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Dual Battery Help Please!

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  • Dual Battery Help Please!

    Hello there,

    I have searched around the forum looking at others topics about dual battery systems and their seems to be many options.

    I own a 2003, MK2 PH2 Renault Clio.

    I want to add a secondary battery for my car pc, firstly I was wondering if I could use a smaller form motorbike battery?

    How long is this to last on average with say, a 1.8 atom system with average specs?

    And whats the difference between relays and isolaters and everything? What's best?

    I want something that when the engine is running and my starter battery is full it will start charging my second battery.

    When my engine is off my second battery wont drain my main battery.

    Any help is appreciated.



  • #2
    You're needing a battery isolator. It will allow the vehicle to charge both batteries, but only drain from one (keeping your main battery charged). Most automotive parts places can get them.


    • #3
      my old atom setup lasted about 3-4 hours, er, that is to say, that was as long as i could sit in my car, with the engine off, and tinker.. as long as i only left the pc running, instead of my fog lights, i had no problem starting the car after that.

      my pentium dual core install is very similar.

      since installing a higher power pentium, i have also installed a voltmeter to monitor battery condition--after a 2-3 hour time, i am usually about 0.2v lower... remember-- car batteries start at about 11-12v, and usually anything below 10v will not allow the car to start. so if i started tinkering while the battery was at 11.8v, it is down to about 11.5 or so after the 3 hr mark.. not dangerous, but getting pretty close..
      My OLD 2001 Mitsubishi Eclipse GT:
      "The Project That Never Ended, until it did"

      next project? subaru brz
      carpc undecided


      • #4
        I suggest searching for the UIBI - often found with my posts on the isolator subject.
        The UIBI is the cheapest and most reliable (and best in most circumstances) assuming you have a charge lamp or some sort of charging/not-charging signal.

        AFIAK there are no systems on the market that charge the first/main battery full before charging the others (aka priority charging etc). A fewer number are still claiming to have it, but that is AFAIK total bullsh - maybe a delayed connection is all they do.
        Besides, why wait to charge the 2nd battery?
        Especially if the 2nd battery is MUCH smaller than the main battery - the argument would be to charge the 2nd ASAP with the first - but I'm merely using their own arguments against them... (LOL! - ie, need to current limit small batteries; want priority charging due to limited alternator output; then we can provide FULL alternator output to the 2nd battery and blow it out of existence; etc etc....)

        First choice is current sensing, but none exist commercially.
        Hence the first choice becomes the UIBI.
        If no charge lamp circuit, the the choice is voltage sensing - ie, smart or intelligent or auto battery isolators - eg, for marine and some RVs, motorbikes etc.


        • #5
          So would using a battery like this: Click Here be okay?



          • #6
            Aside from recharge currents (see below)...

            IMO - no.
            IMO Yuasa - yes, definitely. But that is a flooded cell (vented, wet etc). Usually illegal in non-vented vehicle enclosures like cabins, boots/trunks etc, but fine in engine bays. (But who wants to lose battery life due to hotter engine bays?)

            Get the sealed aka AGM aka VRLA equivalent. They can be tipped without leaking and mounted in any position (though usually upside-down is not allowed due to construction reasons).
            And funnily enough - the AGM equivalent is probably CHEAPER than flooded equivalents. (Yes - AGMs are allegedly superior aren't they? And "far more expensive" than wet cell...)

            EG - down here, 12V 7AH wets (or gel-cell; aka sealed also) are $50; maybe $40 on special.
            The Yuasa NP7-12 (12V 7AH) AGM is $35.
            BUT!!, the NP series is for standby applications (alarms etc), not cyclic use.
            There is the NPH series (High current) but that too might be standby.
            They might be ok for certain cyclic use though... (??)

            So search for AGM cyclic or deep cycle AGM batteries. I could help if looking for the Yuasas (I have catalogs from a recent job).
            Warning - the NP7 has 2 different connector sizes. Both are spades (blades), but one variant is 4.8mm as opposed to the other with the (common automotive) 6.3mm (1/4") size.

            And I could help with sizing if that is an issue - but I'll need what current (drain) and for what length of time? - ie, what battery capacity (AH) is required. Maybe too the peak current though that probably isn't an issue - especially if somewhat oversized (due to long reserve time).

            FYI: The NP7-12 (7AH) was the same price as the NP4-12 (4AH).
            The recent job was the replacement of a house alarm's NP4-12. [ It only lasted 23 years! Is that good or what? (And it's only a 10-year battery.) Would a flooded battery last that long - without inspection and maintenance (not to mention corrosion of alarm circuitry!)? ]
            Finding that the 7AH was the same price, I got that instead, thereby extending reserve time from ~24 hours to ~48 hours, and probably increasing battery life (due to lower drain and recharge current compared to its capacity).

            Recharge Current:
            Batteries have a maximum recharge current. Although car batteries often exceed this, it isn't considered a problem.
            But for small batteries it could be a problem, though - as with normal big batteries - the battery's (internal) voltage usually rises quickly thereby limiting current to acceptable levels.
            But maybe you could suck it and see - especially if the battery isn't too expensive. (Maybe try an NP or NPH if cyclics are more expensive?)
            Though there are doomsayers that reckon current limiting is a must - ie, a 7AH in parallel with a 70AH - but it isn't quite that simple.... The smaller battery has higher internal resistance, therefore lower charge current, etc etc. And people forget the effect of the internal resistance and how the smaller battery internally may be at a higher voltage than the larger battery even though they both have the same terminal voltage. (Batteries: always fun & games!)

            BUT you should use a battery isolator. This can be as simple as a 30A relay if you have a charge light or similar charging indication. (See my UIBI posts.) Keeping any batteries in parallel impacts reliability and battery life and can be a safety issue.
            And then there is under-voltage (battery flattening) protection. (See MW-728; ~$20.)
            And how added relays or inputs can change the UIBI & MW728 into latching or timed systems with manual on or off etc.
            (If only I remembered where I got to in my mp3car posts... ah, more fun!)

            Finally - always check the temperature of any battery - especially AGMs. They heat up if there is a fault like overcharging, high charge current, collapsed cells etc. AGMs can go into thermal runaway which is considered a hazard.
            Being raised to "warm" above ambient temperature during/after charging may be normal, but not too warm or hot. Mind you, how that is determined in a hot engine bay is another issue. (But if it were critical, batteries would have temperature sensors included or on them wouldn't they? Unless it cost that bit extra...)
            Last edited by OldSpark; 04-27-2011, 03:45 AM. Reason: PS... bad spellinks & ikspreshuns.