Using any kind of battery heater without a thermostat would be the height of folly (IMHO)...
LiFePO4 heating pad for cold temperatures
- Thread starter Will Prowse
- Start date
Horsefly
Solar Wizard
To be honest, this doesn't seem like it would be that hard to verify BEFORE you use it on an expensive battery. If @Trabuco has the heater and the aluminum sheet, he can attach them as he intends, and put a tin can full of water on the top of the sheet, to act as a thermal mass simulating the battery. Apply power to the heater and see what happens. Watching it for a half hour or so with a handheld IR thermometer should answer whatever questions there may be.
That's a good test. I'm only thinking of his batteries. Some due diligence is in order to keep them from being fried.
Horsefly
Solar Wizard
It's actually the kind of test I was starting with my heating pads. I was first going to see how it did with no mass on top of the sheet. Once that got to 180°F, I unfortunately had the answer before even trying to put something on the sheet.
Or one of the numerous lead acid batteries I've got laying around...
I'll report back the temps.
Horsefly
Solar Wizard
Hey @Trabuco - I bought that heating pad you pointed to: https://www.amazon.com/gp/product/B0794V5J5H and did a very unscientific test today. May not mean anything, but here's what I did:
I bought a small sheet of steel sheet metal at Home Depot (my intent would be to use Aluminum, but that's not what HD has). I taped the heating pad to the back of the sheet metal with some Kapton tape. I then took 8 of the little 25Ah cells discussed here, organized into a 2p4s 12V battery pack. The ambient temp in my garage was already up to about 40°F but the cells were about 36°F from cooling down overnight (it was something around 15°F outside last night, and that probably means my garage was something close to freezing or maybe even a little below). I hooked up the heating pad to the pack, and let it go. At first I checked it every 5 minutes or so, but not much was happening. After I felt more secure that it wasn't going to cause some catastrophic mess on my workbench, I let it go for about 2.5 hours. By then the temp in the garage was to about 48°F (it was still in the 20's outside), and the cells were about 60°F. The sheet metal on the outside edges of where the cells were sitting was a little warmer, like around 65°F-70°F (different temps in different places I checked).
So I think this little test has some good news and some bad news.
The good news is that this heating pad will not overheat / damage your cells, and it does eventually warm things up.
The bad news is that I think one pad would have trouble warming 4 or 8 of the much larger 280Ah cells. Depending on the ambient temperature you were in, it could take 8 hours or more just to raise them a small amount. I know you said you were going to use 4, so that may do the job (maybe).
I bought a small sheet of steel sheet metal at Home Depot (my intent would be to use Aluminum, but that's not what HD has). I taped the heating pad to the back of the sheet metal with some Kapton tape. I then took 8 of the little 25Ah cells discussed here, organized into a 2p4s 12V battery pack. The ambient temp in my garage was already up to about 40°F but the cells were about 36°F from cooling down overnight (it was something around 15°F outside last night, and that probably means my garage was something close to freezing or maybe even a little below). I hooked up the heating pad to the pack, and let it go. At first I checked it every 5 minutes or so, but not much was happening. After I felt more secure that it wasn't going to cause some catastrophic mess on my workbench, I let it go for about 2.5 hours. By then the temp in the garage was to about 48°F (it was still in the 20's outside), and the cells were about 60°F. The sheet metal on the outside edges of where the cells were sitting was a little warmer, like around 65°F-70°F (different temps in different places I checked).
So I think this little test has some good news and some bad news.
The good news is that this heating pad will not overheat / damage your cells, and it does eventually warm things up.
The bad news is that I think one pad would have trouble warming 4 or 8 of the much larger 280Ah cells. Depending on the ambient temperature you were in, it could take 8 hours or more just to raise them a small amount. I know you said you were going to use 4, so that may do the job (maybe).
I don’t think you want to worry about ‘heating’ cells - they will be heated to target temp once and only once.
At that point, you need to worry about compensating for the Watts of heat loss through your enclosure.
Maintaining a heat mass at a constant temperature requires far less energy than raising the temperature of a heat mass (at least assuming a reasonable job done insulating the heat mass).
