Battery metering

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System
System Posts: 2,511 admin
in Advanced Solar Electric Technical Forum #1
How do I go about measuring the amps in my 12 volt batteries? I saw the shunts and I saw the meter on Northern Arizona's site but I am unsure if that is what I use to meter it with. I have 2 batteries in my system, one is a marine starting battery (I know bad idea, but I didn't know what I was doing and don't want to toss out the battery) and a deep cycle 115 AH battery. They are in parrallel. its also a 12 volt system as I am a hobbyist and just learning. I think I want to measure I think between 0-200 amps. (Not sure what the ah rating is on the marine starting battery).

Thanks ahead of time for help!!!!
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  • Kamala
    Kamala Solar Expert Posts: 452 ✭✭
    #2
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    Re: Battery metering

    Welcome to the forum JB,

    Batteries do not contain amps. They contain an electrical charge. Amps are a measure of how much of that charge is flowing through a circuit at any given instant. But we don't use batteries for given instants. We use them for specific durations. This is where the concept of Amp-Hours comes in.

    Do you want to measure current flowing through a circuit at a given instant (amps), or do you want to measure the capacity of a battery or battery bank (amp-hours)? The former is an instantaneous measurement and the later is a measurement over a specified period of time.

    Kamala
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  • System2
    System2 Posts: 6,290 admin
    #3
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    Re: Battery metering

    Battery Bank Capacity.
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  • Kamala
    Kamala Solar Expert Posts: 452 ✭✭
    #4
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    Re: Battery metering

    AFAIK there is no device that you can connect to a battery bank and get an indication of the AH it contains. There are devices that monitor the charge/discharge activity of a battery bank and are temperature compensated (improving accuracy) which act as a fuel gauge. However, they do require that you input the known capacity of the battery bank. Some of these even learn over time.

    I have heard good things about the TriMetric battery monitor. I have one planned in My Solar Camper Project (see my signature.) A shunt is a very accurate, very low resistance device across which a very accurate voltmeter (and other sensing circuits) is connected.

    If you know the BCI Group size of your starting battery you should be able to come up with a reasonable estimate of it's AH capacity.

    BCI 31 ~ 100 AH
    BCI 24 ~ 65 AH

    K
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  • n3qik
    n3qik Solar Expert Posts: 741 ✭✭
    #5
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    Re: Battery metering

    If both batteries are wet cell, then look at this:

    http://store.solar-electric.com/frhdprhy.html
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    #6
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    Re: Battery metering

    I have been using Trimetric 2020 for years. Probably best bang for the buck. You can program what you want for AH capacity of your system. I have mine programmed to half the actual AH's of my batteries just to give me the motivation not to over discharge my batteries.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #7
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    Re: Battery metering

    I second the TRi-metric.

    I configure mine to read ah from full, and %of discharge,,, so that during the day I can gauge whether or not to run a load now or wait until later.

    Great machine,

    Tony
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  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    #8
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    Re: Battery metering

    Kamala this means what??"A shunt is a very accurate, very low resistance device across which a very accurate voltmeter (and other sensing circuits) is connected."

    A shunt is used to measure the curent flowing into or from the battery. A milivolt meter (or a multimeter set to MV)is then used to read the millivolt drop across it to get a reading in amps..
    ie if a low amp shunt its usually 50mohm which gives 10 mv =1 amp.. if high amp above 50amp shunt it reads 1mv =1 amp

    I have no idea what "other sensing circuits" there are and what they could possibly measure ?????
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #9
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    Re: Battery metering

    The tri-metric,, wired to a shunt,, will first read (very accurately) voltage, amperage and % full (after you have told how big the battery is)

    Then it will also do a cumulative amp/hours DRAWN from the battery, ah from full, time since full, recorded battery highest voltage, recorded lowest voltage, time since eq and a couple of other things I forget.

    The amperage reading is net/net into or out of the battery. So if for example you are charging at 60 amps,, but drawing 20 off to run a load,, the meter would show net 40 amps. I have a second smaller shunt and ammeter so that I can see panel production irrespective of how much is going into the batteries while partially charging/partially loading. If I kill all the loads,, I can compare the two meter readings to see how much mppt gain I get from the charge controller. It is pretty cool to see 18 amps from the panels and 22 into the batteries!

    Tony
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  • Kamala
    Kamala Solar Expert Posts: 452 ✭✭
    #10
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    Re: Battery metering
    john p wrote: »
    I have no idea what "other sensing circuits" there are and what they could possibly measure ?????

    Sorry I didn't make this clear. You are correct John. Nothing other than a millivolt meter is connected across the shunt. The "other sensing circuits" that I referred to are actually part of the Tri-Metric meter and they perform the functions which Tony has described.

    Craig
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