-5C outside so Thinkin about batteries in HOT weather

[Powerplay]

I've read not to try to charge batteries when they are above 33 degrees C (92 F).  What about forcing AGMs to Float when they are hot (That would be about 26.9 Volts in my system at that temp) so then I can use the excess energy to power my loads during the day including the Air Conditioner?  Will discharging a hot battery cause any reduced life span?

[Vic]

I've read not to try to charge batteries when they are above 33 degrees C (92 F).  What about forcing AGMs to Float when they are hot (That would be about 26.9 Volts in my system at that temp) so then I can use the excess energy to power my loads during the day including the Air Conditioner?  Will discharging a hot battery cause any reduced life span?

Heat is the enemy of batteries,  in general.

IMO,  it is not specifically high-ish heat when charging,  but,  really it is the amount of time that batteries spend in warm/hot environments,  that impacts the lifespan of batteries.

Each battery manufacturer dictates the Limits on charging for their specific batteries.

Generally,  cool batteries are happier,  and cold batteries might be better yet,  if they are well-charged at the time,  and the Capacity reduction from the cold does not impact system performance.

Batteries will have longer life at cool/cold temperatures.   But allowing batteries to become hot may well have a larger impact on lifespan than the batteries spending the  same amount of time at a temperature that that is equally below the standard Reference temperature of 25 - 27 degrees C,  as was the time spent at the higher temperature.   And so on ...

FWIW,   Vic

[Powerplay]

True if all you're doing is storing them.  Topping up charging partly negates high temp storage losses.  And at higher temperatures the capacity is raised so the depth of discharge is lower for the same load.  This slightly increases battery life and might compensate a bit.  The more I think about it, the T-comp curve might be wrong for AGMs at warmer temps.  In order to avoid killing the batteries by high temps, electro-chemical runaway which can occur by charging to 14 volts per 12 volt battery on a hot AGM, one must rapidly drop the charging voltage to below 14 V as the temp approaches 40C.  Right now I'm T-comping a 24V battery bank to 29.2 Volts by -.005mVC.  At 40C this results in a dangerous 28.3 Absorb voltage and could cause runaway battery instant death on an AGM.  If I T-comp the same bank to 28.8V at -.008mVC then at 40C we have 27.5 Volts and that is much closer to float voltages and much less hard on the battery.  Reverse to low temps under -.008 and we get -31.7V at -10C and that is probably fine at 25C we're back at 28.8 which would also be fine IMO.

Sounds pretty rambling but I'm just thinking this over.  What o you think?

[Vic]

True if all you're doing is storing them.  Topping up charging partly negates high temp storage losses.  And at higher temperatures the capacity is raised so the depth of discharge is lower for the same load.  This slightly increases battery life and might compensate a bit.  The more I think about it, the T-comp curve might be wrong for AGMs at warmer temps.  In order to avoid killing the batteries by high temps, electro-chemical runaway which can occur by charging to 14 volts per 12 volt battery on a hot AGM, one must rapidly drop the charging voltage to below 14 V as the temp approaches 40C.  Right now I'm T-comping a 24V battery bank to 29.2 Volts by -.005mVC.  At 40C this results in a dangerous 28.3 Absorb voltage and could cause runaway battery instant death on an AGM.  If I T-comp the same bank to 28.8V at -.008mVC then at 40C we have 27.5 Volts and that is much closer to float voltages and much less hard on the battery.  Reverse to low temps under -.008 and we get -31.7V at -10C and that is probably fine at 25C we're back at 28.8 which would also be fine IMO.

Sounds pretty rambling but I'm just thinking this over.  What o you think?

Hi Powerplay,

A few things;

1.  IMO,  just exposing batteries to elevated temperatures  --  those above 25 - 27 degrees C,  elevates Chemical Activity.   This increase in activity,  decreases the battery life,  compared to that which would have  existed if the batteries were stored at or below the 25 - 27 degree Reference temperature.

2.  Believe that using the manufacturer's Temperature Compensation Coefficient when charging,  should not appreciably reduce a battery's lifespan.

3.  Yes,  Thermal Runaway is a factor in Lead-Acid batteries,  but compensation of charge voltage,  and Limits on this compensation is designed to limit this effect.

4.  Temperature Compensation curves are linear,  with a hard Limit at the (ie no further compensation beyond a specified Limit),  for Sealed batteries,  are customary with most charging systems -- at least for the best available CCs.

5.  Also,  IMO,  you might want to check the recommended Temp Compensation value by your battery manufacturer,  as,  many AGM batteries have a recommended compensation value of --3mV per cell per degree C.

6.  Cannot comment on using a --8 mV/cell/C,  as all manufacturer's specs that I have seen ,   recommend a Linear compensation,  with absolute Limits for higher AND lower temperatures above and below  specified measured battery temperatures for most Sealed Lead-Acid batteries.

7.  If you wish to play with the slope of the Compensation curve,   suggest that you might want to play with the absolute Limits that are available with the most advanced CCs,  Like the Classic (and Kid).

