Charging Battery Bank when gone

Philbs4 · June 2013

At what voltage should I have my charge controller set to when I am gone for 6-9 weeks?

Here is a bit of background on my system and it's uses:
My wife and I go up to our off grid cabin every week over the summer and then the school year starts and we only go up every 6 weeks.

The system I built consists of 900W of solar panels, midnite 150 lite charge controller, 2000W inverter with 60A charger, 4 deep cycle flooded batteries - 220AH @ 6V w/ water miser caps.

I know that usually when in absorb you want your charge controller set to about 14.6-14.7V. Usually, anything over 14 causes more electrolysis though (gassing and loss of water). Considering the system is unused during those 6-9, is it necessary to go into this higher voltage range? Or should I simply change the setting so that it absorbs at 14.0V rather than 14.6-14.7V? Overall, my main concern is water loss and figuring out what is the best way to keep my batteries healthy when gone for these long periods.

Thanks,
Phil

mike95490 · June 2013

Re: Charging Battery Bank when gone

I think you can set the absorb time to just a couple minutes, and let it float the batteries while you are gone. Do an EQ cycle before you leave, top the cells up with distilled, and you should be OK.

BB. · June 2013

Re: Charging Battery Bank when gone

As a starting point... Make sure you have no loads on the system (turn off breaker, plus fuses for water pumps/AC inverters/etc.). Loads left on during winter (and a solar array that gets covered by snow for a few weeks) will kill the battery bank.

Don't let the system sit below ~75% state of charge for days/weeks/months at a time.

Your cabin is probably in an area with wide temperature swings... A remote battery temperature sensor will help keep the batteries properly charged (very cold batteries need higher charging voltages).

If you pull some of the array down to lock up when you are gone--Leaving ~1% of y0ur battery bank size (i.e, 200 AH * 12 volt * 1% ~ 240 watt array) behind, or even mounted on the side of a building (shed snow, does not get drifted) will keep the battery bank charged.

You really only need to "maintain" the battery bank which is usually around 13.6 to 13.7 volts at 77F -- Some battery vendors recommend higher/lower voltages.

The Midnite charge controller is very good--But it may try to "bulk/absorb" your battery after midnight--Even if there are no loads. If so, then I would suggest adjusting the absorb voltage down and/or shorten the absorb timer and/or check the termination current (you want the termination current high enough that the controller stops charging quickly).

Realistically, your batteries can probably go 1 month between charging and even longer in subfreezing weather. As long as the batteries are well charged, they should never freeze (you don't want them to freeze as this can crack the cases/damage the plates).

More than likely, "over charging" is going to be the big problem you want to avoid (as long as there are no other loads on the system--security alarm, etc.).

Anyway, my starting point. I have no cabin or battery based system--So others may give you some more/better pointers.

-Bill

BB. · June 2013

Re: Charging Battery Bank when gone

PS: Per Mike's suggestion--Don't over fill the cells. If they gas (or get warm in spring), the excess electrolyte will be spilled out the battery caps. A mess and not really good for the battery either.

-Bill

Photowhit · June 2013

Re: Charging Battery Bank when gone

You've gotten good info, I'd go ahead and turn off the equalizing and manually equalize when you get back.

niel · June 2013

Re: Charging Battery Bank when gone

i will add here that because bb mentioned leaving your batteries at 75% does not mean you can leave them at say 80% or some other % under 100% soc because you should never leave without giving them a full charge as the small float charge would be insufficient to bring the batteries up to a full charge for it was never intended to do so. i might add that due to inclimate weather and shorter less intense solar periods of the winter months that you should have at least around 2-4% of the 20hr ah capacity rating of the batteries for a float charge and make no mistake about it that you can leave all of your pvs hooked up if you like.

H2SO4_guy · June 2013

Re: Charging Battery Bank when gone

Why not just treat them like the backup battery folks do and basically float them. I set the absorb to about 15 minutes and then it will maintain a float charge. I love the Midnite controlelrs and use them every day now that I am living off grid. That's how I handle the batteries that are not being cycled daily and they are still going well even though I got them from a scrapper!

Good Luck,

Skip

vtmaps · June 2013

Re: Charging Battery Bank when gone

Welcome to the forum Phil,

Long term float will kill batteries because of stratification of the electrolyte. You need to run them through a complete charge cycle every week or so. One way to do that on many controllers (not sure on the Midnite classic) is to set auto equalization to once a week and lower the eq voltage to absorb voltage.

