Simple cheap dual battery setup

Submitted: Tuesday, Feb 24, 2004 at 23:21
ThreadID: 10768 Views:2791 Replies:7 FollowUps:9
This Thread has been Archived
Ok everyone this is not a 4wd question but I will ask it anyway.
Firstly my 4wd has a dual battery setup with a decent size fridge. Problem is my missus does not like driving the 4wd but drives her Camry instead. My fridge is too large for her to load into the Camry plus all we need in the Camry is to be able to keep the groceries cold for an hour or two, so no replies on how to install a setup that will last across the desert.
Last night I bought a 12v cooler for $60 (new). You know the type, pieltzer (spelling) device, drops the temp by 20 deg and draws a constant 5 amps. this cooler is 24 litres, plastic and therefore suitable for the missus to carry in & out of the car. All it has to do is keep the food cool for up to 2 hours between shops.
I have a couple of 40 ah sealed lead acid batteries. My idea is to mount one in the boot & connect it as follows. + from accessories connected to one side of a relay coil, plus the make (normally open) contact, plus dioded to the aux battery +. current flow from the main to the aux battery. Aux battery connected to the break (normally open) relay contact. + of a cig lighter socket to the relay common contact.
Neg to the other relay coil contact plus cig lighter socket -. Draw this out to see what I am talking about.
The theory is when ignition is off the aux battery will power the cig socket, ie the cooler when plugged in. When ignition is on or acc the main battery will run the cig socket plus the main battery will charge the aux battery.
As the cooler draws 5 amps plus the battery is only 40 ah I am only talking about using a 30a relay & diode, 25 amp fuse in line to protect.
So shoot me down in flames & tell me what is wrong with this theory, I realise the aux battery will only float charge due to the 0.6 v drop with the diode. I don't think this will be a problem as car is driven every day & cooler will only be used once a week.
from Marty.
Back Expand Un-Read 0 Moderator

Reply By: Rosco - Bris. - Tuesday, Feb 24, 2004 at 23:58

Tuesday, Feb 24, 2004 at 23:58

Pass the Bex .... my head hurts..................;-(Fidei defensor

AnswerID: 47967

Follow Up By: Wayne (NSW) - Wednesday, Feb 25, 2004 at 00:13

Wednesday, Feb 25, 2004 at 00:13

You are showing your age Bex.
I bet more than half the people on this forum don't know what a Bex is.

WayneAlways Out'N About
FollowupID: 309893

Follow Up By: Geoff - Wednesday, Feb 25, 2004 at 06:50

Wednesday, Feb 25, 2004 at 06:50
Bex.............. is better..........
FollowupID: 309900

Follow Up By: tim - Wednesday, Feb 25, 2004 at 08:23

Wednesday, Feb 25, 2004 at 08:23
Repeat the part after this is not a 4x4 question again?????????????
FollowupID: 309904

Follow Up By: Brett - Wednesday, Feb 25, 2004 at 13:46

Wednesday, Feb 25, 2004 at 13:46
I prefer Vincents powder.
FollowupID: 309946

Reply By: V8troopie - Wednesday, Feb 25, 2004 at 02:04

Wednesday, Feb 25, 2004 at 02:04
The cooler is not going to do much better than a simple esky for the hour or two you intend to use it,
UNLESS you fill it up with frozen freezer bricks BEFORE you go shopping.
These Peltier coolers manage to keep things cold, IF the things you put into them are cold AND the cooler's inside is also cold ( has been running for some time or filled with freezer bricks).
Just my 2 cents worth. I have a small Engel permanently in the back for those shopping days, it barely manages to keep cold things at the same temperature for the hour or so its used if I only turn it on as I fill it with shopping.

Ah, yes, your "circuit diagram". I'm afraid its a 3 Becks affair, a bit muddled and overly complicated. Why do you need a relay? If I were to do that I would wire the aux battery to a cig lighter plug and a socket. Plug the cooler in this socket. Plug the Aux battery into the car's cig lighter socket.
The car's cig lighter is off when the ignition is off - Aux battery runs cooler.
Ignition on powers cooler and recharges aux battery from the car's lighter socket.
If the 40Ah aux battery only powered the cooler for one hour or so it should not draw too much charging current as you turn on the ignition.

P.S. I know what a packet of Becks is, have seen people addicted to it, a sorry sight indeed.
AnswerID: 47976

Follow Up By: Member - Bob - Wednesday, Feb 25, 2004 at 09:25

Wednesday, Feb 25, 2004 at 09:25
And I thought you got Becks in slabs. A Bex and a good lie down was standard self stress management from the era when you could scoff a Vincents powder, and the phenacetin stuffed peoples' kidneys.Bob
FollowupID: 309909

Follow Up By: joc45 - Wednesday, Feb 25, 2004 at 13:46

Wednesday, Feb 25, 2004 at 13:46
In serious cases, it was a cup of tea, 2 Bex and a lie down.
FollowupID: 309947

Reply By: Phil G - Wednesday, Feb 25, 2004 at 08:55

Wednesday, Feb 25, 2004 at 08:55
Some people take their esky into the shop, and load it up there - maybe one of those insulated bags, seeing plastic bags are on the way out.