You might want to contact the manufacturer of your batteries,  to seek guidance on how to address Temp Comp values,   and suggested Limits for Compensation.

Many batteries are subject to Thermal Runaway,  to varying degrees.   Simple,  Linear Temperature Compensation seems to deal with this risk,  as well as trying to level the charge rate for the batteries,   verses temperature.   Again,   seeking advice from the manufacturer of our specific battery  should be the best guidance that you can get ...   IMO ...

FWIW,   my stream of "consciousness".     Vic

[Powerplay]

Thanks.  I talked to them (manufacturer) before I built the bank back in September and I suggested -.005 per cell with 29.2V at 25C target.  They replied that is perfect.  Their literature shows a range from 28.8 - 29.6 so I had picked the mid point.  Right now on the classic my hard stop is 31V but that is to keep my cheapo Inverter from shutting off.  I'm just thinking about it because I'm reading about early failures on AGM banks in outdoor enclosures (they can get really hot in the summer) and the possibility the T-comp curve (A straight line is a curve to us    ) could be  a contributor or, if adjusted, could help out a bit.  It is also a flag that AGM charging isn't as well understood as it could be if the batteries fail before the manufacturers think they should.  So I think I'm allowed to kill one battery bank before its time in the interest of science.  Ha ha.  I might try the normal T-comp curve at 25C to -10C and a steeper incline of -.008 with a 28.8V @ 25C target for temps 25-40 and forced float above 40.  I've plenty of time to think it over & it might just be simpler to force float at 35C or so and leave the rest the same.  Also I'm  thinking the T-comp incline might be too flat on the cold side because of how long the absorb phase gets in really cold weather.  But I don't have the guts or the inverter to challenge that one yet.  Thank you for your excellent feedback. 

[CDN-VT]

-5 c now & I take it winter while it can get to 40C as I've read.


Hot transfers to cold , I used the ground & dropped in 4"PVC tubes with rebar & a metal plate on top of bent rebar to transfer heat around Welded some joints for better transfer.
concrete /cement filled the tubes & then made a floating footing for the floor that holds the batterys . the floor stays cool in the summer & is always above  freezing in our -5c winters .
Worked for me Since I had to have my main battery's in the side of the summer sun ..

[Powerplay]

The air temp today is -15C and the batteries are hotter at -5C.  It is an insulated bank & in the shade and fairly consistently stays above the air temp in cold weather.  Ive never seen the batteries go below -10C no matter how cold it got.  In fact -10C is the minimum charging temp the manufacturer gives so there might be something about that temp related to the battery chemistry. 

You're temperature stabilization method sounds very smart and I'll have to see if there is something similar I can do geographically to achieve a more stable temp at the hot side.  Cold is a wash since it extends battery life & I don't need the air then.  I know it is coming in July around here and that is when I'll be also wanting to use the Air Conditioner the most.

One guy I read put his batteries in an old freezer filled half way with water to keep them cool in the summer.  Definitely don't want the water to freeze but it is an interesting (consumptive) way to keep 'em cool.

[Powerplay]

I wonder, has anyone ever measured the temperature of the bare ground wire on a hot day?  The ground rods go down 8 feet and copper is a heat conductor.  I wonder if a cooling coil could be made from #6 bare wire bonded to the ground rods?  It depends if the ground wire is significantly cooler than the air temp I suppose.

[russ_drinkwater]

The batteries in water are an interesting concept and would work quite well in the hotter months.
I have measured water temps in our water tanks here in summer for a comparison. The external/outside temp was 37 deg C and the water temp came in at 26 deg C. So that 5-10 degrees cooler environment may be a battery saver in very hot environments.

[Powerplay]

If you can make 10 degrees difference that's about all you need.  At 25C they're happy campers, 35C not so much.  So I'd need some sort of big tub that has a lid and account for the displacement of the battery volume .  I doubt if I'd go this route but it is an interesting idea.  Evaporation of the water on a hot day would cool things down in the bank.  I think I'll do some temperature measurements and think about dry methods though.  Something about getting a lot of water around the batteries bothers me.   

[Vic]

For several of the power rooms here,  we use priority A/C to cool the batteries and the electronics,  usually as an Opportunity Load.

Also use the Classic CCs to skip 2 - 3 days of full charge.  A full recharge of batteries,  especially Flooded batteries does dump quite a lot of heat into the battery bank.

Skipping full recharges,  and only fully recharging these batteries  does reduce the amount of heat that the batteries experience,  and,  IMO,  is net,  better for these batteries.

Just my opinions,   FWIW,    Vic

[russ_drinkwater]

I have an old raylitte transport box for a 24 volt bank which I use to house my trojans in. It is painted white on the outside and I have
short sheets of corrugated roofing iron screwed to the top which gives a bit of air-space/vent on the top of the box.
It is 29.8C in my lounge room today and the battery temp guage says it is 27.50C for that cell in the battery box!
And considering it is summer that is pretty good and also I forgot to mention the battery box is also under partial cover and gets no real
direct sunlight.