--vtMaps

Philbs4 · June 2013

Re: Charging Battery Bank when gone

Thank you all for your quick responses,

looks like I should equalize before I leave and top off the batteries. Then set it to float around 13.7V (I'll check with vendor on what they think on the voltage). Then have it "equalize" at the absorb voltage every week or two to prevent stratification in the electrolyte.

Thanks again,
Phil

BB. · June 2013

Re: Charging Battery Bank when gone

I would suggest not doing equalizations (or partial equalization) every week... At the most, I would suggest once a month.

But I am not sure it is needed. Stratification, as I understand, appears to only form during charging/discharging cycles. Not during "storage" or during float (at least not much during float).

And cold batteries are "very happy" in storage (roughly, for every 10C below 25C, battery "life" is doubled and chemical reactions slow down).

If you wish to equalize, I would only do manual equalization when you are there. Equalization uses water and generates heat--I would not really want that to happen when I was away.

Again--there are my personal observations. Battery maintenance is as much magic/belief as science (regarding how best to get long life/prevent damage--See desulfators).

These are my "beliefs". :roll:

-Bill

Cariboocoot · June 2013

Re: Charging Battery Bank when gone

From someone who actually does this:

Disconnect inverter and any other DC loads.
Make sure batteries are fully charged and watered.
Leave controller & panels connected.
Come back when you want.

Without loads on the MidNite or Outback controllers will cut back on Absorb time automatically, thus reducing water loss. Most of the time the batteries will be in Float. Your GC2 batteries will not stratify. No EQ cycle will be needed in this time.

Upon return check and charge and water fully before use. EQ only if necessary (difference of more than 0.010 in SG).

I walk away from mine for seven months of the year straight. No over-winter problems in seven years now.

Vic · June 2013

Re: Charging Battery Bank when gone

Regarding Float voltage, to me 13.7 V seems high for Float on FLA batteries. AND, it is often difficult to get specific advice from the manufacturer of GC batteries on charge parameters.

Normally, Vfloat is set to the lowest voltage that will just barely maintain SG.

Also, using Solar as the only source of Floating batteries is different than having the Grid Float batteries continuously, 24/7. FWIW. Vic

Cariboocoot · June 2013

Re: Charging Battery Bank when gone

Vic wrote: »

Also, using Solar as the only source of Floating batteries is different than having the Grid Float batteries continuously, 24/7. FWIW. Vic

This is entirely true. Grid float would be 24/7; solar will only 'float' while there's enough sun to do it. Since this is generally hours of good sun less a brief Bulk & Absorb period you're really looking at floating for three to four hours a day +/-, not 24. Big difference.

I wouldn't worry about the Float Voltage being too high @ 13.7 either. Hitting them with an EQ Voltage is what will 'boil off' water.

Somewhat related thread: http://forum.solar-electric.com/showthread.php?19930-All-s-well

Vic · June 2013

Re: Charging Battery Bank when gone

Cariboocoot wrote: »

I wouldn't worry about the Float Voltage being too high @ 13.7 either. Hitting them with an EQ Voltage is what will 'boil off' water.

Well, Floating a battery, at least FLAs, adds to Positive plate corrosion. So, lowering the Float voltage to that which JUST maintains SG is somewhat better for the battery. Not a huge thing for many systems, but for systems that spend many hours per day in Float it can make a difference, in my opinion. Thanks! Vic

Cariboocoot · June 2013

Re: Charging Battery Bank when gone

Vic wrote: »

Well, Floating a battery, at least FLAs, adds to Positive plate corrosion. So, lowering the Float voltage to that which JUST maintains SG is somewhat better for the battery. Not a huge thing for many systems, but for systems that spend many hours per day in Float it can make a difference, in my opinion. Thanks! Vic

Yes, but as you so rightly pointed out it won't be in Float that much per day due to the nature of solar.

If you're going to Float batteries off AC 24/7 it is indeed a good idea to turn the Voltage down - again just as you said.

Vic · June 2013

Re: Charging Battery Bank when gone

BB. wrote: »

The Midnite charge controller is very good--But it may try to "bulk/absorb" your battery after midnight--Even if there are no loads.

