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  • Power out - surfing on batteries

    Wind storm in the Northeast, trees all over the place, poles and wires down. It's our turn I guess. I'm using car batteries to run my pc and router.

    This question is about the charger. I have a Schumakker charger that does 3A, 5A, 20A, and jump start 75A. I'm charging my 12v car battery now and it was sitting at 55% charging for 45 minutes at 20A. I unhooked the + and rehooked within a few seconds and it jumped up to 95% charged. So is it really at 95% ? It does this both on utility power or generator power. It didn't matter much to me before but now it does. I only have generator power and don't want to discharge the battery too low.

    Has anyone else experienced this?

  • #2
    Originally posted by 01-7700 View Post
    I unhooked the + and rehooked within a few seconds and it jumped up to 95% charged. So is it really at 95% ? It does this both on utility power or generator power.
    The % charged is sort of misleading. Without some way to give the charger some information about your battery, a "smart charger" can only guess at the charge based on the battery response to the charging. There are several ways to evaluate the charge, one of which appears to be checking the voltage right after giving it a charge at a known rate. The voltage will fall off at a somewhat predictable rate and that can be used to guess the state of charge.

    Remove the lead and put it back, and it is likely to start over, and will see the battery as mostly charged before it adds it's own charge.


    That's my guess as to what's going on.

    Dan
    At the end of the project, there is a profound difference between spare parts and left over parts.

    Location: SF East Bay.

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    • #3
      Car batteries have rather low capacity, usually, so it is not impossible it is charged.

      But, percent charged stuff is BS unless the charger knows the capacity and the net current in and out for some time prior to the "measurement". It's guessing based on voltage. Pay no attention to it.
      CNC machines only go through the motions

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      • #4
        Originally posted by danlb View Post
        The % charged is sort of misleading. Without some way to give the charger some information about your battery, a "smart charger" can only guess at the charge based on the battery response to the charging. There are several ways to evaluate the charge, one of which appears to be checking the voltage right after giving it a charge at a known rate. The voltage will fall off at a somewhat predictable rate and that can be used to guess the state of charge.

        Remove the lead and put it back, and it is likely to start over, and will see the battery as mostly charged before it adds it's own charge.


        That's my guess as to what's going on.

        Dan
        Originally posted by J Tiers View Post
        Car batteries have rather low capacity, usually, so it is not impossible it is charged.

        But, percent charged stuff is BS unless the charger knows the capacity and the net current in and out for some time prior to the "measurement". It's guessing based on voltage. Pay no attention to it.
        Makes sense - thank you for the explanations. I had one deep cycle battery powering my router for the past 5 days at 8 watts. Today I decided to use my pc instead of ipad and the consumption jumped to 50 watts. I will switch out the batteries to be charged once a day when I run the generator on the fridge and furnace.

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