Heating pads under battery

[ojoj]

I have placed 30*30cm heating pad in the insulated battery box, then 1mm aluminium plate on it then LFP battery on top of it.

Temperature is regulated by simple thermostat, like this. Temp sensor is taped on aluminium plate.

This pad heats up a lot and fast (300-400W). So i use setting to start heating at 7C and end at 10C. But the temperature on the aluminium plate rises even after that to around 15C, before all the heat reaches sensor.

How do you think about my setup?

 

[pollenface]

I like it.

If you're getting 15c, I guess you could lower it to 0?

 

[ojoj]

I dont understand. Heating is activated at 5C, then few seconds later, when the probe on the other side of aluminium plate detects 7C, it turns the heating off. Temp then slowly keeps rising to 15C.
I couldnt find a heating pad that would heat slower with lower temps.
I was wondering if this constant temperature cycling could be detrimental to battery health.

 

[Samsonite801]

I dont understand. Heating is activated at 5C, then few seconds later, when the probe on the other side of aluminium plate detects 7C, it turns the heating off. Temp then slowly keeps rising to 15C.
I couldnt find a heating pad that would heat slower with lower temps.
I was wondering if this constant temperature cycling could be detrimental to battery health.

You're probably just seeing the effect of the heat still trying to flow (transfer) through the plate and balance from hotspot source to the edge of the plate.

I might monitor and observe the temps seen on the BMS (assuming BMS temp probes are located near top of battery) and correlate heater pad/plate cycling temp readings, along with top-of-battery temperature at BMS, and fine tune the pad thermostat to maintain good average desired low temperature calibration across all readings, keeping lowest temp above minimums, and set to use minimum amount of electricity for the heaters (and still enough margin to hold minimums within requirements), while taking into account the BMS temp readings.

Then watch it for a few days and confirm proper desired effect.

Also consider that pumping too much heat, too quickly might create excessive heat in a hotspot under bottom of battery (before having sufficient time to flow out and up). So part of your testing should include addressing possible overheating of plate with too many watts, too quickly.

 

[S Davis]

How hot is the aluminum plate getting, too high a temperature is not good for the cells. I heat mine with a 25watt silicone heat pad and It gets too hot for direct contact so I heat the air space around the cells. I was going to set the cells directly on it until I realized how hot the pads get. I would think if it is cycling that much you will wear out your temperature control faster as well.

 

[ojoj]

The problem is placing temp sensor under the cells, as the sensor is rounded, so i placed it at the side of cells on the aluminium sheet. I know mid battery hotspots may arrise and temps will be higher, i just dont know how much higher. SmartShunts temp sensor on main + terminal shows constant temp 9-11C, around 50F.

 

[SeaGal]

I'd recommend 15W to 20W for the heating pad, rather than 300W - 400W. Long and slow is much better to get heat into the cells.

I like to sear my tuna, not my LiFePO4's ?‍?

 

[ojoj]

Got some example?

 

[v_green57]

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

 

[ojoj]

Yea, this 10*10cm has 3x Watts/cm (10W/cm) as mine 30*30cm (0,4W/cm), so 3x potential for hotspots. And it has to work 25x longer to output the same heat needed to keep battery at 7C.
I am looking for an exact opposite - a heating pad that has large area (somewhere from 20-30*30cm) and has lower heat output than 0.4W/cm.

 

[Samsonite801]

Yea, this 10*10cm has 3x Watts/cm (10W/cm) as mine 30*30cm (0,4W/cm), so 3x potential for hotspots. And it has to work 25x longer to output the same heat needed to keep battery at 7C.
I am looking for an exact opposite - a heating pad that has large area (somewhere from 20-30*30cm) and has lower heat output than 0.4W/cm.

One could also maybe look into implementing multiple smaller pads in series (wouldn't be as hot with same input voltage in series), and even with a thicker aluminum plate. Could also play with the input voltage of the string to fine tune the watts (btu /hr) they put out when the circuit will be engaged.

Just a quick thought that came to mind...

