Dual Battery using 12v 7AH Battery Bank

[gdunkley84]

Hey guys,

Thought I would run this past everyone first and see what you think. I have put a custom drawer system in the tub of my D40 and didn't really want to have a full sized battery sticking up anywhere.

Through work I have access to boxes upon boxes of new 12V 7AH UPS style batteries.

I was wondering if I could hook a dozen or so of these batteries up in parallel and have one big slim line battery.

The problem I have is can I just use a normal dual battery setup style isolator. I.e. Projecta solenoid to connect these up. I am just wondering about charging amps going into these small batteries as I think they are only rated for something like 1.2A charge current.

Has anyone got any experience in a similar setup and what sort of way can I do this.

Cheers,

Garrick.

 

[Old.Tony]

The short answer is 'yes'.

Hooked up in parallel, these batteries will share the current passed to them, so 10 batteries sharing a 10A feed will only get 1Ah each.

Being gel batteries, you don't want to let them charge at more than 14.4V anyway, so make sure the isolator output (at the tub end of the wire) doesn't read any higher than that. Make sure you measure the voltage with someone holding the rpm at about 1500 just so you can be sure that at cruise you're not over-volting the gels. If they get too much voltage, they cook.

There are issues with them being in parallel. A slightly off battery will cause all the others to try feeding it and the entire bank will flatten itself. In an ideal world, they'll all charge up and discharge at the same rate and work wonderfully, but a bad apple can destroy the bank. Monitoring the temperature of each battery is a big help here, at least for a few working cycles so that you know it's all going to tick nicely.

As for capacity for weight & size ... you're losing there. 10 of those batteries will give you 70Ah and will take up about the same space as 2 gels of 100Ah capacity. But if they're free (or relatively cheap) then you've got little to lose except a bit of spring rate on your rear axle!

Oh, do I need to say that you must use at least 6 sq mm cable, preferably 8 sq mm, from the engine bay into the tub, and both ends must be fused?

 

[Nevyn]

What Tony said!
I was only asking about the same with a sparky the other day. He reckons that the batteries in parallel would work but I would be better off with the biggest battery I can fit under the bonnet because if one of them is a bit off.... then like Tony described.

Cheers Brad

 

[rescue]

Its not a bad compramise but remember you will be limited to the current you can pull from the batteries. Since you have a drawer system then there's probably sapce at the sides where you could get 2 battereies designed for the job to fit with no trouble at all, just some expense

 

[WoodyD40]

Garrick

Like Tony said - it will work. If you are getting them for free (or very very cheap) then do it. Even if you get 3 to 4 years out of the system you will be doing very well.

I did something similar in the back of my falcon wagon for just over a year with no problems.

I now have a 100amp deep cycle in the back of my ute, and tomorrow it gets replaced with a 120amp deep cycle, as the 100amp'er is now at the end of its useful service life. (ie nearly 5 years old)

 

[KraftyPg]

Or if you really want to give the batteries a good home just send them this way. We use both the 6 and 12 volt varieties for many different things from UPS' to prawn lights. We've found them great and always have a minimum of 6 batteries charged and ready for use at all times. I'd never use them as a second battery in my car but i have plenty of other uses for them.