Small heating system to protect LiFePO4 in freezing weather

[PLShutterbug]

First time poster here, thanks for the forum. I'll check back for responses.

Need advice on creating a heating solution for occasional below-freezing weather for my shed lighting system. Basically, what is the right size for my use case.

Background:
I have a small backyard shed, unheated (not necessary), where I've set up a single 3 meter strip of LEDs to a 10 watt solar panel, 10 amp solar charge controller, motion detector, and Lossigy 8AH 12v LiFePO4 battery (no link available) that is 60mm wide x 400mm long x 300mm tall. The solar panel is on the north side of the shed, and the battery case is high up (about 2.5 meters above the floor) against the north wall of the shed as well. When I built the shed I didn't want the solar panel highly visible and was concerned about thermal gain in summer harming the battery, hence my decision to mount at the north rather than the south.

I want light in the shed so I can see. I go in there for maybe 5 minutes at a time a few times per month. On really busy days I may go in and out of the shed 6 times, again for less than 5 minutes each. Take out the lawn mower; mow the lawn then put it back. Search for wood for a project; etc. I didn't want to hook up to house mains power so hooked up the solar-based kit above last spring. I don't want to forget to turn off the light and drain the battery, so set up the motion detector and it keeps the lights on for about 5 minutes before automatically shutting them off. To date this has worked exactly as planned.

I also have a 50 watt solar panel I have never hooked up because the 10 watt powered the LEDs just fine. I could swap out the 10 watt panel for this one pretty easily. It may factor in the discussion.

Problem:
I live near Seattle and a few days per winter the temperature drops below freezing. Last week it dropped to -7° C for 4 straight days. Then we had several days of heavy rain so little chance for the solar panel to recharge the battery. Now it's about 4° C again. Yesterday when I went to the shed the lights did not come on when I opened the door, and my charge controller showed E01 (battery low voltage state). I hit the controller's reset switch and the lights came on again. This morning when I went out to take pictures for this post the lights did not come on, and the controller's screen is blank.

I've verified that my charge controller does not have a low-temperature charge cut-off. So I may have destroyed my LiFePO4 battery by leaving it out there in the cold with a charger that attempted to charge it while below freezing.

Proposed solution:
I'll buy a new battery if I need to ... no problem. However, I don't want to go through this again so I'm thinking of two possible improvements to my system:

1. Install a thermostat-controlled switch that shuts off charging at below freezing, or replace my controller with one with this already there. Probably the simplest and least expensive solution, but it means I give up light in my shed during below-freezing weather. I don't prefer this.
2. Install a thermostat-controlled heating system and upgrade the box to add insulation, so the battery can charge during cold snaps but still be protected against below-freezing weather. I prefer this.

I've looked at polyimide heating pads and silicon heating mats that are the right physical size for this application. From reading here in the forum it seems that a single mat of either of these will keep my battery above freezing if I set it at the bottom of the box, and build a grid to leave a bit of an air gap to the battery above. Does that seem reasonable? Seems the biggest difference is a silicon case on one and adhesive on the other; I'd just attach something to the adhesive so it doesn't trap spiders etc.

I see several 12v 10a thermostat control switches available for very little money. At 120 watts these seem to have enough overhead to accommodate either of my solar panels.

Here is my proposed solution. Is it reasonable?

• Keep the existing charge controller
• Keep the 8AH 12v battery if possible; buy a new one if the old one is toast
• Build a better-insulated box to house the heater, battery, and controllers
• Electrically, hook the thermostat directly to the battery so the heater draws power from it
• Maybe swap the 10 watt panel for the 50 watt panel

Problems to solve:

1. How to cut off charging from the solar charger if the temp is below freezing and the battery is fully discharged?
2. Alternately, how big a battery do I need to ensure all-night heating with 16 hours of darkness in the winter, without having to worry about #1?
3. Will I need to switch to the 50 watt panel to ensure sufficient power during short winter days to charge the battery?

Thanks in advance for any advice. If there are places you can point me to do my power-budget calculations I'll be grateful.

