The Laberinth that is batteries, System design

Fe-Wood
Fe-Wood Solar Expert Posts: 96 ✭✭
in Off Grid Solar & Battery Systems #1
Ok, I've been wading through all the possibilities in putting a system together, this is what I've come up with so far.

My PG&E bill says I use about 8KWH per day on average in the home I presently live in.

I can cater my charging to usage.

I have decided the Magnum Energy MS4024PAE is the unit I will base the system around. Seems like a good system for the money and my needs.

I'm looking at batteries and getting totally overwhelmed. I have around $1000.00 now to spend on batteries. I know you get what you pay for with batteries so I would like to get the best I can.

The system will run on 24 volts. I want to be able to store about 600 amp hours @24 volts. I have plenty of ventilated storage. I have narrowed my choices down to either the Rolls Surrette batteries or Crown Industrial batteries. The fewer the better.

Am I right in my understanding that if you take 2@6V, 300-amp hour batteries and configure them to 12v the amp hour rating stays the same?

20 hour rating, Does this mean it will produce, for instance 350 amps per hour for 20 hours or it will produce a total of 350 amps over the course of 20 hours. Reasoning tells me its the later.

If anyone has a specific configuration/manufacturer- I'm all ears. I'm also looking for distribution in Sacramento, Ca. or Reno, Nv. so I can pick them up to minimize shipping.

Thanks for all the help!
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #2
    Re: The Laberinth that is batteries, System design

    Battery AH rating based on time is amp hours spread over the time. 300 amp-hr at 20 hours means the full capacity of battery will allow 15 continuous amp draw for 20 hours.

    The more the current draw, the lower the AH's will be.

    To achieve 600 AH's at 24 vdc you would need eight 6v, 300 AH batteries. That would be configured as two parallel strings of four 6v batteries in series. Each series string providing 300 AH's at full discharge.

    The longevity of battery will be impacted by depth of discharge you should figure your needs based on about 50% of 20 hr AH rating.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: The Laberinth that is batteries, System design

    Oh the lovely, nutty world of solar electric mathematics! :p

    8 kW hours per day on a 24 Volt system is at least 334 Amp hours, or 668 Amp hour battery @ 50% DOD (not including system losses).

    Step #1: reduce power consumption.
    Step #2: Repeat Step #1 until you get a figure that won't cost an arm and a leg to supply.

    The "20 hour rate" is based on sustained discharge at a fixed rate over 20 hours indicating total Amp hour capacity of the battery. If you drain a 225 Amp hour battery at that rate for 20 hours you'll get 225 Amp hours from it, and scrap metal where your battery used to be because you won't be able to recharge it. Hence the need to limit the depth of discharge.

    We (usually) pick the 20 hour rate for RE use because of the curious coincidental relationship between a day having 24 hours in it and there usually being about 4 hours of equivalent good sun during which the panels supply the power and recharge the batteries. It isn't perfect (nothing is) but it seems to work. :roll:

    A Crown industrial battery in that 24 Volt 600-ish Amp hour configuration is about $3,000. A bit over the budget, I'd say. A similar Surrette configuration is about $1,000 less. Also expensive. 2 Volt cells start are around $300 - $400 each, so that's about a $4,000 option. This doesn't look good. The big 6-Volt units from Crown or Trojan aren't going to do it any cheaper than $2,400 ether. Good ol' golf cart batteries ($135 Crown 6 Volt 225 Amp hours * 12) will get you in under $2,000 - especially if you can get a super deal from a warehouse store.

    Or you can revisit Step #1 until you come up with a battery bank that's affordable. Remember the bigger the battery the more panel you need to recharge it.

    And yes, batteries wired in series go up in Voltage but Amp hour capacity remains the same. Parallel is the other way 'round.
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #4
    Re: The Laberinth that is batteries, System design

    I get so confused by everyone that thinks you must get "off the Grid" to be successful. If you have the Grid, it is the best battery in the world. No maintenance, no charging loses, no replacements costs ...

    Take a look at Grid Tie first! It is the most cost effective solar out there.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: The Laberinth that is batteries, System design
    solar_dave wrote: »
    I get so confused by everyone that thinks you must get "off the Grid" to be successful. If you have the Grid, it is the best battery in the world. No maintenance, no charging loses, no replacements costs ...

    Take a look at Grid Tie first! It is the most cost effective solar out there.

    Absolutely true.
    For some reason I was under the perhaps mistaken impression that he's planning an off-grid system in a location sans grid based on his existing utility-fed install.
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  • Fe-Wood
    Fe-Wood Solar Expert Posts: 96 ✭✭
    #6
    Re: The Laberinth that is batteries, System design

    Yes, It is an off grid system. It is kind of a hybrid system of sorts in that I run my large machines off a generator and would like to be able to run smaller machines, hand tools, lights and music off the batteries.

    I looked into grid tie a couple years ago and they wanted $10K to hook me up.... I can only imagine what they want now.....

    I just added up the base cost of grid tie. This is before the energy flows and after the lines have been run. Thats $28.08 a month just so PG&E can send me a bill. If I put away $28.00 a month I should be able to get a nice replacement set. Lets see- If the batteries last me 7 years, thats $336.00 a year. They haven't supplied 1kw of power yet. Comes to $2352.00 for the 7 years.
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  • RandomJoe
    RandomJoe Solar Expert Posts: 472 ✭✭✭
    #7
    Re: The Laberinth that is batteries, System design
    solar_dave wrote: »
    If you have the Grid, it is the best battery in the world.

    ...until the next thunderstorm / car hitting pole / tree limb in lines takes it out! :p

    While I agree with your point overall, I'd be mightily frustrated this past year if all I had was a grid-tie system. Almost every time it rains, and sometimes for no apparent reason at all, lights out for a few hours...

    And this isn't in the boonies - I'm right in the middle of suburbia around a large metro area. Our grid power used to be so very reliable too, with >1hr outages *maybe* every 3-5 years! The past couple of years I've had numerous >1hr, and a few >4-5hrs.

    So very nice to only notice the grid even went down because a few room lights went out. Everything I care about is instantly switched to inverter (if it isn't already running from it during the day) with no downtime - the lights don't even blink visibly - and I don't have to drag out the generator and extension cords.

    (But yes, I do absolutely agree, grid-tie is far more cost-effective if backup power isn't a primary motivator, as it was in my case!)
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