diy solar

diy solar

LiFePO4 heating pad for cold temperatures

Relion is maybe the only one to do it right, for an internal heater that is internally controlled. Below 0dgC, all incoming charging current is diverted to an internal heater. When the battery is sufficiently warmed, the charging can begin. No battery power is ever used to run the heaters. Perfect! I think Battle Born missed the boat with their system.

Couple ideas here....

1) Do what Relion has done; that when under suitable charge temperatures redirect any incoming charge power to the heaters. Problem is if no charge power coming in batteries are not being heated. The colder they get the longer to get them warmed; and risk to get to a dangerous storage temp if in very cold temp areas.

2) Use battery power, down to some SOC %, to power the heaters to keep batteries in a state they can be charged. This way when chargers come on line you can immediate use that to charge and recover the pack. At some safety SOC cutoff % then disconnect.

The ideal is to combine this I think.

When under a safe charge temp use charger power to heat if it's available or battery power (to a defined SOC %) to heat.

All this assumes you have ample capacity; if your capacity is marginal it could change the approach.
 
When the sun is out, my warming pads will (automagically) get power from the solar charge controller. Otherwise, power comes from the battery bank. When the sun comes up in the morning, my batteries are warm and ready to take a charge. Insulation is key. I think the batteries maintain the heat fairly well.

I have seen a minimal hit to the state of charge from the warming pads. Four pads, each rated at 1 amp, plus two thermostats which don't draw much at all.
 
I have thought about it, but maybe I will reconsider. The heater vent that runs into the pass-thru area is in the middle of my electronics panel - had to cut a hole to let the warm air pass though:

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I count 7 shunts - I presume this is so you can show current info for the 3 packs, as well as solar charge, inverter(s), etc, etc?

In the top left what are the shunts to small bus bars that look like they each feed a pair of breakers? That goes to some Vicron MultiPlus for a 240V 2 phase setup?
 
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When the sun is out, my warming pads will (automagically) get power from the solar charge controller. Otherwise, power comes from the battery bank. When the sun comes up in the morning, my batteries are warm and ready to take a charge. Insulation is key. I think the batteries maintain the heat fairly well.

I have seen a minimal hit to the state of charge from the warming pads. Four pads, each rated at 1 amp, plus two thermostats which don't draw much at all.

Yeah, you have built what I called the ideal case. Do you do much B2B charging or shore power charging?
 
Yeah, you have built what I called the ideal case. Do you do much B2B charging or shore power charging?

No charging from the tow vehicle or shore power. There is infrequent charging from the on-board generator when I fire it up to run the microwave. Charging from solar is 99.9% of where the batteries get their power.
 
No charging from the tow vehicle or shore power. There is infrequent charging from the on-board generator when I fire it up to run the microwave. Charging from solar is 99.9% of where the batteries get their power.

Even on a cloudy day do you still get enough solar production to run the heating pads? I've toyed with using a small panel direct to the heaters without a charge controller at all, just a simple thermostat to turn on/off.
 
Even on a cloudy day do you still get enough solar production to run the heating pads? I've toyed with using a small panel direct to the heaters without a charge controller at all, just a simple thermostat to turn on/off.

I was surprised to see that I was still getting some power with snow in the panels. Not much, maybe enough to cover some of what the pads were using.
 
I count 7 shunts - I presume this is so you can show current info for the 3 packs, as well as solar charge, inverter(s), etc, etc?

In the top left what are the shunts to small bus bars that look like they each feed a pair of breakers? That goes to some Vicron MultiPlus for a 240V 2 phase setup?
You've pretty much got it. I wanted to monitor each pack separately to narrow things down in case of a bad cell or something similar. The Simarine controller lets you monitor them separately and also sum them to a single value. So they show up as "Battery A", "B", and "C", and the summed "Battery Bank" all at the same time.

One shunt for solar, one for the DC-DC converter (only way I can monitor 12V consumption) and one each for two inverters. They are off to the side and not wired yet in that video. Each inverter takes two positive and two negative cables, plus the AC input from the surge protector, so it's a really fat bundle of wire:

IMG_0195.jpeg

Nothing in the trailer is 240V, but it is 50A, so the 120V is in two separate phases and has two separate legs in the circuit breaker panel. By running two inverters, I don't need to bridge them, and I get enough watts to run the air conditioner (3kW for each leg). I did shift a couple of the breakers around to put less other stuff on the side with the A/C.

To get things back on topic... I just got done building the third battery pack, so I will be putting the heating pads on that one and then retrofitting the first two. Just waiting on the aluminum now. Will post pics once I start building the heating assembly.
 
Am using these in my build:


51j73t7rNVL._AC_SL1000_.jpg


$10, 12 watts each, 100mmx120mm.

4 per side of these glued to an aluminum plate will just about perfectly fit my 4 x 280AH cells.

With combinations of parallel and serial connections, I will have the flexibility to drive 96 watts total in parallel, or signficantly less for a more gentle heat without the need for any additional electronics.

Plus I can change after building as needed by only swapping a few connectors,
Just FYI. I ordered two of these from Amazon and they work great. But they cost me around $14 each so I spent $30.10 after tax on Amazon. For kicks I searched AliExpress and found them for about $3 each, and even with shipping (listed at $2.42) that’s a heck of a lot cheaper... I could have purchased 10 of them for that price!

