diy solar

diy solar

LiFePO4 heating pad for cold temperatures

I assume you only have them on one side of the cells, and not the other?
The cells are stacked 2x8. Heat pads on the outer edge of each cell. No way mine will see 140F.

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I am more concerned about an unusual cold snap with inadequate heat.
 
I am using the silicone @140 degrees, I still think it is too hot for direct contact.
I'm not sure how to interpret what you are saying. Are the pads actually heating to 140°? Is that 140°F, or 140°C? Either way, if you are actually getting them that hot, then yeah, you shouldn't put them in contact with the cells. I would actually never recommend that something that hot be used to keep LiFePO4 cells warm.

My point above was that it is a better strategy to heat low and slow: Lower temperature, and for a longer time to allow the thermal mass of the cells to absorb the heat the whole way through. What @time2roll did was to put 4(?) pads in series, effectively reducing the pads heating to 25% of their rated watts.
 
Two in series. Each gets half the voltage and then half the amps for 1/4 the wattage.

I actually drew a diagram to have a second thermostat switch to all parallel for more heat if needed. Have not implemented this.
 
I'm not sure how to interpret what you are saying. Are the pads actually heating to 140°? Is that 140°F, or 140°C? Either way, if you are actually getting them that hot, then yeah, you shouldn't put them in contact with the cells. I would actually never recommend that something that hot be used to keep LiFePO4 cells warm.

My point above was that it is a better strategy to heat low and slow: Lower temperature, and for a longer time to allow the thermal mass of the cells to absorb the heat the whole way through. What @time2roll did was to put 4(?) pads in series, effectively reducing the pads heating to 25% of their rated watts.
Just that I am using the pads but to heat the air around the batteries, i never measured the temperature. I don’t care how hot they get. My system is different in I will never let the batteries get cold soaked, the air is always around 50 degrees and that is all the temperature the batteries will see. And I agree low and slow. I was just using the 60c figure in your post as I never measured temperature on mine.
 
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Just that I am using the pads but to heat the air around the batteries, i never measured the temperature. I don’t care how hot they get. My system is different in I will never let the batteries get cold soaked, the air is always around 50 degrees and that is all the temperature the batteries will see. And I agree low and slow. I was just using the 60c figure in your post as I never measured temperature on mine.
I've not heard the term "cold soaked" but I think I get what you mean. My thermostat is set to keep the cells between 50°F and 60°F. My cells never get below 50°F either, so maybe your system isn't that different? ;)
 
I've not heard the term "cold soaked" but I think I get what you mean. My thermostat is set to keep the cells between 50°F and 60°F. My cells never get below 50°F either, so maybe your system isn't that different? ;)
I work in HVAC and radiant floor heating, so a cold soaked slab is something I deal with which has high mass. It takes more heating capacity to bring a mass up to temp rather than just having to satisfy the heat loss.
 

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I work in HVAC and radiant floor heating, so a cold soaked slab is something I deal with which has high mass. It takes more heating capacity to bring a mass up to temp rather than just having to satisfy the heat loss.
That's interesting! I had Googled it and almost all the results were about aviation, or food. Wings of planes are cold soaked after a flight because of the fuel still stored in the wings, or having been parked overnight in very cold temperatures with fuel in the wing tanks, etc. I had assumed you must be in aviation!

Anyway, I think your point in using the term was that you won't be heating the cells up from being very cold. My point was that none of us should be, if we are using some mechanism (yours, mine, or someone else's) to keep them within a range.
 
50-60*F (10-15*C) degrees or 35-45*F ( 2-7*C)?
Set mine at 2-7*C.
Wondering if there is that much difference, what do you think?
 
For better or worse I set mine 40 on, 45 off. 35 seems like cutting it a bit close. Especially if the system has a high charge rate.

On at 50 would come on more than needed IMO.
 
50-60*F (10-15*C) degrees or 35-45*F ( 2-7*C)?
Set mine at 2-7*C.
Wondering if there is that much difference, what do you think?
My logic to keep my cells in the range of 50°F-60°F was driven by a few things.