If I were doing the cooler thing, I'd just install a bigger battery into the Camry.
AnswerID: 47988

Reply By: The Banjo - Wednesday, Feb 25, 2004 at 08:59

Wednesday, Feb 25, 2004 at 08:59
May have wasted your $ on the cooler....old tech.........we went high tech for summer shopping re persihables.......the portable polymer pack, with modular thermo resistance..... works fine......just keep some heat absorbers in the freezer, fit them to the polymer pack when you go out and the perishables will transfer any energy to the absorbers en route. Simply transfer the heat absorbers back to freezer when home. Might be space age, but its worth the outlay.I need red sand under me.
AnswerID: 47989

Reply By: Member - DOZER- Wednesday, Feb 25, 2004 at 09:02

Wednesday, Feb 25, 2004 at 09:02
You may find by accident that the cig lighter has a fuseable link on its ass that commonly blows in big brother (cruisers) if fridges are plugged into it. The constant draw does it.
My 2 cents worth would be a supercheap battery pack with compreser, light and ciggy lighter attatchment.
Good for pumping up air matresses, supplying power and a light in the tent, and unning fridges. Needs to be recharged by 240 or if you were away camping .... charged by the battery. Their portability is their beauty.
wheredayathinkwer mike?
AnswerID: 47990

Follow Up By: Member - Peter- Wednesday, Feb 25, 2004 at 09:25

Wednesday, Feb 25, 2004 at 09:25

Does the problem with the fuseable link apply to the rear power socket in a 100 series?

FollowupID: 309910

Follow Up By: Member - DOZER- Wednesday, Feb 25, 2004 at 13:27

Wednesday, Feb 25, 2004 at 13:27
Guessing here, i would say yes. It is to stop overloading the wiring loom and causing a fire inside the panels. I would run a dedicated wire from a aux to the rear, fuse it at the battery end and put something better than a cigy lighter in the rear. 32 volt plugs work well, as do 110 and 240 if you dont have anything else.
Andrewwheredayathinkwer mike?
FollowupID: 309941

Follow Up By: joc45 - Wednesday, Feb 25, 2004 at 13:52

Wednesday, Feb 25, 2004 at 13:52
DON'T use 240v plugs, even if they do work well at 12v. I saw someone who had wired their 4wd for these outlets, knowing full well it was 12v. Only problem was someone else used the Engel which had one of these 240v plugs fitted to the 12v lead and plugged it into the mains. Rest is history. Stick to the 2-pin 32v plug if that style suits you. Avail from most sparky shops. There are nicer, smaller low voltage connectors avail from accessory shops or the likes of the 12v shop in Welshpool WA.
FollowupID: 309948

Reply By: Sandie - Wednesday, Feb 25, 2004 at 19:22

Wednesday, Feb 25, 2004 at 19:22
No need for all that.
Just a rely opened when engine is off driven by cig lighter socket grounded all to one point!!
This isolates the 40amphr battery from the main battery, like a switch!! Don't forget that fuse!!

Your using the diode as a control for when the car is running which you don't really need since everthing is running parrallel to the main battery at 14.2Volts. When the engine is off your cutting out the main battery and alternator.
I take it you don't have an aux battery already installed in the Camry.

I use this method to charge my 7.2ah battery while on the go!! Works a treat.

Another way to charge the battery and run the cooler is to use an inverter with a 12V charger connected to the battery. Make sure the inverter has isolation protection. However you will need at least a 5amp charger to cope!

Hope this helps
Marty, yes i'm Marty too, Sandie is my other half!!
AnswerID: 48071

Reply By: MartyB - Thursday, Feb 26, 2004 at 23:48

Thursday, Feb 26, 2004 at 23:48
To the people who made the helpful replies, Thanks for the info.
I have modified my idea slightly & will put it in on the weekend. Yes I do think the diode is necessary, this stops the aux battery from providing current during startup. I doubt my 30a relay or 25a fuse would appreciate it otherwise. Imagine what would happen if for some reason the main battery was not fully charged & the aux battery was trying to provide most of the start current.
To the people with the bex replies. Just because you can't understand it doesn't mean it is a bad idea. I am glad I asked a question about a simple circuit involving only two components. I would hate to be responsible for the brain strain if any more components had been involved.
from Marty

PS I thought land rover owners where smarter than that.
AnswerID: 48305

Sponsored Links

Popular Products (9)