US $3.04 31% Off | Mayitr 12V DC 20W Flexible Waterproof Silicon Heater Pad Silicone Heated Electric Heating Pads 80x100mm

 
Just FYI. I ordered two of these from Amazon and they work great. But they cost me around $14 each so I spent $30.10 after tax on Amazon. For kicks I searched AliExpress and found them for about $3 each, and even with shipping (listed at $2.42) that’s a heck of a lot cheaper... I could have purchased 10 of them for that price!

US $3.04 31% Off | Mayitr 12V DC 20W Flexible Waterproof Silicon Heater Pad Silicone Heated Electric Heating Pads 80x100mm

The Alibaba pads look a bit different from the Amazon ones. Not sure if the difference is enough to matter: 20W vs 12W, and 80mm x 100mm vs 100mm x 120mm. So the Alibaba pads are smaller, and are considerably more wattage. It may be a good idea for someone to try them out, but it doesn't seem like a direct substitute.
 
Just FYI. I ordered two of these from Amazon and they work great. But they cost me around $14 each so I spent $30.10 after tax on Amazon. For kicks I searched AliExpress and found them for about $3 each, and even with shipping (listed at $2.42) that’s a heck of a lot cheaper... I could have purchased 10 of them for that price!

US $3.04 31% Off | Mayitr 12V DC 20W Flexible Waterproof Silicon Heater Pad Silicone Heated Electric Heating Pads 80x100mm

I saw that too.

BTW, they were $8.99, one day Prime until I posted on this forum! I guess the battery crowd buys more than the beekeeper crowd and their algorithm adjusted.

There are lots of sources/resellers that look similar. eBay has them too. I was looking for a quicker shipment and lower wattage, so went with the Amazon one.
 
I am glad we had the conversation about the aluminum and the cases of the batteries - and that someone measured the current.

I went out and cut and rigged my two "heater" bars which will have the small heating pads (coming this weekend) on one side and as you can see I got some rubber tape that will be between the cases of the batteries and the aluminum (just in case). The case I have is made of birch and pine and so the bars will secure to the sides of the case with a bolt or a screw, not sure yet. The temp control unit will be mounted on a wall beside the battery once emplaced. I am also going to use a 12V supply from another battery bank which just so happens to be lead carbon. MTF on that as the parts arrive and I get the bars in place.
 

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Okay folks there arrived today. They are 12V 7W heating elements. Going to the side of battery cells (X8 total of 16) athwartships, one on each side. I calculate this would be ~27W per side to ~54W total which at 12V would be 4.5amps. I am going to run this off a 12V 65AH Lead Carbon battery from bank number 2 - just in case. I produce enough solar during the day to easily combat a continuous run - curious though to see the heat produced and how long it takes to warm the battery. Only time and real data will tell. The long term plan is during storage (non operational use) during the winter is to keep the LiFePO4 warm using the Lead Carbon bank - and during operational use - use the LiFePO4 bank and have the system monitor temps/charge/etc.

After all this discussion on this forum I am wondering if I should take the rubber off the aluminum for better heat transfer?

Have not tested them yet - that will be next week. Going to hook them up to a car battery and see what kind of heat is produced.
 

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Yeah, I would definitely come up with some way to test the heat dispersion of those pads on something representative of LFP cells. I tested those and some similar (but higher voltage / higher wattage) cells here, and found them to get way too hot too fast. That prevents the cells from dispersing the heat through the cells, so you could easily have cooked the edges (bottom, sides, or wherever you have the pads) before the center of the cells get above freezing. I'm not saying those 7W pads are bad, but you should test them some.

As for the metal contact stuff: My personal opinion, and only an opinion, is that you shouldn't even worry about it. I extended my previous test. I have an 8s / 24V pack of Navitas 25Ah prismatic cells. I'm guessing they MUST be similar design (but smaller) than all the cells discussed here. If I measure the voltage from the cell 1 case to the cell 8 case, I see 20+V. If I measure the voltage from the cell 1 case to the cell 8 positive, I see 25V, if I measure between the cell 8 case and the cell 1 negative, I see -23V. If I short between any of these and check the current, it is 0.00A. There is no current flowing between the cell positive / negative post and any case in the string. None.

I have a hard time believing that manufacturers could even sell LFP cells that could be shorted by contacting their cases. That would be a bad day for most EVs or other high C applications.
 
Hey Folks well I put the pads to the test today.

My wiring thoughts were that the pads could be wired in series and draw 7W X whatever number of pads in series. Turns out that is wrong as they are basically resistors so lining 4 up they drew .15amps which is 1/4 of .58amps which should be the draw at 7W 12V. I then wired up a single one and not only did it start to warm up (very slowly on the aluminum) it drew .56amps. Lesson there is to wire them up in parallelly and they should draw about 2.2 amps. That will be tomorrows experiment.

The worry about them getting too hot is gone now. My workshop is not heated and it was -4cel in there today. Tomorrow we should see how warm the strips of aluminum get and how hot they eventually will get to alleviate any concern about heat transfer being too high. Once the aluminum is in contact with the heat sink (battery) I think they will will be a very slow warmer. That is fine if the temp controller is set properly.
 

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An hour I think. The battery I am testing this on is not a high AH battery - but at 2.2 amps for an hour it should be okay.

I am going to use a heat sink of a mass similar to a battery - maybe the little lawn tractor battery itself.
 
An hour I think. The battery I am testing this on is not a high AH battery - but at 2.2 amps for an hour it should be okay.

I am going to use a heat sink of a mass similar to a battery - maybe the little lawn tractor battery itself.

A sacrificial battery sounds like a good plan.
 
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