 

[time2roll]

Use both 10 and 50 watt panels. Is the actual battery temperature going below freezing?
I put my small silicone heat pads to the edge of each cell. I would not leave an air gap between the heater and the cells. Get lower wattage if the heat might be too much. Two heaters in series will produce 1/4 of the heat.

 

[PLShutterbug]

Use both 10 and 50 watt panels.

thanks for the reply. how much underpowered is just the 50 watt panel alone?

Is the actual battery temperature going below freezing?

Good question. I can only assume yes since several ponds in the area froze, it was below freezing by several degrees for several 24-hour periods straight, and we had snow on the ground all that time. The shed itself is not insulated. It’s possible the temp in ther went above freezing during the day but I don’t know - I don’t have a temperature logger.

I put my small silicone heat pads to the edge of each cell. I would not leave an air gap between the heater and the cells.

Is there a danger to leaving an air gap? My thought was to distribute the heat better with a single mat below the battery. I don’t need to het the battery to room temperature, just keep it above freezing. If I don‘t adhere it to the battery then when it’s time to change the battery I don’t need a new pad as well.

Get lower wattage if the heat might be too much. Two heaters in series will produce 1/4 of the heat.

The idea is that thermostat would turn the heating pad on below, say, 2° C and then turn it off again when it hit that temp again. So the pad may go on only 30 seconds at a time, several times per day.

 

[time2roll]

The pad does not need to be adhered to the cells. Air-gap says poor heat transfer to me. If the pad is under the cells I would set it direct on the heat element for best heat transmission. Have seen some adhere the heat pad to aluminum foil or a thin aluminum plate to distribute heat better. Best to otherwise insulate the battery to avoid heat loss and reduce total power consumption.

As long as there is a controller there will not be too much solar. Best to bounce off the top end of charging. If the heater seems to draw too much I would be looking for 200-400 watts solar. But try the 50 and see what goes.

 

[Pi Curio]

To keep it short...

In case you consider making a new setup one day, might be worth reading up and researching more on these devices.

SCC data: https://www.victronenergy.com/uploa...-MPPT-75-10,-75-15,-100-15,-100-20_48V-EN.pdf

If you find 75/10 to be more price-appealing, it will do too.

 

[jjchvl]

Could get an order of these, attach em right to your batteries(or cells)and power them through a thermostatic switch to come on and go off at certain temps. They get pretty hot connected in parallel but each one puts out less and less heat when connected in series with each other. I used two connected in series to make heated slippers. Pads would burn my feet individually connected to a 13v power source but connected in series, they are just enough under "too hot" to work well for geated slippers. For heating lifepo's you could probly connect 3 in series and have each still be warm enough to safely warm up your batteries.

Check this out!
4 PCS Silicone Heater Mat, Icstation Rubber Heating Mat 12V 15W Electric Blanket Flexible Mat Constant Temperature 200℃ Max 50X100mm https://a.co/d/d5Hzazw

 

[v_green57]

I am using this:

https://www.amazon.com/gp/product/B0794V5J5H

and this

Amazon.com: LM YN DC 12V Digital Thermostat Module -58℉ to 257℉ Fahrenheit Temp Display Temperature Controller Board With 20A Relay Waterproof Sensor Probe Dual LED Display Red Blue: Home Improvement

Amazon.com: LM YN DC 12V Digital Thermostat Module -58℉ to 257℉ Fahrenheit Temp Display Temperature Controller Board With 20A Relay Waterproof Sensor Probe Dual LED Display Red Blue: Home Improvement

www.amazon.com

plus some careful programming of my charge controller and insulation of the battery on a 100aH Weize LFP installed in a pit buried below grade to minimize temp swings.

Battery is sitting directly on the heating pad with a concrete paving stone underneath for thermal mass. Temp sensor for the thermostat is attached to the side of the Wieze with aluminum flue tape.

Keeps the battery at 39 - 42F even when ambient temps are in the single digits.

 

[mactech]

I am using this Heating pad with four 100ah 12v LiFePO4 batteries. This pad is placed vertically between them in an insulated box. The batteries are compressed together to maintain good contact but not much pressure. This is my 3rd season using them.

