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Can I charge a deep cycle battery while it is under load?

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  • Can I charge a deep cycle battery while it is under load?

    I have a partial off grid power arrangement, where my facility gets AC power for one or two days a week. I have two 115Ah deep cycle batteries wired in parallel to keep everything running, but I need a straightforward way to charge up the batteries when AC is present.

    My initial thinking was to simply plug in a smart charger that would automatically come on when the AC was on, but after much research it sounds like three stage chargers can get stuck in the absorption stage (14+ volts) because of the load, and cook the batteries. I'd also need the system to kick back on and provide up to full current if I increase the load and start to pull down the batteries.

    My current set-up uses a simple 15A solar charge controller wired to a 15A DC power supply (silly I know)...anyhow, that's not working, it's only providing around 5amp max...not sure why.

    How can I keep my batteries in top condition! Am I just over thinking this, and will a $50 charger do the trick?

    Bruce

  • #2
    I'm assuming flooded cell batteries ? When low, they can generally take a bulk charge rate of C-10 till they are at 80% of capacity, when you have to taper off into absorption rate
    So your 2, 115 AH batteries, should be able to run fine from a charger that can output 23A. If there is a load, it will be supplied by both battery & charger, and that should not affect your batteries, other than the load is eating some of the charge.

    But at 80%, your charger has to throttle back, and reduce the current. Automotive chargers don't often do that.
    Everyone who in their house, disconnects all loads while charging daily, raise your hands.

    First, you need to figure out what happened to your current setup. A solar charge controller needs to see about 17V to charge 12V batteries. Try bumping your power supply up to 17-19V. 15v may only get your batteries to 13V, never getting a full charge into them. New batteries may be needed, if this set was destroyed.

    Second, with recharge only every 3 days, you run the very real risk of sulfating your flooded batteries while they are below 70% charged for that length of time. I don't know what your loads are, or how deep you cycle them, but you may want to consider AGM, which is more resistant to sulphating, and can take a faster charge rate till they are full, when they MUST go to float, or you will vent electrolyte, killing them. AGM require a 3 stage charger.

    Check out the deep cycle battery FAQ:
    http://www.windsun.com/Batteries/Battery_FAQ.htm
    Your battery mfg should have published specs for recharging,
    everything else is just average guidelines.
    Last edited by Mike90250; 06-13-2009, 03:58 PM.
    Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister

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    • #3
      Wow, You must install a switch between your charge controller and the battery bank. Turn the charge controller off when AC power is available. Connect a AC battery charger to the batteries. When the AC power is off unplug the battery charger and turn on the charge controller. You may want to get a book by Calder. "Boat owner's Mechanical and Electrical Manual. You can find this book also at the library. You have just discribed a shore line power situation. I do not know if there is a way to automate this function.

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      • #4
        Originally posted by Off Grid Solar View Post
        Wow, You must install a switch between your charge controller and the battery bank. Turn the charge controller off when AC power is available. Connect a AC battery charger to the batteries. When the AC power is off unplug the battery charger and turn on the charge controller. You may want to get a book by Calder. "Boat owner's Mechanical and Electrical Manual. You can find this book also at the library. You have just discribed a shore line power situation. I do not know if there is a way to automate this function.
        There should be some easy way that you need check the AC input then use the signal to control a relay.

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        • #5
          Bruce, All you need is a on/off switch 200 AMP to turn the batteries off. This is safe and it works. A $50.00 charger will do the trick during AC power times. Just make sure the batteries do not go up to 15 volts. This can overheat the batteries and you could have a fire. Safety first always.

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          • #6
            We had a cutomer up in Nigeria with the same problem. They only saw utility power every second day or so, and then only for a couple of hours. We solved the problem by giving him a 1kw array of PV and a 2kw 24V offline Olson UPS that has a nice 50A charger in it. In the DC line from the panels to the batteries we put a normally closed contactor, and in the AC supply line to the UPS we put a normally open contactor, both operated by the AC. When the power comes on, the PV is disconnected and the batteries are charged with the 50A charger. Clever, no?

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