charging AGM batteries in cold climates

northern_light · October 2012

Hello Wind-sun forum,
I have a question regarding care of AGM batteries in cold climates. My situation is that I am leaving my cabin for a few days to a week unheated in below 0 temps (down to -20C tonight). What I’ve read here is that it’s okay for sealed batteries to freeze if they are fully charged. As the days are rapidly growing shorter, I will soon be charging with my generator. My question: Can I start charging the batteries straight away from their frozen state, before they reach room temperature?

Thanks in advance for any advise on this.

inetdog · October 2012

Re: charging AGM batteries in cold climates

northern_light wrote: »

Hello Wind-sun forum,
I have a question regarding care of AGM batteries in cold climates. My situation is that I am leaving my cabin for a few days to a week unheated in below 0 temps (down to -20C tonight). What I’ve read here is that it’s okay for sealed batteries to freeze if they are fully charged. As the days are rapidly growing shorter, I will soon be charging with my generator. My question: Can I start charging the batteries straight away from their frozen state, before they reach room temperature?

Thanks in advance for any advise on this.

I think that you are misinterpreting what you read here!

It is NOT alright for batteries, whether FLA, AGM or GEL, to freeze. This will cause mechanical damage which will probably not be repairable.

However, the freezing temperature of a fully charged battery is pretty low (-75F) and if it is reasonably insulated it will only need to withstand the average daily temperature and not the nightly low.
In addition, if you connect a small panel and CC to maintain the batteries are full charge, that will also warm it somewhat.

Here is a table of freezing point versus Specific Gravity of the electrolyte, with the corresponding State Of Charge (SOC).
Keep in mind that the SG of a "fully charged" but badly sulfated battery will be lower than that for 100% in the table.

Freezing,
Percent    Specific     Degrees
 Charge    Gravity    Fahrenheit
    100       1.265       -75
     76       1.225       -35
     74       1.200       -17
     50       1.150         5
      0       1.100        18
     <0       1.050        27

niel · October 2012

Re: charging AGM batteries in cold climates

you could, but temperature compensation of the voltage needs to be there. a cold battery will need a higher voltage and just how much should be in the battery specs for your particular battery. once charging starts it will start to warm the battery up and the temp compensation will change as it goes up.

you could also drain a bit of the power too as the battery will not be dead, but the capacity will be lowered by the cold so unless you know what you've got in it for a given temp, i suggest charging it up first.

you are also correct that a battery at full charge will not freeze in the temps you describe. as the battery sits it will lose some of its capacity and would raise the point it could freeze at, but at your described temps you should be fine. even if you left it like that for a month it should be ok. a season too, although i'd hesitate and wonder about them.

BB. · October 2012

Re: charging AGM batteries in cold climates

AGM's are more resistant to damage if frozen (they are not filled with 100% electrolyte, but a glass mat "wetted" with electrolyte--So even if they freeze, the ice may not expand enough to cause damage.

One warning--Never recharge a frozen battery--It must be defrosted first before charging.

However--As stated above, in normal operation (cycling) and with modest precautions (batteries in basement or somewhat insulated box), they should never freeze if properly maintained/charged.

-Bill

niel · October 2012

Re: charging AGM batteries in cold climates

BB. wrote: »

One warning--Never recharge a frozen battery--It must be defrosted first before charging.

-Bill

ok i'll bite, why not? i realize the frozen water will not be converted during the charging process if frozen, but the action of charging will heat the battery some melting the ice and allow charging in time. ideally you are right in that the battery should be thawed first, but i don't see the harm. when there is a bit of water frozen it would just be seen as a battery with a full charge (no water available for conversion) and it will seem like a charged battery to the charging source and just keep the current lower. maybe i didn't think this all of the way through so tell me what you are thinking along the lines of a frozen agm. realize too we are not talking of an agm discharged far as we are talking maybe 3% or 4% per month maximally of self discharge from a fully charged battery to start with.

BB. · October 2012

Re: charging AGM batteries in cold climates

As I understand--The ice prevents circulation of the electrolyte so you end up with "fully charge" electrolyte next to the plates very quickly--And the battery will then begin to gas--generating hydrogen/oxygen which can fracture the ice or the case and spray acid about...

And, obviously, if there is a spark near by (such as ice breaking up electrical connectivity)--worse.