-Bill

Bill, believe that this only applies to systems that have RE inputs that other than PV. The Classic starts a 'new day' at midnight, so if one has Hydro, Wind or other power available during dark hours, the Classic CAN start a new charge cycle at midnight, as noted by Chris. The Classic may look for available power at midnight, but if there is nothing available, should continue sleeping/resting until near Sunrise. My beliefs, anyway. Vic

BB. · June 2013

Re: Charging Battery Bank when gone

Sounds good Vic.

-Bill

vtmaps · June 2013

Re: Charging Battery Bank when gone

BB. wrote: »

Stratification, as I understand, appears to only form during charging/discharging cycles. Not during "storage" or during float (at least not much during float).

Vic wrote:

Normally, Vfloat is set to the lowest voltage that will just barely maintain SG.

My understanding is that stratification does occur during float, and that float cannot maintain SG without an electrolyte circulation system. Some battery chargers (e.g. Iota) will put a battery through a charge cycle every week when maintaining a charged battery, and this is to stir up the electrolyte.

BTW, I admit that my understanding of stratification is somewhat limited:
http://forum.solar-electric.com/showthread.php?17994
The only thing that seems certain (based on my research) is that stratification occurs during charging. I haven't found a source that states explicitly that stratification occurs during discharge, storage, or float. However, I infer from a number of sources that stratification does occur during float. Bill, can you clarify your comment: "at least not much during float" ?

--vtMaps

BB. · June 2013

Re: Charging Battery Bank when gone

Honestly, I am not sure--My uninformed guess is that you need significant charging/cycling to produce the stratification (just electrolyte sitting in a bottle will not stratify--I think--I looked for confirming/conflicting tests reports and did not find anything). And since, ideally, float is only a fraction of 1% of current flowing into a battery vs 10x amount or more for a "recharging" battery, the amount of "heavy" electrolyte produced should be much smaller too (ignoring mixing from thermal and gas driven currents only occur during heavy charging/discharging).

I think the best researched thread is still this relatively recent one:

http://forum.solar-electric.com/showthread.php?17994-When-does-electrolyte-stratification-occur

And it is not clear exactly what is happening from the abstracts and none of them discuss "floating" specifically vs battery life (that I remember).

-Bill

Vic · June 2013

Re: Charging Battery Bank when gone

Well, for the systems that I watch, stratification is NOT an issue at all. These systems get good gassing in the 57 volt range -- 14.25 V/12 V. Have floated FLA batteries for extended periods with no issue. So, for me, stratification is nothing to worry about.

Have also read the (myth to me) that tall-cased batteries are prone to stratification, and therefore must have extended Absorb stage and high absorb V. This, too does not seem to be the case with L-16s here or on the taller Surrette 4KS25s, with their 23-ish inch tall jars.

It is quite possible that the batteries here are of the older high Antinomy variety, and therefore gas more readily.

Every system is different, even if managed by the same person, as usage, recharge and environmental patterns can never be exactly the same for different systems, and so on.

Opinions, Vic

NorthGuy · June 2013

Re: Charging Battery Bank when gone

Vic wrote: »

Have also read the (myth to me) that tall-cased batteries are prone to stratification, and therefore must have extended Absorb stage and high absorb V. This, too does not seem to be the case with L-16s here or on the taller Surrette 4KS25s, with their 23-ish inch tall jars.

My batteries are definitely prone to stratification, and they're tall (24"). I cannot tell whether they're prone to stratification because they're tall.

However, when I did experiments with shallow discharges (5-10% DoD), stratification wasn't a problem at all. Perhaps, for the batteries that do not get discharged, there's no need to worry about stratification.

Cariboocoot · June 2013

Re: Charging Battery Bank when gone

Stratification is a two-part process: first you have to separate the sulphuric acid from the water in order to create the potential fluid density imbalance. This is done by discharging which causes the sulphur to adhere to plates leaving mainly water as electrolyte. At this point there is no stratification because the electrolyte SG is consistent (albeit low) throughout. Then you have to recombine the sulphur and water to raise the SG by recharging. If the stirring action (which occurs mainly at full Voltage - when you get maximum bubbling) is not sufficient to mix the sulphur back in at all levels the higher density electrolyte will remain at the bottom while lower density floats on top. This is the stratification problem.

Small batteries usually do not suffer from it at all; it mainly afflicts larger batteries especially of the "tall case" variety where there is literally physically more height for the mixture to remain uneven in. This is why some forklift batteries have air pumps; to introduce additional stirring action and keep the electrolyte even throughout.

