Dual battery system ideas

[Ando81 2,949]

Hi guys, just wondering if anyone has opinions on dual battery systems. Have been thinking of putting an extra battery in the tub of my xf ute to use for camping. I'm hoping to run some 12v lights and an engel fridge off it without flattening my start battery. There is the cheap solenoid option which can cost under $100 and there is the more expansive dual battery isolator systems worth $100-$500. I am thinking of putting a second battery in a battery box, secured in the tub behind the drivers seat. Or would it be simpler to just take a solar panel out camping to charge the second battery and not bother with dual battery system?

[HIKFRD 11]

Have a look at red arc .... Damn good value for money

[Ando81 2,949]

They look very reasonably priced, and aussie made too

[gerg 10,871]

I've set up our breakdown truck at work to connect the second set only when the engine's running. I've tee'd off the oil pressure light switch to a changeover relay so it powers on when the light goes off. This relay then powers a heavy duty 200A relay to connect the second set of batteries. So simple but very effective. No electronics involved.

 

We've had a bit of trouble with those Redarc relays. The little voltage-sensing module on the side of it can shit itself and not engage the second battery at all, then when you go to use it, it's flat. Also, some of those "smart charge" set ups won't charge a battery from dead flat. I reckon they've tried to be too smart in many ways.

 

Also, if you're going to use the second battery just for 12V devices and not starting engines, get a deep cycle. You can draw them much further down without causing permanent damage. They're not good for jump starting though. A large truck-sized one might start a four-cylinder engine if you're lucky. Lots of capacity but poor cold cranking amps. They hold a charge for a long time too. Just remember to give it a decent charge separately every now and then. They take a slightly higher voltage to charge properly, and will never be completely charged parallel with a standard car battery.

[Ando81 2,949]

Thanks for the tip gerg! That's very helpful. I've heard of a few blokes putting deep cycle batteries in as second batteries also. A fella I know is often charging his second battery with a charger and I didn't know why but a guy on a 4x4 show said that a deep cycle battery needs to charge up to 14.5v and a standard car alternator won't charge that high. Thanks for the warning about the red arc products.

[unfamilia 1,938]

I use a 28ah agm deep cycle as my battery in my 4 cyl datsun. Its only used occasionally and never had an issue in 5 years.

It's great to leave it for weeks on end and jump in and it cranks and starts straight away. Never had that luxury with a lead battery.

Plan on setting a 2nd battery up in my wagon. Do you have a wiring diagram of your relay set up. Sounds way better than the kits

[Ando81 2,949]

This is a basic solenoid setup that costs all up around $60 from repco, I just got the diagram via google. Was thinking I might just go with something like this. The rear battery gets charged when the engine is running but when the engine is turned off it isolates the battery.

[gerg 10,871]

That setup is basically what I was describing but in older cars there is no circuit that's energised only with "engine running". Either a simple manual switch (if you can remember to use it) or like I mentioned, a changeover relay hooked up to the oil pressure warning lamp will do the job.

[Ando81 2,949]

I like the idea of the oil pressure trigger, that way the alternator is guaranteed to be working to charge the battery once the engine has fired.

[gerg 10,871]

Ok I'm nowhere near my computer so I can't draw a diagram. Here's the numbers for the changeover relay:

 

30 - ign+ (i.e. coil +)

85 - looped from 30

86 - oil pressure switch

87 - blank

87a - to solenoid trigger

 

So you can see that when the ignition is off, the relay has no supply. Ignition on, it now has supply and is earthed out by the oil switch and powers on, but power goes to 87, which goes nowhere. Start engine, oil switch cuts out, earth is lost so relay output reverts to 87a which powers out to your heavy duty solenoid.

[Ando81 2,949]

Great explanation buddy, that makes it very easy to understand! No need for technical jargon, just straight to the point and practical.

[unfamilia 1,938]

Does that only work with the idiot light oil system or with the pressure one as well

[Stevemack 122]

Gerg's system would be the most cost effective and would work really well for what youre doing.

[Ando81 2,949]

Does that only work with the idiot light oil system or with the pressure one as well

 

I don't reckon the constant pressure sender would work as gerg describes. That's another thing I need to consider. I like the digital cluster just to keep an eye on the vitals but do like the idea of using the oil pressure switch to switch the solenoid.

[gerg 10,871]

If you're only running a gauge, you can buy a t-fitting to install both a switch and gauge sender. I know VDO makes them, but any 1/4" BSP male to female brass tee will do it. A switch might cost 15 bucks at the most.

[Ando81 2,949]

If you're only running a gauge, you can buy a t-fitting to install both a switch and gauge sender. I know VDO makes them, but any 1/4" BSP male to female brass tee will do it. A switch might cost 15 bucks at the most.

 

Oh ok mate, that makes it easy. That gives me the best of both worlds option! Thanks

[Ando81 2,949]

Thanks for the help lads but I decided to go with a projecta 100 amp dual battery isolator. Am told they do a good job and the whole kit including wiring was only $139. Wired it all in myself and it seems to be working great. I had run my deep cycle battery down to 10 volts and after driving 25 mins, my 60amp alternator had charged it back up to 12.5v. I just need to leave my engel fridge in there for a couple of days without starting the ute to see how long I get out of the battery before it goes flat.