Horsefly
Solar Wizard
I disagree. See my thread about keeping LFP warm. Heating the cells is not "once and only once." I think the idea is you have a thermostat that turns on when the temp gets down to say T, and turns off at T+x. So in the end, whatever heat mechanism we have has to manage to get the temp of the cells from T to T+x. That's what I'm talking about. After that, you want your enclosure to be well enough insulated that it slows the heat loss. Eventually, the temp gets down to T again, and the heat pad(s) (or whatever) turn on and try to raise the temp from T to T+x. So in my simple mind, you have two things to work on: (1) heating up your pack so that it can be charged safely, and (2) building an enclosure so that you don't have to heat so often. Either way, you WILL be heating more than once.
Horsefly
Solar Wizard
No, that's one of the reasons I called it a very unscientific experiment. The sheet metal with the heater on the bottom was just sitting on my workbench in my garage. The ambient temperature in the garage was rising the whole time I was trying to warm the batteries. So - like I said - hard to say it means a whole lot. However, in that environment it did take quite a while to warm these little cells.
This statement is absolutely correct but as long as your heating pad is sufficient to more than compensate for watts of additional heat loss while it is on, the cells will increase in temperature and hit T+x. It is inconsequential whether the cells reach T+x in one minute, one hour, or 10 hours. All that matters is that the cells stop cooling down.
I think we are finding different ways of saying the same thing.
It doesn’t matter how ‘fast’ your heating pad raises the temperature of your cells. It only matters that the heating pad more than offsets the Watts of heat loss through the enclosure.
If the heating pad is not strong enough to offset heat loss through the enclosure and cell temps continue to drop, you’ve got two solutions:
1/ Get a stronger heat pad (or more of the too-weak ones) or
2/ Improve your insulation to the point that Watts of heat lost through the enclosure at target temperature differential are less than the Watts of your (formerly( too-weak heat pad.
Once everything has been designed correctly, the temperature of the cells will follow a sawtooth pattern. The slope down will be determined by the Watts of heat loss through the enclosure (meaning insulation and temperature differential). The slope up will be determined by the Watts of the heating pad. It doesn’t matter how much slower the upslope is than the downslope (as long as it is, in fact, an ‘up’-slope and not a reduced downslope).
Horsefly
Solar Wizard
Well, OK. Seems you have come a long way from "once and only once". Anyway, I think we are in agreement.
By once and only once I meant heating cells from ambient to target. That should happen ‘once and only once’ (every time you turn off your temperature control and allow your cells to cool down to ambient).
Once your pack has been initially heated to target, it should never again cool down to ambient as long as your temperature control loop remains active...
But yeah, I think we can move on. My main point was to avoid purchasing a more powerful heating pad than you need. Among other things like $$$, efficiency, etc..., the heat being generated and the possibility of burning something or damaging something is directly related to the Watts of the heating element.
A heating pad that is on as long as possible each cycle with as low Watts as possible is far better than a heating pad that is on for a very short time each cycle heating everting back to T+x very quickly (with high Watts).
I do a lot of ‘low and slow’ barbecue as well as make a lot of all-grain beer, both of which require temperature control. Heating slowly is best (and safest) as long as it is actually heating (and not just slowing down the rate of cooling).
I used these in my lifepo4 battery, together with a cheap 12vdc thermostat.
I made a case for a battery from 30mm XPS insulation and taped those heating elements to the aluminum sheet (2 sheets for both sides, 2 heating pads on each sheet, taped all around for extra protection from high temp. it's not connected directly to the battery, it is loose between the wall of the battery case and the battery).
It was on all night at about 0C outside. For 15 hours it took about 3A to keep the temperature inside the box at 8C.
I don't really like the idea of the heating pad which heats up the battery ONLY when it starts to charge, because if I'm not driving many days and the battery inside the van was frozen, then temperature sensor can't give the right temperature, because it stays on the side of the battery and it measures the air temp, not the battery. So the battery itself can be still frozen in the center.
With the thermostat, I can keep the proper temperature all the time, and I can charge it from the beginning of the ride or with the first sun and not waste this time for heating up.
Maybe I will make a video or pictures next time when I remove it from the van.
DerpsyDoodler
Solar Addict
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What about something like this, with a thermostat?
