What type of battery should I buy (size wise)?

767mrN#13767mrN#13 Solar Expert Posts: 35
I have "Short-circuit Current (Isc)" of "8.76A." I have two panels in the system. Now I'm trying to figure how much of a battery to buy. Anyone able to guide me?

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: What type of battery should I buy (size wise)?

    Ah, there's nothing like designing a system backwards eh?

    You should determine how much battery you need to run the desired loads, then get enough panel to recharge that much battery.

    As it is having only the Isc rating of the panels tells us nothing. What is needed is the Vmp and Imp ratings as well as the charge controller that will be used (either MPPT type or PWM type).
  • 767mrN#13767mrN#13 Solar Expert Posts: 35
    Re: What type of battery should I buy (size wise)?
    Ah, there's nothing like designing a system backwards eh? GUILTY. That is exactly what is happening. I bought a boxed kit and figured out I was paying a huge premium. About that time I read an article about doing on yourself. Easy as can be, the article insinuated. All you need are panels, a controller, battery, and inverter! Simple as can be...not!

    You should determine how much battery you need to run the desired loads, then get enough panel to recharge that much battery.

    As it is having only the Isc rating of the panels tells us nothing. What is needed is the Vmp and Imp ratings as well as the charge controller that will be used (either MPPT type or PWM type).

    The VMP = 18.9V. The Imp = 8.47 Amps. The controller is a PWM (A ProStar PS-30 designed for use with 12/24 volt systems).
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: What type of battery should I buy (size wise)?
    767mrN#13 wrote: »
    The VMP = 18.9V. The Imp = 8.47 Amps. The controller is a PWM (A ProStar PS-30 designed for use with 12/24 volt systems).

    Good!

    Those panels can be used in parallel on that controller for a 12 Volt system. You will get about 16 Amps peak, meaning it would suffice for 160 Amp hours @ 12 Volts using the 10% rule-of-thumb. Not a lot of battery. On a couple of GC2's (220 Amp hours @ 6 Volts) the peak charge rate would drop to around 7.6% which is still viable providing there aren't any concurrent loads that would drag it down below 5%. Much depends on how you want to use it.
  • 767mrN#13767mrN#13 Solar Expert Posts: 35
    Re: What type of battery should I buy (size wise)?

    Many thanks, Cariboocoot. I'll pick up some gold cart batteries and then get in touch with the sponsors of this site. They helped me pick the stuff that goes from the panel to the controller to the battery. Now all I need is from the battery to the inverter and I can have the electrician out to finish it up!
  • 767mrN#13767mrN#13 Solar Expert Posts: 35
    Re: What type of battery should I buy (size wise)?



    Good!

    Those panels can be used in parallel on that controller for a 12 Volt system. You will get about 16 Amps peak, meaning it would suffice for 160 Amp hours @ 12 Volts using the 10% rule-of-thumb. Not a lot of battery. On a couple of GC2's (220 Amp hours @ 6 Volts) the peak charge rate would drop to around 7.6% which is still viable providing there aren't any concurrent loads that would drag it down below 5%. Much depends on how you want to use it.


    With good weather I'm getting back on trying to finish this project. I have two golf cart batteries. One is a Duracell that is:
    • 20 amp hour rate:215
    • 5 amp hour rate:157
    • 6 amp hour rate:156
    • BCI Group Size:GC2
    • Minutes at 25 amps:395
    • Minutes at 75 amps:105
    • Volts:6
    The other is an Exide that is 6 volts and 100 minutes. Which do people recommend?
  • PhotowhitPhotowhit Solar Expert Posts: 5,023 ✭✭✭✭
    767mrN#13 wrote: »
    [*]20 amp hour rate:215
    This is the Amp hour rating most commonly used in this forum and in the solar industry
    767mrN#13 wrote: »
    The other is an Exide that is 6 volts and 100 minutes. Which do people recommend?
    I suspect there isn't a huge difference between the 2, Whichever can be had cheapest... You can usually find good deals at Sam's Club and Costco's around $80 after core charge. Sam's club can be quite liberal on the exchange for core, my local Sam's club took a lawn mower battery as a core.

    A small 12 volt system shouldn't require an electrician to hook up, indeed an electrician might come out and refuse to hook it up leaving you with a minimum service charge.

    I suspect you will be using a small inverter, with this small system. You can't hook it up to the home's electrical system, if you want to use the electric inside setting up a single power strip to use the energy would be viable.

    What size inverter and plans do you have?

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • 767mrN#13767mrN#13 Solar Expert Posts: 35
    Photowhit wrote: »
    This is the Amp hour rating most commonly used in this forum and in the solar industry


    I suspect there isn't a huge difference between the 2, Whichever can be had cheapest... You can usually find good deals at Sam's Club and Costco's around $80 after core charge. Sam's club can be quite liberal on the exchange for core, my local Sam's club took a lawn mower battery as a core.

    A small 12 volt system shouldn't require an electrician to hook up, indeed an electrician might come out and refuse to hook it up leaving you with a minimum service charge.

    I suspect you will be using a small inverter, with this small system. You can't hook it up to the home's electrical system, if you want to use the electric inside setting up a single power strip to use the energy would be viable.

    What size inverter and plans do you have?

    I can't remember the size of the inverter I have, but the size was approved by the folks that own this site. They were also the ones that helped me pick out what goes between the panels and the controller (wires, breakers). Now I'm going to put the battery specs in and have them tell me what I need between the battery and the inverter. And I have an electrician who does solar who is going to hook it up for me and give me a quick education on the system.

    (Now a faux legal question for the group:I started doing this because of an article by an "expert" who said how easy it was to set up a solar system. All you need are panels, a controller, a battery, and an inverter! It is easy as it can be! Would tracking that author down and sticking him in a microwave for 60 seconds so he could give his "expert" opinion on the amount of electricity going through the compartment be considered unlawful considering the way he suckered me into thinking this would be easy? I'm very appreciative to this forum, Northern Arizona Wind & Sun, and the electrician as there is no way I could have completed this without their help. No thanks to the "expert" who wrote the article.)
  • BB.BB. Super Moderators, Administrators Posts: 29,717 admin
    We are here to help each other. As with many things in life, the details do matter.

    At least the article got you looking at solar.

    -Bill "positive spin" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • 706jim706jim Solar Expert Posts: 270 ✭✭✭
    767mrN#13 wrote: »
    I have "Short-circuit Current (Isc)" of "8.76A." I have two panels in the system. Now I'm trying to figure how much of a battery to buy. Anyone able to guide me?

    Buy some golf cart batteries, at least two, preferably four.

    They are relatively cheap, forgiving and are a manageable weight.

    And if you screw them up by overdischarging, you won't have lost that much money.
    Island cottage solar system with 2400 watts of panels, 1kw facing southeast 1kw facing southwest 400watt ancient Arco's facing south.Trace DR1524 MSW inverter, Trace C40 PWM controller 8 Trojan L16's. Insignia 11.5 cubic foot electric fridge. My 27th year.
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