-Bill

niel · October 2012

Re: charging AGM batteries in cold climates

ok i'm not sure why it would gas as being seen as a fully charged battery due to unavailable water, being it is frozen, it will just reduce in current like trying to bulk charge any full battery would. i'm thinking very little gas would be present to pop the valves.

to northern light i might advise a bit of attention for the batteries in such a case. maybe a small discharge to slightly warm the batteries internally and then hit them with a bulk charge, all assuming the batteries are still out in the cold. we did not get into the aspect of insulating the batteries as this too could help warm them up faster by keeping the heat losses down. don't insulate air tight as i'd make sure there is a bit of room, especially at the top. rigid insulations are good for this, but be careful if there's any foil on it as you don't want that near the terminals or wires. do assume the batteries to be around 50% capacity in that kind of temperature and add to that any self discharge from sitting and adjust any applied loads accordingly. for a week the self discharge will not be enough to worry over.

do keep in mind that a battery has a great amount of mass and it will take a significant amount of heat to bring them to a normal 77 degree f point. with that in mind know your capacity is going to be greatly reduced regardless as batteries out in the winter cold will not reach operating temps to give full capacity even if you use them daily. operating at levels of discharge that are normal for the batteries at 77 degrees f to 50% dod could place the batteries in a severe state of discharge possibly damaging the batteries if below 20% soc or minimally taking some cycle life away from them.

westbranch · October 2012

Re: charging AGM batteries in cold climates

While I agree with the concern about charging a frozen AGM, my experience differs... somewhat

1055Ah AGM sat in an unheated but modestly insulated ( 2" polystyrene), 4x8x8 foot, power room at the cabin all winter for 4 years.
It was always fully charged when unattended.
Temps regularly went to - 30 F* every year, and up to freezing at times.
During winter visits I found the room temp to be near, or above freezing to my amazement! A cheap I/Outdoor thermometer showed the AGM to be above freezing .
Charging time would only be 3 - 4 hours max, with ~1 hr of max input from 4 120W mono panels in Parallel.

So the short version is 'don't sweat the petty stuff'... (from George Carlin) As Long As the AGM's are fully charged and you have a good minimum Array size.

YMMV

northern_light · October 2012

Re: charging AGM batteries in cold climates

Thanks for all the help - I don't have internet out at the cabin, so am delayed in responding. In reply to Inetdog, my batteries are connected to solar panels and a charge controller, but there just isn't enough solar at this latitude at this time of year. Well, there's still some now, but dropping rapidly. So, for the next few weeks, there will probably be enough to keep them almost fully charged while I'm not out there.

In reply to Niel, if I can set my charge controller temporarily to the higher voltage, would this be possible? Then what happens when the temp. of the batteries goes up during the charging? Does the CC automatically drop the charging voltage? My CC is a Blue Sky 25A MPPT Controller.
This is from the Sunextender guide under 'Storage': "...if the temp is below 20C. (68F), compensate the charge voltage by adding 0.04V/cell for every 10C difference." I'm not sure if this is applicable in my situation.

So, to put this altogether, it seems like most think it's okay to let the AGMs freeze...but to not charge them from the frozen state. Question - how can you prevent this, if they're hooked up to the panels, and the sun starts climbing the sky again? (February where I live) And to discharge them a little before charging.

It's good to hear that you've had an experience with the deep freeze scenario, westbranch - thanks for a northern perspective.

BB. · October 2012

Re: charging AGM batteries in cold climates

AGM's will keep for ~3 months without charging (and without a load). In sub freezing weather, their self discharge should drop a lot more--and should keep 3-6 months if you have no charging (again, disconnect/turn off any loads).

As long as the batteries are "put to bed fully charged"--They should never freeze unless the area where they are kept falls below:

Spec. Gravity / SOC / Freeze Temp

1.280 100% -92.0F
1.265 92% -71.3F
1.250 85% -62.0F
1.115 40% +5.0F
1.100 20% +19.0F

BCI Battery Service Manual 1995

If they did freeze (were not stored charged)--You would have to heat them up to "defrost" them (warm cabin, electric battery heater, etc.).

-Bill

niel · October 2012

Re: charging AGM batteries in cold climates

that controller most likely should have a battery temp sensor available for it and i suggest you buy it. if you already have it then connect it up. yes, that info from concorde does apply to you if you have a concorde battery, but if they warm up are you going to babysit them to change the voltage to them? this is too difficult to adjust this constantly by manual means, but you should raise it .1v or .2v (assuming you have a 12v battery) in the meantime just to offset some of the cold temps. remember to change this back to normal charging volts if you put in the bts. in the meantime the .1v or .2v is probably not enough of a raise in voltage, but a warmer day with use could put it to being too high of a voltage for the battery.

charging AGM batteries in cold climates

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