I've read lots of places that the way to think about LiFePO4 cells is that they are comfortable in the same general temperature range that we are comfortable. Part of this "comfort" is the cells ability to deliver intended capacity. The capacity that can be delivered from these cells drops pretty dramatically as the temperature drops. I got 8 cells from @Amy Wan (Luyuan) in early December, and tested the capacity in my garage. It was actually pretty nice weather, but the temperature in the garage was between 50°F and 55°F most of the time. The cells tested out with an average of 10Ah less capacity than Amy had reported. Eventually someone pointed out it was due to the cooler temperature. I didn't realize the impact would start at a relatively high temperature of 50°F, and it gets much worse at lower temps. In the spring and fall our cabin (and the battery) will get quite a bit of use, and yet the temperatures in May/June and Oct/Nov will still be a bit chilly. This is especially true in the basement where the batteries will live. I expect the heater will stay on in the spring and fall, and I want to know that they are able to meet capacity needs.

My test has also shown me that staying in the 50°F to 60°F range doesn't actually require much power from the battery. Even when it is 10°F to 20°F in the surrounding air, I can keep the cells in my insulated box warm with between 2-3% of the battery capacity per day. Assuming that I get solar now and then to top it back off, maintaining the temperature will be really easy.
 
My thought on the temperature range is that 5° F is too small. Besides the heating system coming on more frequently, perhaps the batteries are not getting thoroughly warmed. I use a 10° F spread. Would 15° F be even better? Maybe. To really answer that question, I would need a lot of temperature sensors in different locations on the battery. I will say that the temperature probe on the surface of the battery between cells two and three (4s battery) reports a value that is very similar to the temperature probe on the positive post of the battery.

My batteries are not in a sealed container. I tried to isolate my temperature probes from the ambient air but I've noticed that the temp drops a bit after the warming system shuts off. I suspect that my temperature probe needs a bit more isolation and I need to seal up the insulation around the batteries better. A warming system wasn't part of my initial battery fixture design, so the insulation is retrofitted in as best I could. I don't have much room to work with and that greatly limits my options.

I don't use my batteries hard when they're at the lower temperature ranges. When I'm camping at cold temperatures the furnace is cranking and the batteries are usually sitting around 60° F due to having a furnace duct running through the same compartment where the batteries are located.
 
My thought on the temperature range is that 5° F is too small. Besides the heating system coming on more frequently, perhaps the batteries are not getting thoroughly warmed. I use a 10° F spread. Would 15° F be even better? Maybe. To really answer that question, I would need a lot of temperature sensors in different locations on the battery. I will say that the temperature probe on the surface of the battery between cells two and three (4s battery) reports a value that is very similar to the temperature probe on the positive post of the battery.

My batteries are not in a sealed container. I tried to isolate my temperature probes from the ambient air but I've noticed that the temp drops a bit after the warming system shuts off. I suspect that my temperature probe needs a bit more isolation and I need to seal up the insulation around the batteries better. A warming system wasn't part of my initial battery fixture design, so the insulation is retrofitted in as best I could. I don't have much room to work with and that greatly limits my options.

I don't use my batteries hard when they're at the lower temperature ranges. When I'm camping at cold temperatures the furnace is cranking and the batteries are usually sitting around 60° F due to having a furnace duct running through the same compartment where the batteries are located.
I don't think I've mentioned it here anywhere, but I made some changes to my insulated box, and got some happy results.

I was changing the heater-control electronics, and decided to pull the cells out and change some of the bus bars and such. In the process I decided to seal up the box more. I already had a big roll of mastic-lined foil tape for sealing heat ducts, so I used that to seal up all the seams in the XPS and between the XPS and the plywood inserts in the box. I also put a good amount of weather stripping between the box and the lid to prevent air flow when the lid is on (I knew I had a leak there).

When I reassembled the battery with the new electronics and ran a test last weekend (really cold temps from 31-Jan until the morning of 3-Feb) I was pleasantly surprised. The change was not huge, but it was obvious. Temperature fell more slowly between heat cycles than I would have expected, and it took a bit less time to arm the cells back up to 60°F than before.
 
I don't think I've mentioned it here anywhere, but I made some changes to my insulated box, and got some happy results.

