long term battery storage?

hillbilly

Hi all,
just wondering if any of you out there had some suggestions regarding the best solution for being able to leave our battery bank "unattended" for a four month period or so. We're off grid, and using surrette "rolls" type flooded lead acid batteries (700ah@ 24V), and I would like to keep them in good working order durring the winter while my wife and I will be away. I don't need to have them actually running anything, I can easily shut the house down for the winter if need be.
I was thinking that it would be fairly easy to keep them in float most of the winter, but not sure if having them not discharge at all would be a problem or not. I could attatch a small load or two, if that would help. Biggest issue would be no easy way to EQ them for about 4 months, as we only have about 450 watts of PV (STC) currently we have to use our generator to do the EQ'ing. Any ideas???
Thanks a bunch for your helpfull input...

mike95490

Re: long term battery storage?

What is the size of your battery bank (Volts & A hours)
What is your charge controller ?
" 450W of PV "

I'd say as a first guess, equalize the day you leave, and let the PV's and controller keep it topped off . maybe switch off the controllers equalize function - since you won't really be cycling the batteries, and wont be able to refill any boil-off.

hillbilly

Re: long term battery storage?

Thanks Mike:
the battery bank is 8 6V batteries in 2 strings of four; rated 700ah at 24V.
The Charge controller is an MX60 (still growing the system at the moment), right now we just do the EQ manually via the Generator and inverter (vfx3524). I am not so worried about the watering per se, as it seems that our batteries don't tend to need that much ever (we have water miser caps).
I just don't know enough about batteries to decide if it's better to have them connected to some small loads so that they have some exercise, and how big of a deal it would be for them to not recieve an EQ charge for that long of a time. I could allow the CC to do an automatic EQ, albeit a VERY minor EQ charge, and this day happened to fall on a cloudy day (will be away from NOV-FEB)... then it would do nothing at all.
If you think that it's a big issue would could possibly look into having a friend come over once or twice and run the Genny to do an EQ charge... (would have to show them how the whole system works, to do this).
cheers... thanks again for the help

mike95490

Re: long term battery storage?

Don't worry about the loads, the batteries are quite happy staying at 100%

Give them an EQ before you depart, and fill the cells with water, and let the PV's keep them on float charge all winter. FLOAT, not Eq , and no automatic EQ's all winter, with no cycles, no need to Eq.

That's the best I'd suggest, AFIK.

--
“Science has achieved some wonderful things of course, but I'd far rather be happy than right any day."
-- Slartybartfast

niel

Re: long term battery storage?

i'll second mike's advice to you. just eq it before you leave and let it float charge all winter long without any automatic eq charges. the full charge in the batteries will keep them from freezing if your house should get that cold and no time degradation will occur either seeing as how they will receive float charges. even if they go for a week without a float charge because of snow, this will be fine as it will not loose that much in that time and the pvs if at a good winter angle also helps in keeping any applicable snow off of them. (assuming you get snow) the pvs do warm up quickly and melt snows even when accumalated, but then i would imagine you already observed that if you get snow.:roll:
do inspect everything so as to prevent any possible mishaps while you aren't there. ie shorts.

icarus

Re: long term battery storage?

I have a second system of very similar size on another site. The battery bank often sits for 6-9 months at a time with nothing but the panel and controller. We see temps routinely -40. The batteries are in an insulated box and they never freeze. I top them off before I shut them down, and check them when I turn the system on. I am on the 11th year and so far so good. As long as they are charged they should be fine.

Icarus

System2

Re: long term battery storage?

My system at the camp sits over winter. I'm able to get to it with difficulty in winter. Year before last, toped it up, shut everything down and about once a month went back and turned on PV's and controller for a few days to top them up. Last year, I set the Morningstar TS-60 to the lowest voltage and just left it.
Wish it could have been set to permanent float however, as they did somewhat overcharge. 660 watt PV and six L-16 flooded in 12 volts.
They did survive however. 4 years so far and I don't yet see any drop in capacity.
Perhaps the TS-60 can be set to permanent float, but I know nothing of it.
Wayne

niel

Re: long term battery storage?

wayne,
the lowest setting on that controller is 14v and that's too high for float charging even with dimished current through pwm action for longer time spans. i might suggest you place a diode or 2 in series that can handle the current you'd need for float charging. this will drop the voltage lower to allow a better float charge level as it should be around 13.5v or so (check for your specific battery float voltage specs) and if too much current flows you may need to obtain a different controller to accomodate the long term float charging. bluesky specs a good float current rate of 100ma per 100ah of battery capacity with no loads on. if you are seeing rates like 500ma per 100ah then you know it is still overcharging the battery(s) some.

System2

Re: long term battery storage?

Thanks for that info Niel, much appreciated. The diode thing is so simple. Just never thought of it. Stunned dunce. Yes, they were getting overcharged, no question.
But, hopefully last winter was the last for that. If all goes well, I'll have the whole system moved and set up at home before the snow flies, but for sure I'll remember your advice. That 100 ma per 100 ah is very interesting too.
Thanks again
Wayne