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Thread: Load Testing Deep Cycle Battery

  1. #1
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    Load Testing Deep Cycle Battery

    Doing a load test on a 115Ah AGM deep cycle using a 12W (12V, 1A) work light. Battery started at 12.64 volts, and dropped at a linear rate to 11.98 over 63 hours. Does this indicate the battery is fine? Should I remove the load when testing the volts?

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    Hi Ranga, and if I get the time, I will try to work out the exact amount of SoC your battery is at, when at 11.98v with a 1a load.


    But can I suggest you recharge your battery and leave it on float an additional 12 hours and then retry the test with a 6 amp load.


    A 6 amp load would be very close to a standard C20 test, the normal capacity test used to determine the SoC of a battery.


    Your 1 amp load would actually be like a C150 test which is just not accurate enough to determine the true condition of your battery.


    NOTE, at 63 amperes for a 50% discharge of the battery, while not bad, it does seem a bit short of the optimum.


    Again, it is not a bad result.


    If you would like more detailed info on simple load testing of your battery, ask away.

  3. #3
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    Thanks Tim,
    The 12W lamp was all I had at the time, so I'll take your advice and find a higher voltage device to test with. Appreciate your assistance.

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    How about running an Engal Ranga ?


    and,,
    While you are here Tim...

    Does the load testing procedure change for Lithium?
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    Quote Originally Posted by Ranga View Post
    Thanks Tim,
    The 12W lamp was all I had at the time, so I'll take your advice and find a higher voltage device to test with. Appreciate your assistance.
    Get a few old brake/stop lamp bulbs and solder the 21w filaments in parallel. Four in parallel will give 84 watts or
    approx 6.6 amps (assuming 12.7 volts at the battery)

    Re voltage measurement, yes, leave the load connected when measuring the terminal voltage.

    I made up a discharge tester with multiple old stop/tail lights**. It has a 12 volt computer fan blowing air over the globes. There's a voltage switch which operates a relay to disconnect the load when the voltage reaches a preset point and also switches off a clock. I read the discharge time directly from the clock (I set it at 12 before the discharge starts).

    ** get a flat piece of wood, bore multiple holes in it to fit the globes. Push the globes through and use heavy copper wire to link them together.
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    Quote Originally Posted by Pedro_The_Swift View Post
    Does the load testing procedure change for Lithium?
    Hi Pedro and yes, you can use the same test on lithium batteries, but you can also use different load sizes on lithium batteries and still get the same total amperes.

    All lead acid batteries have an effect known as Purkerts Exponent, where the high the current load applied to the battery, the low the total available energy is from that battery.

    Basically and this is exaggerated for example purposes, put a 1 amp load on a 100Ah lead acid battery and it might take 120 hours to discharge the battery to 0% SoC. ( total amperes = 120 ).

    Apply a 5 amp load ( the equivalent of a C20 test on a 100Ah battery ) and the battery will take 20 hours to discharge 0% SoC. ( total amperes = 100 ).

    Apply a 25 amp load and the battery will take about 2.5 hours to discharge 0% SoC. ( total amperes = around 60 ).

    With a 100Ah lithium battery, because they do not suffer from Purkerts Exponent, a 1 amp load, a 5 amp load a 25 amp load, even a 50 amp load, will all return a total amperes of 100.

  7. #7
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    I use a small (300W) inverter driving a 240V, 70 watt incandescent bulb.
    That draws about 6.4 amps from the battery. Add another lamp to change the current.
    More convenient than 12 volt lamps.

    Cheers Peter
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    Hi Peter, and with inverters, you will need to measure the current being drawn from the battery at the start of the discharge cycle and at the end of the discharge cycle.

    There can be quite a difference.

    With loads like light globes, as the voltage in a DC circuit reduces, so does the current being drawn by the light globe, and over a 1v difference ( between 12.6v and 11.6v ), the current reduction would only be in the 100s of milliamps but it does occur.

    With inverter, the exact opposite occurs where the INPUT current draw increases as the INPUT voltage drops.

    But unlike globes, an inverter’s current draw will increase quite substantially, like your 300w inverter might start with a current draw of 6.4 amps at 12.6v but could easily be as much as 7.5 amps at 11.6v.

    Where possible, with all devices, but for more accuracy when using an inverter, it is better to measure the total watts used.

    But if you can measure the amps being drawn at the beginning and the end of the discharge cycle, you can still get a fairly accurate result by averaging out the two current measurements.

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    Quote Originally Posted by drivesafe View Post
    Where possible, with all devices, but for more accuracy when using an inverter, it is better to measure the total watts used.
    I forgot to mention I have a wattmeter in circuit, too. One of those cheapies off evilbay. It also measures current.

    Watt_Meter_1_990x.jpg
    Ron B.
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    Quote Originally Posted by p38arover View Post
    I forgot to mention I have a wattmeter in circuit, too. One of those cheapies off evilbay. It also measures current.

    Watt_Meter_1_990x.jpg
    I have a stack of these, but they are the original ones that came out of the States.

    They are very good for this type of LOW CURRENT monitoring but while they are marketed as being able to monitor currents up to as much as 140 amps on some, I personally would not use them for CONSTANT current loads greater than about 15 amps.

    They will monitor current SPIKES way in excess of 15 amps but I have found that they tend to both heat up and introduce an unacceptable resistance level for CONSTANT current loads over 20 amps.

    They are excellent for monitoring solar, both before and after the solar regulator, and for monitoring fridge power usage and the likes.
    Last edited by p38arover; 15th January 2021 at 11:01 AM.

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