Minimum Battery Capacity for 185W Panel?

I have a 10Ah 24V battery pack consisting of two 12V SLA batteries. I would like to charge it using a 24V 185W solar panel. I've purchased a 12/24V 20A solar charge controller to do it. But before I hook it up I'm worried the solar panel will pump out too much current for the battery to handle!

As I understand it this size battery should be charged with a charger under 2A. The solar panel is capable of putting out at least 7A at full power. Do I need to worry about this? Will the charge controller limit the current from the solar panel without knowing the capacity of the battery? Won't it just let all the current through that's available, even if it means frying the battery?

I have found nothing anywhere that talks about what happens when you hook a high wattage panel up to a low capacity battery. I guess they assume the battery will be at least 40Ah or more, which then would be fine to charge at 7A, right? Am I crazy?

Thanks for any help you throw my way!


  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Minimum Battery Capacity for 185W Panel?

    i guess we are all crazy to one degree or another, but on this you are correct as it will pump more current than the battery is meant to handle during the bulk stage. you need something that'll limit the current to the battery if you've already bought all of this. a power resistor can be employed for this. another way maybe to place a 1 or 2a light bulb in series with the + output to the battery as this was an old method of current limitation on some motorola walkie talkies. i don't remember if you'll need the bulb to be rated the voltage of the supply or not being it is in series and not parallel. i guess it should be rated for 24v so 2 12v automotive type bulbs in series may do the trick unless somebody says it needn't be so.
    to be sure you understand what it is i am saying is that the + of the controller's output to the batteries shall have this output go to a 24v bulb rated at an amp or 2 and the other side of the bulb's connection shall go on to the + of the battery and not to -.
  • icarusicarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Minimum Battery Capacity for 185W Panel?

    Flooded batteries like to be charged between 5~15% of ah capacity. So if your panel puts out ~7 amps it would like a battery between ~40 and 125 ah.

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Minimum Battery Capacity for 185W Panel?

    About Niel's idea for the light bulb balancing act ...

    If I recall correctly (and I probably don't), it's same Voltage at a Wattage that will "use up" the "extra" Amperage capacity. So to get 7 Amps down to 1 Amp it'd be 72 Watts? (6 Amps @ 12 Volts)

    No, somehow that doesn't sound the least bit right. :confused:

    How about a rheostat such as used for automotive heaters and an Amp meter? Turn the resistance down 'til the meter reads maximum allowable Amps for your battery. Of course to do this you need full output from the panel and batteries drawn down enough to take full charge rate. But at least you could keep an eye on it.
  • mike95490mike95490 Solar Expert Posts: 9,582 ✭✭✭✭✭
    Re: Minimum Battery Capacity for 185W Panel?

    Stick Post-It notes on your panel, till you meassure a "safe" charge current. Or sheets of notebook paper. Experiment.

    Resistors get hot when running 2A through them
    Powerfab top of pole PV mount | 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

    gen: ,

  • System2System2 Posts: 6,290 admin
    Re: Minimum Battery Capacity for 185W Panel?

    Wow, thanks everyone. Those are good ideas. And they seem to confirm my concern about hooking up this big panel to a small battery. Has anyone heard of a charge controller where you can set a maximum for the charge current? Or perhaps dial in the battery capacity and it figures out the maximum rate for you? I suppose that would be getting out of my price range...

    So back to the ideas of reducing amperage. I like the ideas of the resistors/lights/rheostat/post-it notes, but I wonder how well they will interact with the charge controller? Won't they affect voltage as well as current, and thus mess with how the controller is designed to work?

    And how about what happens when the panel is putting out less than 7A or 2A, will these current limiters know that I still want 2A to go through if available? Doubtful. It's almost like I need an ammeter connected to a variable resistor that will decrease the resistance down to zero as the current tends towards 2A. Anyone seen a circuit like that?!?

    Lots of questions, I know, but better to discuss it here than blow up my batteries over there! Keep the ideas coming!
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