Nor is it a problem if the batteries are kept charged because there is no chemical separation and recombination process to lead to the different densities being available to stratify. I've never heard of it occurring just from allowing a battery to sit, although they will self-discharge when idle and so the overall density will alter. Inevitably one would recharge the battery at that point, wherein you can get the disparity.

Vic · June 2013

Re: Charging Battery Bank when gone

N G,

In the Summer, our cycles are shallower. But, DO regularly cycle these banks to 75 - 80% SOC, and to about 55% SOC on occasion. Just have not ever seen stratification on these systems, nor has a neighbor who also uses the 4KS25 Surrettes. All of these banks have been in service for more than seven years. They all gas readily at 57 volts and above, this may well have to do with the Antinomy content. I have no way of knowing.

But, am quite familiar with the issues with your Trojans, and am quite happy that the systems here do not have stratification issues. We get to worry about other things -- always something to worry about. YMMV, follow the label, avoid excessive use. Vic

vtmaps · June 2013

Re: Charging Battery Bank when gone

Cariboocoot wrote: »

This is done by discharging which causes the sulphur to adhere to plates leaving mainly water as electrolyte. At this point there is no stratification because the electrolyte SG is consistent (albeit low) throughout.

Marc, I have no idea whether you are correct (that discharge does NOT cause stratification). To my (often wrong) way of thinking, the heavy acid produced by charging sinks to the bottom of the jar, and the light water produced by discharge should float to the surface.

Can you explain (physics) why discharge does not cause stratification?

--vtMaps

Cariboocoot · June 2013

Re: Charging Battery Bank when gone

vtmaps wrote: »

Marc, I have no idea whether you are correct (that discharge does NOT cause stratification). To my (often wrong) way of thinking, the heavy acid produced by charging sinks to the bottom of the jar, and the light water produced by discharge should float to the surface.

Can you explain (physics) why discharge does not cause stratification?

--vtMaps

It's not that discharge does not cause stratification; it is part of what causes it. On discharge the chemical change in electrolyte is fairly even (providing the battery isn't suffering any other problem). The stratification comes about when the charging process re-introduces the sulphor to the mix; being heavier, once it is pushed off the plates it tends to sink. That is why you need the bubbling at the higher Voltages to stir things up.

I can not see any reason why left on its own electrolyte would separate out (except for long-term gravity effects which wouldn't apply in practical situations). Chemical solutions tend to remain stable if not acted upon, unlike chemical suspensions which will naturally settle out.

SandyP · June 2013

Re: Charging Battery Bank when gone

Some suggestions from Victron re charging with reference to 4 step charging when away :

Attachment not found.

Coach Dad · June 2013

Re: Charging Battery Bank when gone

Cariboocoot wrote: »

Stratification is a two-part process: first you have to separate the sulphuric acid from the water in order to create the potential fluid density imbalance. This is done by discharging which causes the sulphur to adhere to plates leaving mainly water as electrolyte. At this point there is no stratification because the electrolyte SG is consistent (albeit low) throughout. Then you have to recombine the sulphur and water to raise the SG by recharging. If the stirring action (which occurs mainly at full Voltage - when you get maximum bubbling) is not sufficient to mix the sulphur back in at all levels the higher density electrolyte will remain at the bottom while lower density floats on top. This is the stratification problem.

Small batteries usually do not suffer from it at all; it mainly afflicts larger batteries especially of the "tall case" variety where there is literally physically more height for the mixture to remain uneven in. This is why some forklift batteries have air pumps; to introduce additional stirring action and keep the electrolyte even throughout.

Nor is it a problem if the batteries are kept charged because there is no chemical separation and recombination process to lead to the different densities being available to stratify. I've never heard of it occurring just from allowing a battery to sit, although they will self-discharge when idle and so the overall density will alter. Inevitably one would recharge the battery at that point, wherein you can get the disparity.

Coot... Great explaination. What about AGM batteries since they don't have water and there is no stirring action. Any thoughts?

niel · June 2013

Re: Charging Battery Bank when gone

no stratification on agms.

Coach Dad · June 2013

Re: Charging Battery Bank when gone

niel wrote: »

no stratification on agms.

That's what I like to hear....

Charging Battery Bank when gone

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