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  • Battery Charger Question

    Hello everyone, I have a battery related question and need some advice.
    I have a battery bank, 12V, deep cycle lead acid batteries and they are connected to a "smart" AC-sourced charger, which eventually switches to float mode. I believe that's around 13.2V.
    At the same time I have an inverter connected that unfortunately draws about 1A at standby. Because the charger appears to have a current limit (I am guessing), the battery voltage eventually drops and stabilizes at 12.55V.
    Is this too low for long-term storage? I guess my batteries aren't fully charged anymore at that voltage, are they?
    My understanding is the batteries should be kept at around 13V float voltage.
    Should I add another charger or some power supply to bring the voltage up to 13V?
    Or is there another charger I should buy that can supply more current at the float stage?
    Thank you!

  • #2
    Hello FN1 and welcome to Solar Panel Talk

    I feel comfortable with a battery voltage around 12.7 after it has been charged and rested. Maybe you can figure out a way to turn off the inverter when it isn't needed so you have no load on the battery.

    I know that is sometimes a PIA but that is what I do. I usually check the battery voltage first, then turn on the inverter and then plug in my loads. More work but the battery seems to have lasted a lot longer not under any load for extended periods of time.

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    • #3
      Originally posted by FN1 View Post
      Or is there another charger I should buy that can supply more current at the float stage?
      OK this is where you get lost and do not understand terms and what is going on. If you have say a 20-Amp 3-Stage, Bulk/Absorb/Float it is a 20 amp charger in any mode, even Float. Assuming the charger does not shut down after Float which it should not, Once in Float mode will hold regulate the voltage to what ever you have Float Set to like 13.2 up to 13.8 volts. Whatever you want. So if you have a load or a vampire bleeder of 1-Amp found on your Inverter, the charger supplies the power, not the battery because it is Floating and not doing anything. As long as the load current demand is equal to or less than the Charger Current Limit of 20 amps in this example, all power comes from the charger, not the batteries.

      All Bulk, Absorb, and Float means is a voltage set point.

      The term Float means is the battery voltage is regulated on the batteries after they have been fully charge and hold them at 100% SOC until ready to use. The batteries are neither charging or discharging, they are Floating. You can store batteries a really long time at Float Voltage and is exactly what is done by pros to store batteries. It keeps them fully charged until ready to use.

      So unless your charger shuts down after charging, Float will supply as much current as any other stage. Only current limitation on a charger is maximum the charger is ratted for. When a Charger is in Float mode is a regulated DC Power Supply.
      Last edited by Sunking; 11-16-2018, 01:05 AM.
      MSEE, PE

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      • #4
        SunEagle + Sunking thanks for your replies.
        I went down and pushed 'charge' on the charger. It's a 25A Stanley charger with display. The batteries were at 12.55V.
        It starts charging with 25A and it took about 45 min roughly to get to float. The duration alone makes me believe the batteries were definitely already drained to some degree. I checked some time later, the voltage was already down to 13.05V after 'float' stage was reached for about 15 minutes.
        When I go back in the afternoon I bet the voltage will be down again to 12.55V.
        Hence, what Sunking describes is probably not the case here. If the charger can provide as much current as necessary, and the load is only about 1 to 1.5A, the batteries should definitely remain way above 13V, around 13.2V perhaps.
        So either I need a different charger or perhaps switch to a 'inverter/charger' that handles its own batteries, like the triplite APS1012SW for example.

        What I need is unattended operation. If the batteries are drained, the inverter shuts off (obviously). But then, when AC mains returns, I want it to immediately provide AC to the load and charge the batteries automatically. Like a standard UPS.
        Unfortunately the AIMS inverter I have right now is good but requires a manual 'switch on', which sucks a mile and a half. That's why I have to keep it on all the time. I called them about another 'inverter/charger' they have and they told me that the charger won't start after the batteries are drained, even though it has a built-in charger. There has to be a minimum voltage on the batteries for the inverter to start up. Also, it won't switch AC back to mains if the batteries are drained, so someone will have to first charge or replace the batteries for the damn thing to start up. I think that makes it useless for my application.

        The APS1012SW sounds like it will do the job but I have to confirm with them all scenarios. At $500 it's not cheap either. Would you guys know of another product I could use? I need about 1KW pure sine with auto transfer switch and charger function for 12V lead acid.

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