12v battery bank question

comknight
comknight Registered Users Posts: 7 ✭
in Solar Beginners Corner #1
I have 7 Deep Cycle batteries that I want to hook up together in parallel but I am not sure what gauge wire to use between the batteries. Will 4 gauge wires be fine or should I use 2 gauge?

Thanks in advance,
Robert
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: 12v battery bank question

    Welcome to the forum Robert.

    First, don't do it. The chances of current being shared equally between seven parallel connections is just about nil. That means they will neither discharge nor charge evenly. Ultimately this shortens the life of them all as some 'sit idle' while others 'do all the work'.

    Second, the wire size used depends on the amount of current expected. Batteries can produce enormous amounts of current, but usually there is an anticipated load.

    Third, if you can provide us with some details of what exactly you're trying to do we may be able to come up with a better solution.
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  • comknight
    comknight Registered Users Posts: 7 ✭
    #3
    Re: 12v battery bank question
    Cariboocoot wrote: »
    Welcome to the forum Robert.

    First, don't do it. The chances of current being shared equally between seven parallel connections is just about nil. That means they will neither discharge nor charge evenly. Ultimately this shortens the life of them all as some 'sit idle' while others 'do all the work'.

    Second, the wire size used depends on the amount of current expected. Batteries can produce enormous amounts of current, but usually there is an anticipated load.

    Third, if you can provide us with some details of what exactly you're trying to do we may be able to come up with a better solution.

    Sorry, I should have given more specifics. I have 2 235 watt panels that I will be hooking up to an MPPT controller to charge my battery bank of the 7 deep cycle batteries. I plan on using the battery bank to run some lights, tv and dvr for starters then work up from there.

    I have another question about the MPPT controller that I bought here in Houston is a "Model MPPT-3015A Battery System Voltage 12V/24V Auto Recognition Maximum Solar Input 12V DC 420 Watt / 24V DC 840". Since my total watts is over the 420 watt for 12V, does that mean I have to run a 24V setup for my battery bank so that my I will be under the maximum solar input?

    Thanks again,
    Robert
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  • Marc Kurth
    Marc Kurth Solar Expert Posts: 1,148 ✭✭✭✭
    #4
    Re: 12v battery bank question

    What are the batteries?
    I always have more questions than answers. That's the nature of life.
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  • comknight
    comknight Registered Users Posts: 7 ✭
    #5
    Re: 12v battery bank question
    Marc Kurth wrote: »
    What are the batteries?

    Right now I have my batteries in my garage. I am building a cabinet for them and not sure if I want to leave them in the garage or out in the shade on the side of my house. I have not hooked up anything yet. Just have the batteries sitting there till I figure out what to do next.

    I am confused on my MPPT controller. It says max solar input is 420 watts at 12v but 840 watts at 24v. Since I will have 2 - 235 watt panels to hook up to it, I am leaning at setting up 2 batteries in a series to get 24v then setting up the 3 groups of 2 in parallel. I might get me another battery so that I will have 4 groups in the battery bank. Then I know I will have to get me a new 24v inverter and I believe the 4 guage battery cables between the batteries should be okay.

    My hopes is to one day get my frigerator, lights, tv, dvr and computers on solar and my AC unit and electric dryer and dishwasher will stay on grid. Not sure if I could get enough panels on my roof to be truly grid free but reducing my bill will be nice and having it for emergency use is another reason for doing it.

    Robert



    Robert
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: 12v battery bank question

    Robert;

    The question is actually "what capacity are the batteries?" because you've got 470 Watts of panel which is enough for a couple of GC2's on a 12 Volt system. More batteries = more capacity = poorer charging from a given amount of PV, right down to the point where it no longer works.

    The model number of that charge controller sounds like it is a Blue Sky unit (looked it up; it's a 'SUN'), unfortunately known for having operational quirks (such as not being good at down-converting higher Voltage or losing current capacity when it does). The input Wattage rating relates to this: 420 Watts on 12 Volts is roughly 27 Amps output. The 470 Watts you have would be roughly 30 Amps. This would max out the controller, and I don't know how well it would deal with that situation. If you were to go with a 24 Volt system you'd have 15 Amps available.

    If this is equipment you've got that you're experimenting with for learning purposes, adjust the number of batteries to best suit the potential output current at either 12 or 24 Volts. If this is going to be a system you need to depend on, it's time to reevaluate the needs and start over.
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  • comknight
    comknight Registered Users Posts: 7 ✭
    #7
    Re: 12v battery bank question
    Cariboocoot wrote: »
    Robert;

    The question is actually "what capacity are the batteries?" because you've got 470 Watts of panel which is enough for a couple of GC2's on a 12 Volt system. More batteries = more capacity = poorer charging from a given amount of PV, right down to the point where it no longer works.

    The model number of that charge controller sounds like it is a Blue Sky unit (looked it up; it's a 'SUN'), unfortunately known for having operational quirks (such as not being good at down-converting higher Voltage or losing current capacity when it does). The input Wattage rating relates to this: 420 Watts on 12 Volts is roughly 27 Amps output. The 470 Watts you have would be roughly 30 Amps. This would max out the controller, and I don't know how well it would deal with that situation. If you were to go with a 24 Volt system you'd have 15 Amps available.

    If this is equipment you've got that you're experimenting with for learning purposes, adjust the number of batteries to best suit the potential output current at either 12 or 24 Volts. If this is going to be a system you need to depend on, it's time to reevaluate the needs and start over.

    The batteries I have right now are rated 160 each.