 

[Powerscol]

If you happen to have a Victron Cerbo GX in your set up with a temperature probe attached you can have one of the programable relays it contains turn the pad on and off and not need a separate thermostat

 

[Brightside]

Yea, this 10*10cm has 3x Watts/cm (10W/cm) as mine 30*30cm (0,4W/cm), so 3x potential for hotspots. And it has to work 25x longer to output the same heat needed to keep battery at 7C.
I am looking for an exact opposite - a heating pad that has large area (somewhere from 20-30*30cm) and has lower heat output than 0.4W/cm.

Maybe use a seedling mat. They have a much lower heat density. There are all kinds of sizes.

https://www.amazon.com/dp/B016MKY7C8/ref=sspa_dk_detail_1?psc=1&pd_rd_i=B016MKY7C8&pd_rd_w=XEVZt&content-id=amzn1.sym.dd2c6db7-6626-466d-bf04-9570e69a7df0&pf_rd_p=dd2c6db7-6626-466d-bf04-9570e69a7df0&pf_rd_r=72XDXS6Y0FTQ0ZC59E5W&pd_rd_wg=hgdoY&pd_rd_r=c76630a7-6d34-4bc3-afdd-ad5ee8e20896&s=lawn-garden&sp_csd=d2lkZ2V0TmFtZT1zcF9kZXRhaWxfdGhlbWF0aWM&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzVTA4U0FZVjg5TFoxJmVuY3J5cHRlZElkPUEwNjk2MzM1SFJKTzdXRkxSNzdTJmVuY3J5cHRlZEFkSWQ9QTA3NTE2OTUzVlFEN0Q5Q0g1SkFPJndpZGdldE5hbWU9c3BfZGV0YWlsX3RoZW1hdGljJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==

 

[Samsonite801]

If you happen to have a Victron Cerbo GX in your set up with a temperature probe attached you can have one of the programable relays it contains turn the pad on and off and not need a separate thermostat

I do that too right now using the Victron BMV-712 Battery Monitor (which has temp probe and internal relay), it works well to trigger a larger 40a relay and turn on/off my little 12v 200w space heater...

 

[Saturn_up]

Maybe use a seedling mat. They have a much lower heat density. There are all kinds of sizes.

https://www.amazon.com/dp/B016MKY7C8/ref=sspa_dk_detail_1?psc=1&pd_rd_i=B016MKY7C8&pd_rd_w=XEVZt&content-id=amzn1.sym.dd2c6db7-6626-466d-bf04-9570e69a7df0&pf_rd_p=dd2c6db7-6626-466d-bf04-9570e69a7df0&pf_rd_r=72XDXS6Y0FTQ0ZC59E5W&pd_rd_wg=hgdoY&pd_rd_r=c76630a7-6d34-4bc3-afdd-ad5ee8e20896&s=lawn-garden&sp_csd=d2lkZ2V0TmFtZT1zcF9kZXRhaWxfdGhlbWF0aWM&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzVTA4U0FZVjg5TFoxJmVuY3J5cHRlZElkPUEwNjk2MzM1SFJKTzdXRkxSNzdTJmVuY3J5cHRlZEFkSWQ9QTA3NTE2OTUzVlFEN0Q5Q0g1SkFPJndpZGdldE5hbWU9c3BfZGV0YWlsX3RoZW1hdGljJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==

I second the seedling mat. I have (3) 7.5 watt mats for window sill planters. They're about 20" x 4" strips.

 

[scpanish]

I too use a seedling germination heat mat under a single BattleBorn 100AH battery for my observatory. 18W. Temp sensor is taped to the side of the battery near the top. Battery is swathed in fiberglass insulation and in a box with a 600W Samlex inverter and an Outback FlexMax 60 controller. No problem keeping the battery up to 65deg even in the -15F weather of the past 2 days. Very gentle heat, and it does take some time to fully propagate through the battery.

 

[ojoj]

Will these germinating pads bear weight of cells around 100 pounds? 43kgs? How sturdy are they built? The silicone pad i use right now is very dense and compact and it handles weight no problem so far, but it is a little too hot/too much Watts.