I was changing the heater-control electronics, and decided to pull the cells out and change some of the bus bars and such. In the process I decided to seal up the box more. I already had a big roll of mastic-lined foil tape for sealing heat ducts, so I used that to seal up all the seams in the XPS and between the XPS and the plywood inserts in the box. I also put a good amount of weather stripping between the box and the lid to prevent air flow when the lid is on (I knew I had a leak there).

When I reassembled the battery with the new electronics and ran a test last weekend (really cold temps from 31-Jan until the morning of 3-Feb) I was pleasantly surprised. The change was not huge, but it was obvious. Temperature fell more slowly between heat cycles than I would have expected, and it took a bit less time to arm the cells back up to 60°F than before.

That doesn't surprise me at all. I need to go back to the drawing board on my insulation approach. Something that is assembled and sealed outside the trailer and can then be lowered over the batteries would work a lot better. The insulation needs to come off for non-winter operations, so sealing what I have in place now would mean breaking the seal each spring and putting it back in place each fall.

If I'm going to redo the insulation I should do it now while all the wiring is in flux for the Multiplus install. Two steps forward, one step backwards.
 
So @HRTKD, I get the impression (from here and other posts you've made) that you are going mostly all-in on upgrading your system to Victron? I've been helping someone do that for their boat, and I have to admit I like Victron stuff.
 
So @HRTKD, I get the impression (from here and other posts you've made) that you are going mostly all-in on upgrading your system to Victron? I've been helping someone do that for their boat, and I have to admit I like Victron stuff.

Yep. I already had the two Victron MPPT 100/50 and the BMV-712. I'm adding the Multiplus, Smart Battery Protect 65, Cerbo GX and the Touch 50 in this phase. The common bus bars are getting upgraded as well. The old four stud bus bars were likely brass, not copper and I needed more posts. Here's a post from a couple weeks ago showing my progress with the new bus bars.

 
Coming from a gardening perspective of keeping delicate seedlings at just the right temp makes all the difference. These mats are cheap, durable and come in all sizes. This size only consumes 17.5 watts. If you have 120vac close by this might be an option?
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Coming from a gardening perspective of keeping delicate seedlings at just the right temp makes all the difference. These mats are cheap, durable and come in all sizes. This size only consumes 17.5 watts. If you have 120vac close by this might be an option?
View attachment 82797
Those do look nice, but for most of us using 120VAC would require firing up the inverter, which would be counterproductive. My little 12V 12W heating pads fit my needs better.
 
Those do look nice, but for most of us using 120VAC would require firing up the inverter, which would be counterproductive. My little 12V 12W heating pads fit my needs better.
I sure don't disagree since I haven't taken a survey, but in my case, as in the case of anyone coming from a grid connected world, there has always been a 120vac outlet in my shop close to where my DIY back-up system will be set up so it's just a no brainer in my case. I already own these and most of the time they just lay in a drawer unused. My batteries will remain snug and warm, plus that way they can also serve double duty when gardening time arrives. I'm just trying to keep it in the true spirit of DIY? ;)
 
I sure don't disagree since I haven't taken a survey, but in my case, as in the case of anyone coming from a grid connected world, there has always been a 120vac outlet in my shop close to where my DIY back-up system will be set up so it's just a no brainer in my case. I already own these and most of the time they just lay in a drawer unused. My batteries will remain snug and warm, plus that way they can also serve double duty when gardening time arrives. I'm just trying to keep it in the true spirit of DIY? ;)
My situation and use is similar. I had a seed starter heating pad and figured, why not.
?
 
Those do look nice, but for most of us using 120VAC would require firing up the inverter, which would be counterproductive. My little 12V 12W heating pads fit my needs better.
I sure don't disagree since I haven't taken a survey, but in my case, as in the case of anyone coming from a grid connected world, there has always been a 120vac outlet in my shop close to where my DIY back-up system will be set up so it's just a no brainer in my case. I already own these and most of the time they just lay in a drawer unused. My batteries will remain snug and warm, plus that way they can also serve double duty when gardening time arrives. I'm just trying to keep it in the true spirit of DIY?
My situation and use is similar. I had a seed starter heating pad and figured, why not.
Well, well...maybe I should take a survey? ;)
 
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