24v panel -12v battery questions

Please forgive me if I sound like a complete novice...I am.

I'm setting up a simple system using batteries(12v) and panels (24v) both found locally on craigs list. My intention is to have one assembly put together for Emergency backup at my home and an identical setup at a remote cabin completly off the grid. I may at some point combine the 2 assemblies into one at the cabin.

Batteries: six Powerware PWHR12500W4FR (wired in parellel)
Panel: 1 Mitsubishi PV-MF170EB3
inverter: Powerdrive 2000

I read on another thread here where a Sunsaver MPPT was reccomended to regulate power between the higher voltage panel (in my case 29.9 VOC) and the 12v battery bank.

I've read thru the IOM of the Sunsaver and this seems to be great advice.

I'm wondering if there are other options I ought to consider?


  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: 24v panel -12v battery questions

    Welcome to the forum.

    When using a higher Voltage panel to recharge a lower system Voltage you have two choices:

    1). Use a PWM charge controller and lose up to 50% of the panel's power.
    2). Use an MPPT controller and get the maximum potential from the panel.

    That's the short version anyway. ;)

    I have to say I'm a bit concerned about your proposed set-up. You have a lot of batteries there. I don't know what their capacity is, but I do know that one 175 Watt panel would deliver perhaps 10 Amps at best into a 12 Volt system and wouldn't be good for charging more than 100 Amp hours of battery, 200 Amp hours tops. Having six batteries wired in parallel presents some problems too, even if you did have enough panel to recharge them. Namely keeping the current flow in and out even for all batteries.
  • System2System2 Posts: 6,290 admin
    Re: 24v panel -12v battery questions

    Thanks! I appreciate the quick reply.

    Battery specs: Eaton 12v 6 cells each
    These came out of a large UPS system for a call center.
    Capacity 500w @15minute rate of 1.67 per cell @ 77 deg F

    Would I be better charging them as 24v in series/paralel?

    Or should I charge them 2 batteries (12v in parallel) at a time?

    I want to keep it simple so as to not do alot of connecting/disconnecting between charge (panel) and usage (inverter).
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: 24v panel -12v battery questions

    It should be interesting to see how suitable for RE those batteries are. The ratings given are not of the type normally used for powering inverters, because that was not their original intended purpose. I tried to convert the information given to the "20 hour" rate normally associated with deep cycle batteries in an RE application - got three different answers! One of them might be correct, but I wouldn't hazard it. We need someone who's better at math than me. :blush:

    As for system Voltage, if your inverter is 12 Volt you need to charge the bank as 12 Volt. There's no practical way to charge at one Voltage and draw at another; you'd have to rewire between charging and discharging. That would be possible for back-up power, but for an off-grid situation you're usually charging and using at the same time.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: 24v panel -12v battery questions

    Searched the 'Net for more info. Good ol' Google! They would appear to be 130 Amp hours each.
    Six of them definitely would not recharge properly off one 175 Watt panel.

    Do you have any kind of power target in mind? Usually you base an off-grid system on expected Watt hours of use. It's much more practical to know how much power you need and then try to supply it than it is to find out how much power you've got and then see what you can do with it.
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