How to size battery

[bbradb]

i am experimenting with setting up a simple solar system to replace electric meter at entrance sign to home development.

Been successfully with an older pair of 40 watt panels that I got free plus a 10 watt LED flood that is adequate for sign. I added good charge controller from Renogy and am using a 12v 54AH AGM battery that I acquired for nothing. Actually have two of these batteries that were removed from service after 5+ years of use as battery backups or something. I have tried individual batteries and also have parallelled them to try that. Obviously I am not getting full charge on these since I run sign light each night. I also feel my 2 panels would be best replaced with an 80 to 100 watt modern panel (mine are probably 10 years old or so from a portable RV charging system).

With poor sunlight this time of year I am struggling to keep enough charge in batter(ies). I'm thinking I should get a new battery properly sized. Since I've read that an 80 watt panel under the best conditions can charge a 35AH battery in a day, is this the proper size battery I should buy? Question is... Is it better to have an oversized battery in AH or one that is correctly sized for panels and load?

 

[rmaddy]

Size the battery based on the amount of power you need. If you are powering a 10W light for 14 hours a day then that is 140Wh/day. How many days do you need it to last to account for cloudy days? Let's say 3 days as an example. Then that's 3 x 140Wh/day = 520Wh. Since an AGM should only be used to 50% you would need 2 x 520Wh = 1040Wh. At 12V that would be 1040Wh / 12V = 87Ah. Adjust all of the numbers as needed to work out the battery size you really need.

For the solar you basically do the same. If you need 140Wh/day and this time of year you get maybe 1.5 solar hours you would need 140Wh / 1.5h = 93W of solar to replace the 140Wh you used in a day. But this needs to be adjusted up since you rarely get 100% out of solar panel and if you have a few cloudy days you would need more solar to replace all of the battery capacity used during the cloudy days.

 

[Zil]

To start, I would grab a marine deep cycle AGM from Walmart. Get the highest ah you can. Shoot for 100 amp hour. Old batteries are most likely a problem. You need enough solar to return the ampere hours you use from the battery.