 

[Saturn_up]

Will these germinating pads bear weight of cells around 100 pounds? 43kgs? How sturdy are they built? The silicone pad i use right now is very dense and compact and it handles weight no problem so far, but it is a little too hot/too much Watts.

I'm sure they would be fine, as long as they're not folded and getting creased it should be a non issue. If you were to put 1000lbs on one that may be another story, but even then if its layed flat and has a static load on it, it should be fine.

 

[Horsefly]

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

I'm pretty sure that is the one I used. I put two in series for my 8S 24V battery.

My design put emphasis on low and slow heating. I had experimented with a couple of other heating pads, and decided they heated up to quickly and went to a higher maximum temperature than I wanted.

I show some plots of the temperatures at the top of my cells and at the bottom of the cells here: https://diysolarforum.com/threads/horseflys-cabin-solar-lifepo4-upgrade.27472/post-419828 The rest of that thread talks about the insulated battery box and controls that I built.

 

[ojoj]

Alternatively, could i use 24V pad (30*30cm, 350W), but supply it with 12V power? This should halve the heat production right?

 

[Horsefly]

Alternatively, could i use 24V pad (30*30cm, 350W), but supply it with 12V power? This should halve the heat production right?

That's still 175W at 12V, which seems like lots to me. I was doing 24W for twice as many cells.

There really isn't any reason to try and heat things up quickly. You are just trying to maintain a temperature somewhat above freezing. If you use a lower number of watts, the heater may be on more of each 24h, also consumes less power and keeps the temp where you want.

 

[Leeds]

Interesting. I'm just a few days away from testing my latest battery warmer and this topic is very much on my mind.

This is the first time I've warmed a 5kwh battery so I've scaled up the wattage to 50W to accommodate the additional mass.

I don't like the concentrated heat of the heat mat approach so I always wind my batteries with resistor wire.
The 50w is spread over roughly 30 feet of wire so I should be heating at a rate of a watt per 7.2 inches of wire, give or take an inch. Once I start the testing phase I'll watch it with the Flir to verify the wrapping job.

In my opinion this "crock pot" approach is the way to go, although I can see how a big chunk of metal can help mitigate the problems that arise when you go with pads.

 

[Horsefly]

Interesting. I'm just a few days away from testing my latest battery warmer and this topic is very much on my mind.

This is the first time I've warmed a 5kwh battery so I've scaled up the wattage to 50W to accommodate the additional mass.

I don't like the concentrated heat of the heat mat approach so I always wind my batteries with resistor wire.
The 50w is spread over roughly 30 feet of wire so I should be heating at a rate of a watt per 7.2 inches of wire, give or take an inch. Once I start the testing phase I'll watch it with the Flir to verify the wrapping job.

In my opinion this "crock pot" approach is the way to go, although I can see how a big chunk of metal can help mitigate the problems that arise when you go with pads.

That may work, but... Seems to me the width of that wire is what, maybe a few mm? So you are heating a few mm wide spot, and counting on the cells themselves to spread that heat out. How many wraps will this wire have around the entire pack? You didn't say how big your cells are or how many cells there are.

To each his own, but I still think an aluminum plate - essentially a heat sink - distributing the heat across one surface of all the cells is better.

 

[ojoj]

I will try to place heating pad on a L shaped aluminium sheet (kapton insulated towards cells), this way it is impossible to get any hotspots under the cells. Also with the temp sensor in between heating pad and cells, you can control precisely how hot the aluminium can get.

 

[Saturn_up]

I think you're over thinking it. You could really just throw it on top of the batteries or tape it to the side of the box. If the box is well insulated with space all the way around the battery it would work. Just has to stay above 32°. Put some feet on the box to thermally break it from the ground. You'll only get hot spots it the heating pad is directly touching the battery.

2nd option
Heating pad on bottom, batteries on some small risers ( empty soup cans, some rocks, legos, bottle caps, ect..) To keep it from direct contact. Thermo cube either inside or outside the box. Inside will stay between thermo cube temp range, generally 35° on 45° off. If its not touching the batteries you could use a larger range of wattage heating pad since its heating the air in the box and not the batteries.