diy solar

diy solar

Heating pads under battery

ojoj said:
Alternatively, could i use 24V pad (30*30cm, 350W), but supply it with 12V power? This should halve the heat production right?
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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.
 
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.
 
Leeds said:
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.
Click to expand...
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.
 
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.

L heating.png
 
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.
 
ojoj said:
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.
Click to expand...
That will probably work.

I know most people will instinctively know this, but it is worth saying: The pack of batteries has a pretty high thermal mass. That means that it takes quite a while to warm up, and once it's warm it takes quite a while to cool down. In the graph that I posted earlier in this thread you can see that the temperature at the top of the cells lagged 5-10°F behind the temp at the bottom of the cells (my heating pads and aluminum were beneath the cells). This means you may need to do some experimenting to see if the cells really get warm enough before you turn off your heat. It also means that if you heat too quickly the cells area furthest from the heat could still be too cold when the area nearest the heat could get dangerously warm.
 
How does heating the air work? Placing heating on the side wall of the battery box without it making any direct contact with cells/aluminum plate. Any experiences?
 
It really is simple. I have been using a seed germination pad under the battery with the temp sensor taped to the side of the battery near the top. This is small, just a single 100AH 12V battery to power my observatory. The battery box is insulated and contains the MPPT and 600W inverter to provide a bit of warm air in the winter. I open the battery box once the weather is warm. the battery itself is swathed in more insulation. Works just fine - as noted the battery has lot of thermal mass so using an 18W heat pad is a very slow temperature rise, and after it shuts down the temp will go up about another 4 or 5 degrees F. A couple times in winter there have been 3 or more dark and cold days and I have forgotten to shut down some equipment and not caught it until the battery BMS shuts it down. The battery has never gone below 32F. This is central NH, so there is some pretty cold weather.
 
ojoj said:
How does heating the air work? Placing heating on the side wall of the battery box without it making any direct contact with cells/aluminum plate. Any experiences?
Click to expand...
See my previous post.
 
ojoj said:
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.

View attachment 136114
Click to expand...
Consider a second heat pad on the other side. Run these two pads in series for 150 to 200 watts total.
 
ojoj said:
How does heating the air work? Placing heating on the side wall of the battery box without it making any direct contact with cells/aluminum plate. Any experiences?
Click to expand...
Very well if the cells have plenty of insulation around them. Mine have just 15W of heat and easily maintain around 20 degrees C above ambient.
 
SeaGal said:
Very well if the cells have plenty of insulation around them. Mine have just 15W of heat and easily maintain around 20 degrees C above ambient.
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What would plenty of insulation be? I will have 5cm/2inches of PIR all around the box. R14.
 
SeaGal said:
Very well if the cells have plenty of insulation around them. Mine have just 15W of heat and easily maintain around 20 degrees C above ambient.
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So you are saying that on a cold night, you just turn on the heat and leave it on?
 
I used these :


I simply taped one pad on each of the 4 sides of each battery.

I then used these :


to control the heating pads.

I placed the sensor for each control unit on top of each battery it controlled the pads for.

I believe I have them wired 2 in series on each of the 4 pads but I may have all 4 in series I cant remember which worked out the best.

I set the controllers to 37F turn on 40F turn off. I havent noticed much in the way of shortened battery life with this setup on my lifepo4 batteries.

So far using a infrared thermometer the batteries have stayed above 34F at all times. My batteries will function to 0C and they have a 32F limit on charging with the bms they have. So I have it setup to keep it over 34F to ensure they will charge each morning no matter how cold it gets.

That said it doesnt get that cold here in Alabama with 20F being the normal lows most of the time in the winter.
 
bajajoaquin said:
So you are saying that on a cold night, you just turn on the heat and leave it on?
Click to expand...
No, it is temperature controlled?️ My software monitors temperatures from the BMS, which has 3 probes on the cells, as well as an air temp inside the insulated battery compartment. On / off thresholds are configurable, currently set to on at 20C and off at 23C. Last night, for example, cells maintained temp of between 22.2 and 24.7 degrees C with the ambient temp in garage dropping to 3 degrees. Heater was on and off 3 times over 7 hour period overnight.
 
Crowz said:
I set the controllers to 37F turn on 40F turn off. I havent noticed much in the way of shortened battery life with this setup on my lifepo4 batteries.

So far using a infrared thermometer the batteries have stayed above 34F at all times. My batteries will function to 0C and they have a 32F limit on charging with the bms they have. So I have it setup to keep it over 34F to ensure they will charge each morning no matter how cold it gets.

That said it doesnt get that cold here in Alabama with 20F being the normal lows most of the time in the winter.
Click to expand...

All sounds good. My only thought though is that 34F is only about 1.5C isn't it? If so, then EVE's recommendation [which presumably applies for all LiFePO4 cell chemistries) for charge current (see table below), is to only charge at 0.1C rate below 5 degrees C and 0.2C below 10 degrees C if the SOC is more than 70%... (too many C's there!).

So, in order to safely charge at my max charge rate of about 0.25C early on a cold morning, I prefer to keep my cells above 15 degrees C.

121396-aaa8031196902f39c008caf746bd2086.jpg
 
SeaGal said:
All sounds good. My only thought though is that 34F is only about 1.5C isn't it? If so, then EVE's recommendation [which presumably applies for all LiFePO4 cell chemistries) for charge current (see table below), is to only charge at 0.1C rate below 5 degrees C and 0.2C below 10 degrees C if the SOC is more than 70%... (too many C's there!).

So, in order to safely charge at my max charge rate of about 0.25C early on a cold morning, I prefer to keep my cells above 15 degrees C.

121396-aaa8031196902f39c008caf746bd2086.jpg
Click to expand...
I may up the temps on the controllers before this coming winter but at the same time I dont even know if I need the heaters anymore since the tp6048 units put out so much heat I doubt my workshop this is installed in will ever get that cold in the winter time now.

I was just going by the labels on the batteries. I never took into consideration the max charge rate varying by temperature. I was just figuring bms cutout point.
 
SeaGal said:
No, it is temperature controlled?️ My software monitors temperatures from the BMS, which has 3 probes on the cells, as well as an air temp inside the insulated battery compartment. On / off thresholds are configurable, currently set to on at 20C and off at 23C. Last night, for example, cells maintained temp of between 22.2 and 24.7 degrees C with the ambient temp in garage dropping to 3 degrees. Heater was on and off 3 times over 7 hour period overnight.
Click to expand...
Is there a resource here that explains how to diy that? Or could you?
 
It's BMS specific... I use some software running on an ESP32 using code that communicates with my Overkill (JBD) BMS to get (amongst other data) the temperatures. Then the microcontroller simply switches a relay on or off at the required temperatures to power the heaters.

But other forum members have just used a cheap commercially available digital thermometer (e.g. STC-1000) type device, widely available, to turn heaters on or off.
 
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