Battery Bank Sizing

stmarstmar Solar Expert Posts: 328 ✭✭✭
Using a system for grid backup is bank sizing that critical? What I mean is if you are on a fairly reliable grid and only have brief occasional outages, say a few hours, then do you need a lot of battery bank? When the grid goes down we cut way back on power usage. We do not "sell" power back to the grid. My system is Trace SW4024, 4 PVX2580 Concordes, 12 solar panels. As posted in earlier threads I am contemplating replacing my battery bank and if I go to 6 volt I am going to have to reduce AH (410) if I go with 4 PVX4050 batteries or go with 8 PVX2560 batteries to stay withing my current range of 516ah, with the PVX2560 I will be at 512ah. Also the 8 PVX2560 batteries are about the same $$ as 4 PVX2580 12 volt, a little less but that will be used up with extra cabling and fusing I assume.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,518 admin
    Re: Battery Bank Sizing

    Yep, sizing is critical... A battery bank performs differently over time (getting "better" the first year of life, and then slowly dieing for the next 3-7 years or so).

    We try to size the battery bank to reliable support your loads. What good is a system that may support those loads for the first year, but black out or only work for a few years for the next 5+ years).

    And, a too small battery bank can have problems (over voltage with too much sun from the array, micro cycling, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery Bank Sizing

    Well obviously you need enough capacity to supply your requirements when the grid goes down. That's a given. Another issue is the actual size of that "12 solar panel" array as you don't want the maximum charge current to be potentially hazardous to the batteries. With the AGM's you have a pretty wide leeway on that, though.

    From 410 Amp hours to 516 Amp hours you're looking at a difference of about 0.5 kW hours @ 25% DOD.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Battery Bank Sizing
    stmar wrote: »
    if you are on a fairly reliable grid and only have brief occasional outages, say a few hours, then do you need a lot of battery bank?

    It sounds like your batteries will spend most of their life floating. That is not good for the type of batteries that many of us use (and that you read about on this forum).

    There are deep cycle batteries that are optimized for that type of life and have service lives of 20 years. Of course, there is a trade off... these batteries do not have a very high cycle life. Sometimes they are called 'stationary' or 'telecom' batteries.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • stmarstmar Solar Expert Posts: 328 ✭✭✭
    Re: Battery Bank Sizing

    It is hard to determine load, from what I have seen lately it averages about 5 amps so I guess that would be 600 watts. The only time it spikes is when I have my horse water tank heater plugged in and then it goes to about 18 amps but if the grid was down I would not use that device. It looks like I will have to do an audit of consumption and production (panel output).
    I did increase the ah when I replaced the 6 volt GC2 batteries (8 X 232) = 464 @ 24 volts with the PVX2580 (4 X 258 ) = 516 @ 24 volts and this was the only change I made to my system. I have been running this system for a total of 15 years; 5 with the 6 volts and 10 with the PVX AGMs but I really could not tell you if it is functioning properly. All I know if that it works when the grid goes down. The only problems I have had was a lightning strike and a fried AC main transformer in my SW4024. And you are correct in that most of the time they stay in Float.
  • stmarstmar Solar Expert Posts: 328 ✭✭✭
    Re: Battery Bank Sizing

    I did some calculating and think that my consumption on the inverter/alternative system is about 400kwh per month and I went heavy on this number and used winter months because those are the most. My water heater and well are on the grid and they are the energy users, the only time I would put the well on the inverter would be an extended grid outage and I do have the capability.
    My question is: What size battery bank is optimal for a backup system based on my SW4024 taking into consideration that the grid is fairly reliable?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery Bank Sizing

    400 kW hours per month is about 13 kW hours per day. That's a lot of kW hours for an off-grid system. 541 Amp hours used on 24 Volts. If you tried to supply that entirely from batteries ... big. 1000 Amp hours plus.

    So what you need to do is reason out the things you absolutely have to keep running when the grid fails. Refrigeration, for example. Measure those items with a Kill-A-Watt and add them up. Then you'll know something like "I've got to have 2kW hours reserve power". That should be fairly easy because that's about 100 Amp hours used and times four for 25% DOD (gives you 2 days maximum power out) you're in the 400 Amp hour range.

    If that amount of power will do I'd go for whatever AGM's can give you the Amp hour range in a single string (to minimize the problems from wiring).
  • stmarstmar Solar Expert Posts: 328 ✭✭✭
    Re: Battery Bank Sizing

    Still have some things to think about but I am sure I can get it down to a minimum usage, probably in half to 200 easily. That may be the way to go and just get 4 of the PVX-4050, that would put me in the minimum range at 400 but would be a single string to get my 24 volts. Thanks for all the help, batteries are problematic because you can't add to a bank once you put it in so you have to make a firm decision up front.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery Bank Sizing

    You can add to the bank if you figure out fast enough that you need to. Usually paralleling a second string within a year will not cause any problems. Its when they've been used a lot over time and the actual capacity has gone down that the issues arise because you are in effect coupling lower capacity (older) batteries to higher capacity (newer) ones. The greater the difference the greater the chance of difficulty with the resulting bank.
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