PTC Battery Heater?

Username1

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I want to buy a Dualtron City scooter for year-round use in frigid Canada, so I'm investigating battery heating. The idea is to have the heating entirely powered by, and contained within the removable battery itself. This is because the battery is sealed whereas the compartment isn't (it has drainage holes), and I plan to frequently swap between two batteries.

I'm looking at using a PTC film heater placed directly under the cells. This is because PTC heaters don't need temperature sensing or electronic control, as they self regulate by becoming a resistor past a specified temperature. This makes them very safe and simple.

I think I'll need a film that runs on the battery's nominal 60v, and produces roughly 15-20°c maximum temperature. Is such a film available, and does this seem like a viable idea? I'm not experienced in this so any advice is greatly appreciated.

Dualtron_City_Electric_Scooter_4__11715.1650656360.jpg
 
I've been looking on Alibaba and it seems they're sold as floor heating rolls, cut to your desired size. Most are made for 240v and heat up to 60c.

This is obviously too hot for batteries, but that's assuming 240v which is 4x the battery voltage. However the seller said that at 60v the power output would drop from 240w/m to roughly 16w/m2. I'm not sure if it's a coincidence or not, but that's the square root.

I guess that means the decrease in maximum temperature wouldn't be linear either. Even if it was (providing a usable 15°c), the power output would be too low anyway with only a few watts for the size I need. It seems I'll have to keep looking.

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I found another seller with 110v films available with various surface temperatures. They say the 110v versions have the same temperatures as the 220v options listed. Perhaps one of these would be suitable with the 60v battery, but I still don't have an understanding of how to determine temperature/power when using voltages below the rated value.

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The PTC output will not be affected a lot by voltage in its working range since the change in resistance is huge around the temperature setpoint. An example here:
37736B55-41CC-455E-8265-DCE53946BE96.jpeg
Decreasing voltage from 110V to 60V will halve the initial current and (EDIT) quarter the power but at the temperature setpoint the resistance increases 1000-10000 times so voltage will have small impact on the temperature - as long as the setpoint is reached.

You will need to know that the initial power of the ptc at your voltage is enough to increase your battery temperature enough to reach the thermal setpoint and that depends on the thermal transfer and insulation of the battery casing.

I’d do some assumptions and try it:
It would be easy to try a heating element on a heat spreader (like some 12V20W light bulbs in a box with water inside your casing), put it in the freezer or refrigerator (if that’s a relevant temperature for your minimum outside temps) and check what temperature the water reaches. If it doesn’t reach your ptc setpoint temperature then increase number of bulbs/power or increase insulation until it does. When you reach the setpoint temperature then you know minimum power of the PTC. Try to mimic the layout of your battery so the thermal transfer is similar when setting this up.

For me in sweden and since i ride no matter the temps i would do my first test with freezer at -15 deg C, 15deg C wanted temperature/setpoint with a guesstimate of 60W input (this wattage depends on how much insulation you can get around the battery)

This assumes there is a 15 deg C PTC out there.. don’t get a 40 deg PTC and drive it with lower voltage to get a lower temp because then the stability of the PTC temperature control is no longer there. Under the setpoint temperature it’s basically a resistor.
 
larsb said:
The PTC output will not be affected a lot by voltage in its working range since the change in resistance is huge around the temperature setpoint. An example here:

Decreasing voltage from 110V to 60V will roughly halve the initial current and power but at the temperature setpoint the resistance increases 1000-10000 times so voltage will have small impact on the temperature - as long as the setpoint is reached.

If that's the case, why did they say their 240v film would drop from 240w/m2 to 16w/m2 when using 60v? That's not a linear drop in power as you're describing.


larsb said:
This assumes there is a 15 deg C PTC out there.. don’t get a 40 deg PTC and drive it with lower voltage to get a lower temp because then the stability of the PTC temperature control is no longer there. Under the setpoint temperature it’s basically a resistor.

Does lower voltage have any effect on the maximum temperature, or is it just too tiny to matter? I guess either way I need to select a model with the temperature I intend to use.

The lowest temperature I've found so far is the 28c 110v model listed in the chart above. That's not ideal but could be used. It would consume more power than a 15-20c heater, and I'd need an off switch so it doesn't continually draw power when sitting inside at room temperature.
 
Does lower voltage have any effect on the maximum temperature, or is it just too tiny to matter? I guess either way I need to select a model with the temperature I intend to use.

Sure, voltage(=power) will matter how hot it gets below the transition temperature, then it will find a thermal balance at a temperature where power in=power out which will differ more depending on temperature. It might work but you will miss the biggest benefit of using a PTC (foolproof&simple). I'd add a thermostat for 15 or 20 deg C if you go with the 28 deg C foil.
 
I've realized these PTC films have power densities that are way too low. I would need a non-film solid PTC heater, which either transfers heat directly (surface type), or by using a fan. These are either too bulky or have the wrong specifications off the shelf. I'd need to use a conventional non-PTC heating pad with electronic control.

It's too bad manufactures don't include heated batteries from the factory. It wouldn't take much to insulate the battery case and add a safe PTC heater. Then again, many refuse to even water proof their products, so what can you expect?
 
It’s probably not that easy since the standard PTCs seem to have min 60 deg C transition temperature. It’s likely that the 28deg C foil you found is actually a 60 deg C foil that’s underdriven (so to say).

If the power density is too low then you could use multiple foil layers?

I've used silicon heating mat with thermostat. I think it was "Keenovo" site on aliexpress i ordered from.

https://keenovo.store/
 
Username1 said:
I've realized these PTC films have power densities that are way too low. I would need a non-film solid PTC heater, which either transfers heat directly (surface type), or by using a fan. These are either too bulky or have the wrong specifications off the shelf. I'd need to use a conventional non-PTC heating pad with electronic control.

It's too bad manufactures don't include heated batteries from the factory. It wouldn't take much to insulate the battery case and add a safe PTC heater. Then again, many refuse to even water proof their products, so what can you expect?

Thermal insulation would not really be a desirable thing in warm weather, it would trap the heat in.
You can get all sorts of different heater elements and materials. You could wire 5x 12v heaters in series and wind them into your battery.
Another option would be an aluminum pcb with some resistors soldered on to make heat. (This picture is for my sla printer to heat the resin vat.)
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When the heat isn't needed the aluminum could help dissipate battery heat. You could probably use a thermal snap disc to regulate temps and keep it from overheating. The battery drain and when it turns on stuff would have to be figured out.
 
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