    AutoCraft Marine / RV 12-Volt Deep Cycle Battery, Group Size 27, 600 CCA
    Battery Type: Lead Acid
    BCI Group Size: 27
    Cold Cranking Amps: 600 amps
    Cranking Amps: 750 amps
    Deep Cycle/Starting: Deep Cycle
    Height: 8 13/16 in
    Length: 12 3/4 in
    Maintenance Free: Yes
    Reserve Capacity: 160 min
    Terminal Type: Combination Auto / Stud Terminal
    Voltage: 12 v
    Weight: 50 lbs
    Width: 6 5/8 in

    I am doing this as my first use of solar so dont expect to run much right now because this is my experiment to see what I can get way with on energy use right now with what I have put together on my budget. Using some promotion codes from Advance Autoparts, i was able to get the batteries at about $89 each which met my budget for my little home solar experiment. I spent more than I thought I would right now but excited at playing around with solar.

    Robert
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
    Re: 12v battery bank question

    Oh dear. RV/Marine batteries are not really deep cycle despite the makers' claims. They are automotive batteries with thicker plates. Expect them to behave as such (i.e. die quickly in an RE application).

    Given the batteries, charge controller, and panels you have the most efficient set up would be two of those batteries in series giving 160 Amp hours @ 24 Volts. That would match fairly well with the 15 Amp output available from the charge controller at 24 Volts. But first check the charge controller's input limits because the Voltage from those panels is bound to be Vmp around 30, which is not sufficient to properly charge a 24 Volt system. The panels would need to be in series, which pushes the charge controller input Voltage up to 60 and the maximum Voc over 70 (probably).

    If the charge controller can't take this you'll have to go with two batteries in parallel for a 12 Volt system and run the panels in parallel and hope the controller can manage 30 Volts in for a 12 Volt system. Too often these inexpensive controllers have severe limitations on input/output which make them useless in the real world. Some require 'standard' Vmp for input (i.e. 17.5 for 12 Volts, 35 for 24), making them work no better than a PWM type controller and essentially removing the main advantage of MPPT.
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  • Rybren
    Rybren Solar Expert Posts: 351 ✭✭
    #9
    Re: 12v battery bank question

    Note that the batteries have a reserve capacity of 160 minutes and NOT 160AH.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #10
    Re: 12v battery bank question
    Rybren wrote: »
    Note that the batteries have a reserve capacity of 160 minutes and NOT 160AH.

    True. Probably 90 Amp hours.
    The lack of an Amp hour rating is a strong indication of how unsuitable they are for an RE application.
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  • comknight
    comknight Registered Users Posts: 7 ✭
    #11
    Re: 12v battery bank question
    Cariboocoot wrote: »
    True. Probably 90 Amp hours.
    The lack of an Amp hour rating is a strong indication of how unsuitable they are for an RE application.

    Man, you guys are raining on my parade. I guess I can take the batteries back since I have not done anything with them and look at some true golf cart batteries. I just got them yesterday.

    I just got so excited about solar that I just jumped in without checking the water level.

    Robert
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #12
    Re: 12v battery bank question
    comknight wrote: »
    Man, you guys are raining on my parade. I guess I can take the batteries back since I have not done anything with them and look at some true golf cart batteries. I just got them yesterday.

    Batteries are the LAST thing you buy when building a system. They start deteriorating immediately after manufacture. They deteriorate even faster when they sit around, uncharged, waiting to be used.
    comknight wrote: »
    My hopes is to one day get my frigerator, lights, tv, dvr and computers on solar and my AC unit and electric dryer and dishwasher will stay on grid. Not sure if I could get enough panels on my roof to be truly grid free but reducing my bill will be nice and having it for emergency use is another reason for doing it.

    More rain on parade... you need to be very clear about this: Any power you generate for a battery based system will cost you several multiples of what your power company charges.

    In a battery-less grid tie system there MAY be a reasonable return on investment (depends on local policy).

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • comknight
    comknight Registered Users Posts: 7 ✭
    #13
    Re: 12v battery bank question

    I appreciate all the feedback and advice. I ended up returning all but 4 of the batteries and will use them as my learning experiment. I decided to run them as a 12v system with the 4 batteries as I dont plan on running any high amp appliances anytime soon. Right now what I have will be perfect for short term experimenting and learning how solar works.

    I really love this forum as there is a lot of great information.

    Thanks all for the great feedback,
    Robert
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,590 admin
    #14
    Re: 12v battery bank question

    Robert,

    One of things about solar RE power is that it is pretty predictable how well it performs (on average--a bunch of days with dark cloudy weather--Solar panels will not output hardly any power).

    I always like to start with your loads (peak watts, average watts*hours per day of power usage, AC or DC, etc.). It is much easier to design a system to meet your needs that way--Or at least you do a few paper designs and figure out that the original plans were not going to work and/or are not cost effective).

    A Kill-a-Watt type meter is great for measuring/understanding your 120 VAC needs. There are DC Watt*Hour/Amp*Hour meters too.

    Another neat tool is the AC/DC Current Clamp meter. Sears has one that "works good enough" for our needs.

    Home Depot had a General Tool unit on sale for $30 this weekend (don't know anything about it).

    Understanding your loads (how Volts, Amps, Watts, etc.) all work together is a big help towards understanding solar.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Marc Kurth
    Marc Kurth Solar Expert Posts: 1,148 ✭✭✭✭
    #15
    Re: 12v battery bank question

    For $60.00 or so, that little Sears clamp-on meter is a great tool for getting an idea of what is going on in a system. I've given about six of them away, along with a battery bank purchased by customers that I just knew would need to learn about how their system works!

    Mine generally reads pretty close to my very expensive, annually calibrated bench top test system designed for aircraft battery load testing. I can't begin to tell people how valuable it is to know what is flowing in/out of your system at any given time!

    For the money, it just makes senses to provide a "sanity check" even if you have other instrumentation.

    Marc
    I always have more questions than answers. That's the nature of life.
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