Single 12V battery recommendation for trial PV system

rice81 · October 2015

I'm setting up a simple PV system to provide power for an enclosed bumper pull trailer. Not to burden you with other details of the system, for this discussion I just want to get recommendations on a battery for the system. I have space for a standard to large size "automotive" 12V battery in a heavy plastic battery box. I understand that automotive batteries have a very poor design for PV storage use due to plates which are optimized for cranking amps and not longevity or repeated or higher depth of discharge. So my guess is that storage batteries designed for 'deep discharge' may give me the best battery life. I am willing to spend up to ~$200 for the battery BTW.

I see a variety under the listings for 'marine deep cycle' batteries. There are also the 'Optima' brand of batteries with the spiral cells and they are advertised as deep cycle with enhanced battery life. I think these are AGM.

I am a little confused when I look at specs. I would expect that a truly thick plated battery would have lower 'cranking amps' and fewer watt hours, reflecting a lower capacity but more durable design, but I don't see much of a derating of this spec with a so-called deep cycle battery. This makes me suspicious that the battery is not as durable as I might think or the specs are wrong.

If it matters, I have a Blue Solar 75/15 MPPT charge controller and I have 3 x 25 watt mono PV panels (in series should be under the max voltage for the CC)

Please make a recommendation, be it a specific battery brand / model or specific technology to look for or any general comments as well. Thanks for reading.

BB. · October 2015

75 Watts of solar panels is pretty small. Assuming 5% to 13% rate of charge (for an off-grid solar power system):

75 Watts * 0.77 panel+controller derating * 1/14.5 volts charging * 1/0.05 rate of charge = 80 AH @ 12 volt battery maximum
75 Watts * 0.77 panel+controller derating * 1/14.5 volts charging * 1/0.10 rate of charge = 40 AH @ 12 volt battery nominal
75 Watts * 0.77 panel+controller derating * 1/14.5 volts charging * 1/0.13 rate of charge =31 AH @ 12 volt battery minimum

If the panels are just there to help on weekend camping trips, and you will use an AC battery charger to bring the batteries back up when you get back, then the above limits/recommendations may not apply.

And--We like to size the battery bank based on the loads--How many Amps*Hours (or Watt*Hours) per day do you plan on using. What region (how much sun)? How many days of camping--Is electric power needed (laptop, work, cell phone, photography)--Or just a nice to have to run a fan, a few lights, and radio?

Flooded cell batteries are usually cheaper and a bit more forgiving/easier to use (check water levels, use a hydrometer to understand state of charge). AGMs do not check electrolyte levels, but are easier to over charge and kill (but cleaner, no fumes during normal operation). And AGMs are usually much more expensive.

-Bill

rice81 · October 2015

Thanks, Bill. The panels will be mostly to keep up with low levels of discharge from a security system and some low power security lights. So, in effect, you could say that I may have a bit of overkill on the battery, but I would like to have the reserve available. Thus, the 75 watts will probably be OK for now but I may upgrade the panels if needed.

Do AGM batteries do OK on a decent charge controller like the Blue Solar 75/15 MPPT. It has three level charging with float. I presume it should not overcharge. Since this is mostly a learning experience I would love a log of continuous energy I/O at the battery level. Sounds like a good Arduino project . . . monitor and log battery voltage and put a current sensor on the battery line.

But, back to the battery question, is there a good, 12V, automotive form factor, long life battery suitable for this sort of PV app?

Johann · October 2015

Is there a reason why you want to stick with an automotive battery that will croak/die in about 1 year if used in an PV system?
For about $84 you can get a sams club 6 volt 215 ah battery. Get 2 of those and series them together to get 12 volts out of them.

rice81 · October 2015

Are those Sam's batteries suitable for this sort of storage app? What are they normally used for? Golf carts? Have others had good luck with them in PV applications? I do not want an automotive battery as per the original post. Thanks for the info.

Mountain Don · October 2015

75 months on our golf cart batteries we got from Sam's Club. Part time recreational off grid, PV powered, cabin use. Seldom drawn down past 22% and never past 45%. They're doing very well.

We used the same in RV's since the mid seventies. Those all did well too.

But it does mean 2 batteries to get to 12 VDC.

rice81 · October 2015

With the help of the suggestions above and from looking at reviews from solar users and marine users, golf cart batteries seem to be a sort of super value due to the popularity of golf carts.

Here is a youtube that sort of summarizes some things about the spectrum from pure automotive to marine deep cycle to golf to solar batteries. It is also an ad for the company that produced the video but the info seems legitimate.

https://www.youtube.com/watch?v=xELqluPUsuQ

From this I might conclude that I should ditch the idea of even using a 'deep cycle' marine battery which the video claims is a beginner mistake as the battery is mostly a modified automobile battery.

Mountain Don · October 2015

rice81 said:
From this I might conclude that I should ditch the idea of even using a 'deep cycle' marine battery which the video claims is a beginner mistake as the battery is mostly a modified automobile battery.

Absolutely!!

rice81 · October 2015

I will go with the golf batteries on my next project. Meanwhile, is there anyway to get 12V of good PV worthy cells into a box that holds a large automotive battery? Thats the space I'm working with at the moment.

mike95490 · October 2015

rice81 said:

...... is there anyway to get 12V of good PV worthy cells into a box that holds a large automotive battery? ....

Nope. Any loose cells you buy on fleabay, are production rejects. They may "look" pretty, but electrically fail something.

Are you asking for a PV panel that can fold up and fit into a battery box ? I don't know of any.

Photowhit · October 2015

The video was informative, but there are 12 volt true deep cycle flooded batteries out there. Try searching for Trojan T1275 I believe it is a true deep cycle 12 volt battery. Also the video makes it sound like knowing the 100 hour rating makes 'RE' batteries better. Trojan pubishes the 100 hour rate for most/all of their 'Deep cycle" batteries. Here's a list;

http://www.trojanbattery.com/pdf/ProdSpecGuide.pdf

The generic golf cart batteries are a better value, in my opinion, but if you need a smaller footprint. These have @150 Ah at 12 volts.

rice81 · October 2015

Thanks, Photo! Good option. The Trojan T1275 looks like a good one. Looks like they are based around southern California. I'm in Dallas. The freight, hazmat and $25 core fee they charge come to quite a bit. I may try some marine stores. Perhaps they carry them.

Cheers . . .

BB. · October 2015

For the core fee... Try to find an old lead acid battery somewhere to take in (neighbor or similar). Some places will even take a small motorcycle battery in as core (core charges and polices can vary state by state).

rice81 · October 2015

BB, good point. I believe even a little Pb battery here in Texas has the same mandatory disposal / core fee as the big ones. I have been annoyed in the past when I bought small SLA batteries for an alarm system and they charged me the same core and disposal fees as for an automotive battery.

Once again, I really appreciate the responses. This is really the best solar forum.

Single 12V battery recommendation for trial PV system

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