Lifepo4 heating pads - circuit design question / diagram - relay to relay?

Hello! If anyone has any time to help me confirm the validity of my very simple battery heating pads circuit, I would be VERY grateful.
Here is the diagram I drew. I will have this W1209 thermostat switch (also attached in photos) activate when the temperature has dropped towards 0c freezing, to protect my Lifepo4 battery.
My primary concern is that the W1209 already has a 20 amp relay on it, and since my heating pad load will be reaching a total of 60 amps, I am thinking about putting in a 60 amp relay coming off of the W1209 20 amp relay to switch the 60 amps of heating pads on. Makes me nervous, but I do not yet know why.... I just do not know enough yet.
Per Will's advice, I will install these heaters on some sheet metal and surround the battery to disperse the heat instead of directly on the battery.
THANK YOU in advance if you could give me a nod of approval!
Photos attached below.

You can absolutely use a relay to trigger another relay.

I think its a good thing you have multiple heaters adding up to 60a rather than one large one. I may be wrong but im thinking 750w+ of heat directly to an enclosure is actually a lot and you may end up needing less. Having multiple elements gives you the option of series vs parallel, or leaving one or more out.

Depending on how big the battery enclosure is i might not be comfortable just putting 700w directly into it without supervision, at least the first time.

Vigo said:

You can absolutely use a relay to trigger another relay.

I think its a good thing you have multiple heaters adding up to 60a rather than one large one. I may be wrong but im thinking 750w+ of heat directly to an enclosure is actually a lot and you may end up needing less. Having multiple elements gives you the option of series vs parallel, or leaving one or more out.

Depending on how big the battery enclosure is i might not be comfortable just putting 700w directly into it without supervision, at least the first time.

Click to expand...

Thanks Vigo! You soothed my unfounded worries.

I want someone to prove to me that adding heaters to a battery is worthwhile because I think it's ridiculous.
Surely the charging and discharging of the batteries plus heat from the BMS would keep the batteries above freezing in an insulated container.
Of course I have the opposite problem here in Florida I need to cool the batteries and I am more than happy with that.

Don't do it if you are not sure what you are doing and, with all due respect, I'm not convinced you do.

Few things:-
- 700W sounds way too much. More like 20W - 30W should be more than sufficient with sufficient insulation. @upnorthandpersonal uses 7W IIRC - see https://diysolarforum.com/threads/lifepo4-heating-pad-for-cold-temperatures.5/page-30#post-379481
- 60A would likely drain your battery pretty quick too.
- Your diagram is not clear. I agree with what @Vigo said above; "you can absolutely use a relay to trigger another relay.", but the contacts of one relay will not power another one, as your diagram implies you are connecting it.
- Be careful where you measure the temperature - you don't want to cook the cells before the air around warms up the temp probe.

Quattrohead said:

Surely the charging and discharging of the batteries plus heat from the BMS would keep the batteries above freezing in an insulated container.

Click to expand...

Nope. The batteries themselves stay almost cold at typical solar applications, and unless you're pulling large currents, the BMS stays cold as well (10W dissipation at 100A draw or something - I made the calculation before). Charging currents are already minimal in winter. An insulated box and 7W polyimide heating pad coupled to an aluminium spreader and a basic thermostat does the job.

upnorthandpersonal said:

Nope. The batteries themselves stay almost cold at typical solar applications, and unless you're pulling large currents, the BMS stays cold as well (10W dissipation at 100A draw or something - I made the calculation before). Charging currents are already minimal in winter. An insulated box and 7W polyimide heating pad coupled to an aluminium spreader and a basic thermostat does the job.

Click to expand...

Sounds way more like it.
700w of heating for a battery sounds like a youtube video of "hold my beer and watch this"

THANKS PEOPLE! I am realising now that I calculated the amperage wrong. I kept mistaking the 15w per pad for 15a per pad. oops. SO. each pad is only 1.25 amps in fact, and that means I do not need a 60a midi fuse there, instead I only need a 5amp fuse if I use all 4 pads. I can skip the relay to relay and go straight into the thermostat 20a relay. Apologies for the confusion, my fault, all a part of my learning process.
pads are as pictured below:


and I will wire it up as instructed in this video:

upnorthandpersonal said:

Nope. The batteries themselves stay almost cold at typical solar applications, and unless you're pulling large currents, the BMS stays cold as well (10W dissipation at 100A draw or something - I made the calculation before). Charging currents are already minimal in winter. An insulated box and 7W polyimide heating pad coupled to an aluminium spreader and a basic thermostat does the job.

Click to expand...

Are you only using one of those 7W pads? I'm using 4 of them for a 460Ah battery with great results.

100 Proof said:

Are you only using one of those 7W pads? I'm using 4 of them for a 460Ah battery with great results.

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Yes, only one. I used to have two but it wasn't necessary.

Quattrohead said:

I want someone to prove to me that adding heaters to a battery is worthwhile because I think it's ridiculous.
Surely the charging and discharging of the batteries plus heat from the BMS would keep the batteries above freezing in an insulated container.
Of course I have the opposite problem here in Florida I need to cool the batteries and I am more than happy with that.

Click to expand...

i don't have proof, but the way it makes sense to me is this:

the internal resistance of LiFePO4 cells are generally quite low, like 0.001 Ohm (1 milliOhm)

heater pads are usually like almost 1 Ohm or so.

so it makes sense to me that the cells resistance would not generate as much heat as a pad.

sorry if this actually does not make sense haha

^I think that’s exactly right.

Glad i wasn’t crazy thinking 700w was gonna overheat something even at >0c ambient. Itd have to be huge to ‘lose’ anything close to 700w to need to put 700w in it to keep up.

15w is a lot less dangerous. It wouldn’t be hard for me to believe that a large enclosure would lose >15w of heat continuously at low ambient temp and require multiple heating pads to keep up.

Other than that i think the problem conceptualizing how much heat is required is just based on us tending to think in terms of heating humans for comfort vs ‘lack of death’ which requires us to take 100+ lbs of mass WAY above freezing. When keeing a battery in the 10s of lbs just ‘above’ freezing by a small amount, it takes a fraction of the wattage we’re used to thinking about for anything else.

Considering the lifepo4 are reasonably happy up to 120f, i think the risk of overdoing it with 60w is relatively small, but like upnorth said it’ll probably turn out not to require more than 1 or 2 of the pads in the end to keep the batteries above freezing.

Just educated guesswork though. It rarely goes below freezing here. ?‍

Remember that the battery cells act as a large thermal mass. The reason a small hater works is because you store the heat into this thermal mass. It takes a lot to cool the batteries once they are warm inside an insulated box, and you only need a small heat source to keep them warm. You're not warming air in an empty box.
You could calculate this in the same way you do that for a house - like I did here for mine. Just take the size of the box, R value of the insulation, and temperature delta. This will give you the power required to keep something at temperature.

This is how the calculation looks like:

- Assume a battery box, roughly 0.9 meter by 0.5 meter by 0.4 meter.
- This should give space for 10cm of insulation all around
- Total surface area of the box = 2m²
- Assume insulation, PIR, U value 0.19W/m²K

Total heat loss for the box per K = 0.38W

Assume -20C outside, battery 5C --> delta of 25C. Heating needed to keep the temperature = 25*0.38 = 9.5W.

Considering the battery box itself is also located in an insulated building, the 7W element is fine. If your box is outside, add a second heating pad and you should be fine.