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PV and battery voltage high, but battery drops voltage when connected to load

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  • PV and battery voltage high, but battery drops voltage when connected to load

    Hi all, I live in West Africa way out in the jungle, working as a teacher. About 6 months ago I recieved a 120w 12v panel, a LD2420C charge controller, and a 12v 100Ah deep cycle gel battery from a friend who was going back to the states. She had them for about a year before I got them and used them the same way as I do: to charge a laptop with a 200w inverter, to run a few DC lights, and to charge a phone with a DC to usb car plug. It worked perfect for me for the first 5 months that I've had it (I could charge the laptop once a day and run the lights for an hour or so).

    The problem started about a month ago - right around the time that the real dry season began. The PV voltage (as reported by the controller) runs to about 16v during the 8 hours of good sunlight and the battery voltage stays around 14v when there is no load connected. The controller reports that the battery is fully charged. Whenever I try to charge my laptop, it can charge for about 5 min before the controller cuts off the load due to low voltage (I have the controller set to not allow the battery to drop below 12v). About 30 min after the load is disconnected, the controller reads that the battery is full. I have let it sit for the past 3 weeks without connecting any load to it and it still does the same thing. I have checked all the connections and everything is fine. I have a suspicion that either the battery is at fault, but I find it strange that the change in operation happened pretty suddenly at the same time as a dramatic change in the intensity and length of sun hours.

    Based on all this, what do you all think is the problem? I would appreciate any ideas for next steps and tests; this is my first solar setup. Thanks to you all!

  • #2
    First of all a GEL battery, deep cycle or not, is NOT the right battery type for solar. The GEL battery can deliver a fairly high current under load and does not require topping off water or other maintenance effort. But it is not designed to charge quickly. For that you need an open (flooded) battery or a Absorbed Glass Mat (AGM) sealed battery. Both of those can accept a charge at the C/8 rate, or higher in the case of AGM. That means that for a 100AH battery you can use a charging current of 12.5A or more without damage. A typical GEL battery, on the other hand, can be permanently damaged by charging current above the C/16 rate. For a 100AH battery that would be 100/16 = 6.67A. Any higher current, especially when the battery is hot, can cause it to vent gas, permanently losing electrolyte and therefore charge holding capacity.
    Your 120W 12V panel can, in the best high sun conditions, deliver far more than 7A.
    Chances are that the battery has been slowly losing capacity over time from this abuse.
    And here is the final nail in the coffin: Say the battery has slowly lost capacity until it is, for practical purposes, a 50AH battery. That means that the safe charging current has been reduced to a little over 3A.
    So a more or less total failure happening when the solar intensity and sun hours increase rapidly is just what is to be expected.
    PS: 16V under charge is also far too high a voltage for battery survival. Your friend probably survived the incorrect charger setting because the battery we discharged enough that at the end of the day it did not yet reach 16V.
    Your best bet is to replace the battery, this time with an AGM type or with a flooded cell (caps on the cells, keep it upright) battery. To have kept your GEL battery happy you would have had to program your charge controller to limit the current to about 6A. With the right new battery you can just let the charge controller run full out.

    You could also, in theory, switch to using a Lithium battery pack, but that would absolutely require a different charge controller and more knowledge on your part about the system operation.
    Last edited by inetdog; 01-11-2019, 06:44 AM.
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

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    • #3
      inetdog thank you so much for the info! I suspected I was ignorant of some crucial bit of technical knowledge.

      I'll probably go for an AGM battery, since it sounds like it would do better with surviving the 9 hr motorbike ride from the capital to my house 😥

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      • #4
        You might be able to get a new FLA battery that is "dry charged" along with a bottle of electrolyte, and fill it when you get home. That's how my airplane batteries are shipped to me. You'd need to keep some distilled water handy if you ever need to add a little.
        Last edited by sdold; 01-11-2019, 08:55 PM. Reason: Clarified

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