Silicone pad battery heaters: need a simple battery thermometer to avoid overheating

thorsness · April 2015

We have a remote cabin, off the grid, north of Anchorage, Alaska. We have two solar panels, two AGM batteries, an inverter, Morningstar Controller, auto transfer switch, Vector charger and use a Honda 2000i generator for power tools, wintertime charging, etc. In the winter when we roll up on our snowmobiles, the cabin, and the batteries, are at zero or single digits Fahrenheit. While the woodstove warms the cabin in several hours, the batteries take five days or more. Accordingly, they have very little capacity (if that's the word) and are spent quickly (I liken it to a water bottle mostly full of ice; not much water goes in or out until the ice melts). Last winter I installed two orange silicone battery heater pads under the batteries (the pads sit on top of a foot square piece of flooring tile). I run the heater pads with the Honda and after 5-6 hours, the batteries are as warm as summertime and run my nighttime lights, and other items just fine (I used to have to run the Honda all night so I'm very happy).
My question: the other day I ran the heaters too long and got a warm silicone smell. The bottom quarter of the batteries where quite warm (not too hot to touch) but I believe I ran the heater pads too long. I know that if you overheat batteries you can ruin them so I want to get a remoted thermometer that I can clamp onto a battery post to make sure the temps stay below 90* (?). I have not been able to find one and the model of controller I have does not have that capability I am told. Any reccommendations?
Thanks. I have been away from this forum for quite a while and I am glad to be back.

John

BB. · April 2015

Welcome back John!

Your off grid system is working well--Always nice to hear. If you want to (in this thread or another) tell us what you did, how you use it, lessons learned, etc... I am sure lots of us would enjoy reading about it.

Your question... The "standard temperature" for a lead acid battery bank is ~25C/77F.

For every 10C (18F) over 25C, the battery ages 2x faster. But, don't have a heart attack. If the battery bank was at 95F for 2 days--It aged "4 days"--Not the end of the world.
Th
And, of course batteries have different aging/failure mechanisms. There is "aging"--Say the battery will last ~6 years--Running at 95F it will last ~3 years.

And there is cycle aging... Discharge to 50% it may last 1,000 cycles. Discharge it to 25% (state of charge), it may last 2,000 cycles. But if you have a 2x larger battery bank (so that it only discharges to 25% state of charge), it will cost ~2x more to replace.

In theory, for a deep cycle lead acid battery, cycling should be to ~80% state of charge to 20% state of charge--But really recommend that you don't go often below 50% state of charge--Deep cycle batteries are supposed to be "ok" cycled deeper--But they do not seem to last like they should--Plus it will take a lot of sun+genset to get the battery bank back fully charged.

On person here found some small perfume vials that he was able to drill and glue into a spare battery cap. Put a standard remote read thermometer in the glass vial.

http://forum.solar-electric.com/forum/solar-electric-power-wind-power-balance-of-system/advanced-solar-electric-technical-forum/23724-different-current-in-2-strings?p=300510#post300510

Another thing you can try is simply take a piece of Styrofoam block, hollow out a pocket for a the thermistor/thermocouple and tape/glue to the side of the battery bank. Will be more than accurate and responsive enough for your needs--You really do not need to have direct metal/electrolyte contact as the foam block will stop air cooling/heating of the temp sensor.

Generally, never allow the AGM battery to exceed 130F/55C... Here is Concord's Lifeline battery manual:

http://www.lifelinebatteries.com/manual.pdf

-Bil

mryimmers · April 2015

You could cycle the heating pads on and off with a timer

froggersix · April 2015

why not use a battery charger from the gen instead? it would use less power warm the batteries from the inside and make sure they are charged too. agm can take a lot of amps charging. if the solar goes you can still charge too. solar left on should keep them warm. maybe insulate around them.

waynefromnscanada · April 2015

froggersix wrote: »

why not use a battery charger from the gen instead? it would use less power warm the batteries from the inside and make sure they are charged too. agm can take a lot of amps charging. if the solar goes you can still charge too. solar left on should keep them warm. maybe insulate around them.

Problem is, AGMs do not react well to overcharging, they tend to vent, dry out, and die.

froggersix · April 2015

waynefromnscanada wrote: »

Problem is, AGMs do not react well to overcharging, they tend to vent, dry out, and die.

why would he overcharge them? i didn't say leave it on till they fry. you always have to use the right charging. its just better than trying to warm them up from outside.

BB. · April 2015

I believe Wayne's point is that AGM batteries are very efficient during charging (and discharging), so they generate little internal heat during operation. Flooded Cell batteries generate most of their heat when equalizing (gassing)--Which you cannot do with AGMs (without damaging them).