It has its own thermostat, as near as I can tell. But where is the thermostat? Is it on the controller or on the pad?
For my situation, an AC-120v solution won't work because my inverter isn't on unless I'm actively using the RV.
Those are AC; maybe not the first choice for RV usage.
I bought a couple of those to use on my 32 cell bank at home.
I'll report on that later, but currently (pun intended) they are helping lettuce and onion seeds to germinate.
Edit: There is a temperature probe which sits on the pad and is connected to the controller.
I bought a couple of those to use on my 32 cell bank at home.
I'll report on that later, but currently (pun intended) they are helping lettuce and onion seeds to germinate.
Edit: There is a temperature probe which sits on the pad and is connected to the controller.
DerpsyDoodler
Solar Addict
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I wonder if there aren’t other “seedling” mats with thermostat that can take a dc power source.
Kristof
1977 Airstream Off Grid Trailer
Hi I have been following this thread actively.
I have built two LiFePO4 batteries to date - both 24V 8S2P using 100amp cells. I built a box out of 1/2in birch for the cells and there is some air and insulation in the box. In addition it is in an Airstream trailer which when operating has a hydronic heating system. I also have a bank of Lead Carbon batteries (4 X 12V 65amps) which gives me 130AH at 24V. They also handle the cold way better and they are now taking the load at the trailer while in storage for the winter. This is an off grid setup.
This spring I am going to rewire and redo the system to support two banks - one is the LiFePO4 battery and the other would be the Lead Carbon batteries. This gives me easy redundancy through a switch - I have lost cells from the first LiFePO4 battery due to a closed loop through the inverter and my hydronic heating system - in essence I drained them too quickly and about 4 of them never recovered. Expensive to keep replacing them to say the least.
I just bought a temp controller and some smaller 12V 7W heating pads (8 in total). Here are the links:
So far this experiment seems to be able to work. If I wire the pads in series I would be drawing about 5amps at 12V. The LiFePO4 battery should provide me with around 400AH at 12V so at 8 hours of draw the battery would be at 85-90% SOC in the morning. I have 1 KW of solar which at peak can generate 800 watts here in Colorado parked in my lot. Over the sunny part of the day I normally generate in excess of 1.5 Kilowatts of power which is distributed by a VICTRON solar controller at a high end of 38 amps.
The pads have a sticky side and then some closed cell foam - I was thinking of sticking them directly to the cells.
Anyone think I am nuts or doing something dangerous? I am feeling confident that 7W on each pad on each cell is not a show stopper.
Request Advice - brutal if required!
Kris
I have built two LiFePO4 batteries to date - both 24V 8S2P using 100amp cells. I built a box out of 1/2in birch for the cells and there is some air and insulation in the box. In addition it is in an Airstream trailer which when operating has a hydronic heating system. I also have a bank of Lead Carbon batteries (4 X 12V 65amps) which gives me 130AH at 24V. They also handle the cold way better and they are now taking the load at the trailer while in storage for the winter. This is an off grid setup.
This spring I am going to rewire and redo the system to support two banks - one is the LiFePO4 battery and the other would be the Lead Carbon batteries. This gives me easy redundancy through a switch - I have lost cells from the first LiFePO4 battery due to a closed loop through the inverter and my hydronic heating system - in essence I drained them too quickly and about 4 of them never recovered. Expensive to keep replacing them to say the least.
I just bought a temp controller and some smaller 12V 7W heating pads (8 in total). Here are the links:
So far this experiment seems to be able to work. If I wire the pads in series I would be drawing about 5amps at 12V. The LiFePO4 battery should provide me with around 400AH at 12V so at 8 hours of draw the battery would be at 85-90% SOC in the morning. I have 1 KW of solar which at peak can generate 800 watts here in Colorado parked in my lot. Over the sunny part of the day I normally generate in excess of 1.5 Kilowatts of power which is distributed by a VICTRON solar controller at a high end of 38 amps.
The pads have a sticky side and then some closed cell foam - I was thinking of sticking them directly to the cells.
Anyone think I am nuts or doing something dangerous? I am feeling confident that 7W on each pad on each cell is not a show stopper.
Request Advice - brutal if required!
Kris
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