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Thread: Dual battery setup questions

  1. #1
    Newbie
    Join Date
    Aug 2008
    Location
    Cranleigh, UK
    Posts
    36

    Dual battery setup questions

    Not sure if this belongs in 'power supplies' or 'general', mods please move it if necessary.

    So, by trade I am a computer engineer, I spend a lot of time in the field with various equipment and gadgets in the car, mostly running from an inverter. Sadly, this has led to several flat batteries and those not-so-pleasant phone calls to my mother to deliver a spare battery and jump leads...

    I am finally now looking at a permanent PC (well, tablet) install, I would like to make it all as efficient and low-powered as possible, and hopefully have a dual-battery setup. I am gradually replacing as many bulbs as possible in the car with LEDs to reduce the power usage and hopefully lighten the load on the alternator.

    I've got a 2007 reg Suzuki Swift with wires all over the place. I can spare enough space in the trunk for a couple of extra batteries and necessary cables/adapters. I think it was this forum I posted on about a year ago asking about using sealed lead acid batteries in a dual setup, from what I can remember, I was advised that they would cook in a fairly short space of time when hooked up to the same circuit as the alternator if they did not have a charge regulator.

    I would like:
    - Car and all standard accessories running from normal battery
    - Anything else including tablet pc and whatever else I decide to install running from a 65Ah SLA battery (or 2) in the trunk (I can get these for 5 a piece so I'm not too fussed about the life span of them)
    - 65Ah battery (or two) to be charged from the alternator circuit
    - If possible, to have 2 SLA batteries in addition to the standard vehicle one

    I have:
    - 2x 65Ah SLA batteries
    - More wires than you can shake a stick at
    - A techy friend who can get (or make) pretty much any circuit or components I may need

    I would like to know:
    - If I had 2 batteries, would this be of any use as a charge regulator? If not, could you suggest one?
    - Will the stock alternator (from Google, 55 amps, from experience, just about enough to jump start a 2.5 litre diesel van) be up to the job? If not, how much would I expect to pay for one that is?
    - How long would it take to charge the extra battery(ies) from say 50% to full capacity?
    - Do SLA batteries need ventilation, can I build an enclosure around them to look neat and tidy?
    - Am I being stupid even thinking about it?

    I'm not sure if it's really worth it but I thought I'd throw a few questions out there and see what responses I get.

    Any thoughts on this are appreciated

    Cheers
    Rob

  2. #2
    Newbie
    Join Date
    Feb 2011
    Posts
    40
    IDK what a Suzuki Swift is but I can try to help where I can.

    I run two batteries in my Dodge ram truck. They are both acid celled 1000 CA / 850 CCA batteries and are under the hood. My stock alt is 136A. When I added my second battery I took the (-) and grounded it to the engine block and frame. I used a battery isolator to separate the batteries. It has three posts on it. One goes to the (+) of the main battery. The second goes to the (+) of the second battery and the last post goes to a switched (+) power source. So when the key is on the battery isolator allows both batteries to be used and charged. But when the key is off the batteries are separate and my power hungry toys rub off my second battery.

    I have never had a problem jumping anything off but my setup may be bigger than yours?

  3. #3
    Raw Wave
    Join Date
    Nov 2009
    Posts
    2,116
    You can also search my threads for info.
    A common theme is the UIBI if you have a charge light.

    Older posts explain using whatever isolator as a mast switch to control slave isolators - ie, from main battery to other batteries, and between those other batteries (assuming you do not want them paralleled when not in use or not being charged).
    And I mention using low-voltage cutouts, and relays or changes to make them latching or drop-out, add manual on & off etc.

  4. #4
    Raw Wave
    Join Date
    Nov 2009
    Posts
    2,116
    That depends on the charger. Even a normal car battery charger should not exceed 14.4V unless it is equalising etc.
    15V etc is typical of cheap chargers where the user is expected to pay attention and reduce current or disconnect once ~14.4V is reached.

    Granted, wet cells can boil off above 14.4V which AGMs can't do, nor Gels past a limited extent. But that does not mean >14.4V is good for wets, nor that AGMs & Gels cannot be equalised.

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