So, the heating pads are a neat idea to get the battery bank warmed quickly--Then daily cycling and being in a warm cabin will generally keep the battery bank at a nice/efficient working temperature.

To automate the system and reduce the chance for human error... There are relatively cheap/simple mechanical temperature controllers--Just get one with a remote probe (bulb type on copper tubing), but next to batteries with foam insulation block to keep the probe at battery temperature, and set it for ~70F--That will turn off the heating pads when the battery is to temperature.

A second method would be a simple mechanical timer that fits in a wall switch box. Twist it to 5 hours (or what ever is best), and it will turn off--Check battery temperature and turn the switch on again, if needed.

-Bill

thorsness · April 2015

Gentlemen,

Thanks for all the excellent suggestions.
The timer is a great idea (simple ideas usually are!). I do run my Vector charger but when it's real cold, they just won't accept much (maybe 2.0) and after about three minutes I get the 'FULL' notice and the charger shuts off. Conversely, as I warm the batteries with the pads, the voltage (amps?) they accept increases so when warm, they'll take as much as 9.0 and more. Before I installed the heater pads, in the winter I had to run the Honda all night (which it does nicely) but now with the pads, depending on how late we roll in, I can get the batteries warmed up in less than a day.

Yesterday, I came across one of those hand-held lazer thermometers. Surprisingly inexpensive on Amazon ($14). Any thoughts?

Thanks again. It's good to be back.

John

waynefromnscanada · April 2015

thorsness wrote: »

Yesterday, I came across one of those hand-held lazer thermometers. Surprisingly inexpensive on Amazon ($14). Any thoughts?

Have one and find it surprisingly accurate, within a couple or so degrees, and at times quite useful for a variety of purposes. Think I paid around $30 a few years back.
The lazer points roughly to the spot being tested, but I'm sure you already know that.

BB. · April 2015

I have a version of the IR Thermometers--They work very nicely.

However, if you get a standard LCD type thermometer and remote cable (imported units can be cheap--cost more for shipping than the unit itself)--You can just leave it connected to the battery bank 24x7 (under a foam block). You will probably get more consistent readings than with an IR thermometer.

-Bill

Vic · April 2015

thorsness wrote: »

... I do run my Vector charger but when it's real cold, they >(the batteries)< just won't accept much (maybe 2.0) and after about three minutes I get the 'FULL' notice and the charger shuts off. Conversely, as I warm the batteries with the pads, the voltage (amps?) they accept increases so when warm, they'll take as much as 9.0 and more ... John

The Vector charger probably does not have a temperature sensor, and if it does, likely that does not have the ability to attach to the battery.

So, perhaps what you are seeing as battery Acceptance issues is just that COLD batteries look much more fully charged than they really are -- the battery voltage rises with decreased in temperature -- and warm batteries look less charged than they really are.

Opinion, FWIW, Vic

waynefromnscanada · April 2015

Vic wrote: »

perhaps what you are seeing as battery Acceptance issues is just that COLD batteries look much more fully charged than they really are -- the battery voltage rises with decreased in temperature -- and warm batteries look less charged than they really are.

Opinion, FWIW, Vic

Have been thinking the same thing. Thanks for bringing that forward. With proper battery temp sensing, the controller should raise the charge voltage high enough to "force" charging if charging is necessary.

thorsness · May 2015

Thanks for the comments and advice gentlemen. I will look for an LCD thermometer as well. I have ordered the lazer-IR thermometer (which I can use on my woodstove chimney to monitor stack temp as well) and report next winter when I have a chance to compare them. I also ordered a timer that I'll plug the heating pads into. All great ideas that will make my battery bank (2 AGM''s) more useful in the winter and give my little Honda 2000i a break!
Thanks again.

John

thorsness · February 2017

Follow up: the heaters worked good for about three years. Too good since they overheated due the airspace allowed by the rectangular-cross hatched plastic structure on the bottom of the batteries. This burned a few small holes in the pads and decayed the plastic on the bottom of the batteries. I noticed this when one AGM battery pooped out and we replaced them. Don't know if the overheating caused battery failure (they were about 8 years old) but I reckon it didn't help.

I replaced my 2 12 volt AGM's with 2 6V 220 AH Silicon Salt batteries (JH Energy but I cannot find them on the web). So far I am quite pleased. These new batteries claim to be almost impervious to low temps which is key here in the north. There is a discussion about silicone salt batteries and I have put my experiences on there. My solar guy says that Nu Global NRG makes similar batteries and they have graphs and spread sheets that sing the praises of this new technology. I will put my experiences going forward on that discussion.

Thanks. I appreciate the advice and comments. JB

Silicone pad battery heaters: need a simple battery thermometer to avoid overheating

Comments

Categories