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Help with Battery Testers

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  • Help with Battery Testers

    Sup guys, I received my battery isolator in the mail today and I'm on my way to get a battery for the trunk(only hooking up power inverter/ computer/ external HD).

    How would I go about testing the life of the battery to see how much time it would last for? I'm pretty sure it goes by Amperage per hour(correct me if I'm wrong), but do they make a tool that shows hours left at current amp drainage? or something similar?

  • #2
    they do, they are called amp-hour meters and are typically expensive. they use them alot in renewable energy applications for measuring the energy left in a battery bank.


    • #3
      You could look into the Watt's Up meters made by RC Electronics. These 50 dollar thingies show accumulated Ampere hours until reset but not time left. But, these have their shunt (current sensing resistor) in the negative lead.

      I use two modified (internal shunts removed) Watt's Up meters, one for charge Ampere hours and one for discharge Ampere hours on my aux battery, along with a single external 1 milli-Ohm shunt that's hooked up directly to the battery's negative pole. This has been a fairly involved project however.

      Most people don't need power inverters, because efficiency and reliability are bad. Instead people use car power supplies from Opus and Mini Box.


      • #4
        The problem with a battery capacity test is that it is no indication of how long it will last life-cycle wise - it merely indicates "it just lasted x minute to 80% capacity" etc.

        The best indicators are impedance measuring devices, but even then....

        IMO the best is a simple capacity test with a typical load.
        EG - headlights on, measure voltage and time.

        And every vehicle should have a voltmeter, and the user just a bit of electrical & battery awareness (eg, 12.7V full & 11.3V empty (OC = Open Circuit); never above 14.4V (longterm), etc).

        But FWIW, to describe my recent situation (stranded 1 hour from home at night with no alternator)....

        Battery is 38AH. (C20 rate)
        I estimated ~15A current drain (low beams, engine; averaged stop & indicators; no stereo; and the girlfriend NOT to use her hairdryer).
        I estimated a 2 hour capacity for a good battery.
        I wondered if a 10-year old battery (with 10-year design life that I picked up when it was 5 years old but I had only just started using) was still a "good battery?

        Ignition On.
        LED voltmeter shows 12.7V. Good - she's fully charged.
        Turn on headlights. Voltmeter dips to 12.3V. That means a (12.7-12.3=) 0.4V drop under load - ie 12.3V with headlights is a full battery.
        Crank and start. Still 12.3V. (good; eliminates running current being high, and no major cranking discharge).

        I decide that if I can get to point-x (half way time-wise) and have over 11.8V, I should make it.
        That assumes a linear discharge voltage drop (which is normal except for stuffed batteries; or constant power loads) and that each 0.1V = 10% discharge.
        IE 12.3V - 50% = 12.3 -(5 x 0.1) = 12.3-.5 = 11.8V.

        Anyhow, I get home no problems. The voltmeter read about 11.8V which suggested a 50% discharge.
        I later confirmed the OC voltage to be ~12.2V, hence ~50% discharged.

        Hence also suggesting hat the battery was still as good as new. IE - I estimated a 2 hour reserve at my estimated load, and it was 50% discharged after 1 hour.

        I could do something similar with a known load (say headlights, or constant power load etc) maybe with a ammeter - but definitely with voltmeter and a clock. However I couldn't be bothered. I felt the above unintentional test was good enough. Besides, since I have a spare battery in the back, who cares?

        Just some other comments and caveats:

        I was prepared to discharge 100%, but that is destructive (to the battery). (Cranking batteries should not go below 70% capacity; deep discharge anecdotally not below 50% despite 30% or 10% claims. Having said that, I only recently retired my 8 year old wet-cell cranker which has been 100% discharged a few times!)

        An old wet cell would probably have discharged very quickly; maybe hit 50% and then dies, etc. My 8 year old wet-cell despite being a healthy 12.7V probably only had ~5% of its original 40AH capacity.

        The 10 year old battery I was using was an AGM, hence I was reasonably confident that it could make it. It was a 10-year life UPS battery that was preventatively-maintenance discarded after 5 year; then 5 years with no use but with annual voltage check and top up.
        The fact that it was a Yuasa battery gave me almost certain confidence. (UXH38-12)
        In fact, with 100% discharge (to ~10.5V internal) and immediate recharge, it would barely have been effected.

        PS - I thought those Watt energy meters were for measuring the power used - NOT the power remaining.
        The only way to calculate the energy remaining is a voltage test and knowing the capacity of the battery - it has NOTHING to do with the current they are putting out - especially is attached to chargers (solar panels etc).