When outside was -10F, the temp of the outer edges of one of the batteries plastic case was always 45-50F with medium setting.
A small thermal switch turns it off when it reaches 50F. Not the most efficient being AC vs. DC

 

[jjchvl]

I am using this:

https://www.amazon.com/gp/product/B0794V5J5H

and this

Amazon.com: LM YN DC 12V Digital Thermostat Module -58℉ to 257℉ Fahrenheit Temp Display Temperature Controller Board With 20A Relay Waterproof Sensor Probe Dual LED Display Red Blue: Home Improvement

Amazon.com: LM YN DC 12V Digital Thermostat Module -58℉ to 257℉ Fahrenheit Temp Display Temperature Controller Board With 20A Relay Waterproof Sensor Probe Dual LED Display Red Blue: Home Improvement

www.amazon.com

plus some careful programming of my charge controller and insulation of the battery on a 100aH Weize LFP installed in a pit buried below grade to minimize temp swings.

Battery is sitting directly on the heating pad with a concrete paving stone underneath for thermal mass. Temp sensor for the thermostat is attached to the side of the Wieze with aluminum flue tape.

Keeps the battery at 39 - 42F even when ambient temps are in the single digits.

Yup. Pretty much exactly what I was trying to describe ? The listing on those pads say they are 12v and 12w so dont draw much more than an amp. The 20amp rating on your temp switch is a bit over kill unless youre running multiple pads through it. On the other hand, you cant really go wrong going a little overboard when it comes to amp rating. Fortunately, my lithium battery compartment doesnt drop lower than 46.8f no matter how cold it gets so I dont need worry about heating solutions for my batteries. The reason I had looked at these pads originally was for a possible solution to melting snow from my solar panels. I had thought to use one of the longer, narrower versions of the orange heating pads to go across the bottom part of each panel. My theory had been that the rest of the snow would slide off my panels pretty easily once the bottom area was clear and the rest lubed by some water from what the heating pad warmed. I never did try it though.

 

[jjchvl]

I am using this Heating pad with four 100ah 12v LiFePO4 batteries. This pad is placed vertically between them in an insulated box. The batteries are compressed together to maintain good contact but not much pressure. This is my 3rd season using them.

When outside was -10F, the temp of the outer edges of one of the batteries plastic case was always 45-50F with medium setting.
A small thermal switch turns it off when it reaches 50F. Not the most efficient being AC vs. DC

I suppose whatever works, works but the orange pads work really well for the same purpose, one can simply be placed under each of your batteries, they're 12v, very inexpensive(like $15 to $20 for four!) and, depending upon the size of the pad, draw an amp or less. You can decrease their total current draw by connecting 2 in series as well. Hooked alone to 13v the pads can heat up to 200 degrees. Kinda hot. But connect 2 in series and their heat output decreases to something much more appropriate to warming batteries AND the current draw would be the same as running just one pad. So basically, you warm two batteries for the price of one! Sort of lol! Hopefully you get the picture. But yeah. Youd do well to change over. Use a thermostatic switch to activate/deactivate. Switches a really inexpensive too.

 

[mactech]

I suppose whatever works, works but the orange pads work really well for the same purpose, one can simply be placed under each of your batteries, they're 12v, very inexpensive(like $15 to $20 for four!) and, depending upon the size of the pad, draw an amp or less. You can decrease their total current draw by connecting 2 in series as well. Hooked alone to 13v the pads can heat up to 200 degrees. Kinda hot. But connect 2 in series and their heat output decreases to something much more appropriate to warming batteries AND the current draw would be the same as running just one pad. So basically, you warm two batteries for the price of one! Sort of lol! Hopefully you get the picture. But yeah. Youd do well to change over. Use a thermostatic switch to activate/deactivate. Switches a really inexpensive too.

Those orange pads look good. I wish I had found those before but I am going to try switching over to these next winter.

 

[Ekard]

You need to run a couple strips of LED lights a few minutes at a time, a couple times per week? I have a cell phone charging brick I keep in my truck, and plug my enclosed trailer's lights into it when I need lights inside the trailer.