper leads if the brain gets it wrong.
Seems the fuses are essential in case of accident & shorting of the cables.
AnswerID: 59183

Reply By: Eric Experience. - Tuesday, May 18, 2004 at 22:53

Tuesday, May 18, 2004 at 22:53
You are spot on with your thinking, you will be much better of connecting to the alternator direct. the alternator does not sence battery voltage it just puts out a constant voltage regardles of how many batteries are connected. Yes you will drian your other batteries if you don't use an isolation switch. Eric.
AnswerID: 59192

Follow Up By: -OzyGuy- - Thursday, May 20, 2004 at 11:38

Thursday, May 20, 2004 at 11:38
Connecting to the + positive terminal on the First Battery from the alternator as I suggested gets you the same result as connecting to the Alternator direct and has the benifit of being able to disconnect the storage battery simply and without interupting supply to existing battery system.
FollowupID: 321048

Reply By: Lachlan - Wednesday, May 19, 2004 at 09:23

Wednesday, May 19, 2004 at 09:23
Thanks to everyone for the helpful advice.
I have one last question. If I were to connect third battery direct to alternator with a manual isolation switch (and fuses) do I need to hook up some device to discharge the battery when the fridge is not connected (and third batterey not isolated) to prevent overcharging. Most of the time the 4wd will be driven around the city and the third battery will not be isolated (ie being charged). Am I correct in assuming that if it is connected to the alternator there will be devices (such as the alarm) drawing on it (and the other two batteries) or should I just connect the alarm straight up to the third battery so there will be some draw on the third battery when vehicle is not being driven.
My limited 12v knowledge (and previous posts) would say that if the third battery is connected to the alternator and no battery is isolated they are all effectively in parrallel and it would not matter which battery I connect the alarm to, it will draw on all three batteries.
I hope this makes sense.
AnswerID: 59219

Reply By: Hoffy - Wednesday, May 19, 2004 at 13:13

Wednesday, May 19, 2004 at 13:13
Lachlan. Why not consider an Arrid "Twin Charge" unit powered from the auxilary battery. These units give a proper charging regeime with an input voltage from around 8volts to 15volts. Charging current can be up to 20 amps. To stop it still working when the engine is turned off you could use a heavy duty relay on the input side that is energised from a curcuit that only has power on it when the engine is running. Disadvadvantage they cost around $300. I use one for a third battery in my caravan and it works well.
AnswerID: 59251

Follow Up By: Rosscoe - Wednesday, May 19, 2004 at 15:02

Wednesday, May 19, 2004 at 15:02
I've been looking at these but couldn't get much detail on how they can claim 100% charge. I know they use pulse technology but was lead to believe by a lady at the 12V shop in Sydney that they only put out 14V and they are not "smart".

Sounds like you're happy with yours but from what I can see all their main function is to compensate for voltage drop between the vehicle voltage regulator and caravan battery and save you rewiring the caravan.
I can't see how they can fully (100%) charge the deepcycle van battery at 14V.
Can you tell me more.
P.S. I've been quoted $78.00 fitted by an auto electrician for a device with similar claims. I am confused.

FollowupID: 320901

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