Cold Weather Regen Suggestion

Cold Weather Regen Suggestion

It has been noted that when cold, the regen does not work because the battery cannot take the charge till it reaches a certain temperature. This can surprise the driver, and even if the driver expects it, is an operating inconsistency that the car could do without.

It seems to me that a simple solution under these conditions would be to direct the power from motor braking to a simple heating element. This has two advantages. One is that the accelerator pedal would work the same under all conditions, providing "regen" braking even when cold. The second advantage is that this heating element could be thermally connected to the battery coolant circuit, so the battery would warm faster in initial cold conditions. Some heat would be generated just getting out of the typical parking lot.

A heating element, basically a resistor like that found on an iron, is a very cheap part that would not add much to manufacturing cost. There is some R&D involved for the integration and software, but this seems like a simple yet helpful upgrade for cold regions, improving both driving experience and battery performance.

mthanos | April 9, 2013

I completely agree. Why waste that energy heating up and wearing out the brake pads and rotors when it could be used to heat up the battery to optimal charging temperature with no waste whatsoever? Resistance coils can be very light and very powerful just like in your typical water heater. Just need to make sure the coils stay covered or they will overheat. But the batteries have the exact same issue and Tesla has worked that out.

Joel N. Weber II | April 9, 2013

If you're concerned about heating the battery pack on cold days only, just building the heating element into the battery pack might be the optimal solution. However, if you want regen to work with max range charge on a hot summer day, you need another approach.

It's worth noting that a standard diesel locomotive on a typical railroad has some approximation of regenerative braking, but with a resistor array that converts all the energy from braking to waste heat, so the resistor array for wasting energy from regenerative braking is probably a technology more than half a century old.

danielccc | April 9, 2013

Yup, the diesel locomotive braking is where I got the idea from. I claim no originality.

The reason the heating element should not be built into the pack is precisely so it can be used for max range charge situations as well. In that case, you would just radiate the heat, rather than send it to the battery.

I also think that a very hot metal element should not be inside the pack for safety reasons, plus the thermal control engineering in the pack is already quite developed and it would probably be costly to modify it.

Go_Peddle_4_me | April 10, 2013

Or better yet, build the element into the coolant system and have a cut-off switch when the battery temp stabilizes to allow full regen to take place. That way in the summertime the heating coils wouldn't even heatup. All the energy would go to regen purposes.

Since the coolant system must have some type of thermostat to regulate the battery temp, all you'd need to do is shunt the braking regen to the coolant heaters until they are stabilized.

jat | April 10, 2013

Do you realize how much heat would be generated by dissipating 60kW into a resistor? That is the equivalent of 600 100W lightbulbs, and would be .3MJ in only 5 seconds.

Personally, I think a better approach would be to blend in friction braking to make up for the lost regen, but I doubt that is going to happen since one of the benefits of the current system is that the hydraulic brake system is completely conventional. (People that argue for one-pedal driving should really be arguing for doing away with the brake pedal and having friction brakes apply as well when you lift your foot completely off the pedal).

BTW, it doesn't disable regen, it only limits it.

danielccc | April 10, 2013

Yeah, it's 40 hair dryers or 10 Tesla cabin heating elements. That's why I think it's worth exploring. That's significant energy going to waste in winter.

It gets a little trickier in summer with max charge. Still, though significant, it is also brief. An aluminum heat sink, properly ventilated, could handle it. Your 0.3 MJ would bring 3kg of aluminum to maybe 150 degrees C, starting from 35 C (a hot day). That's hot, but not red hot.

I am guessing the existing cooling system could be modified to handle the extra load.

Definitely some engineering is involved, but if the numbers work, it would be cheap to include.

gianni.terragni | April 10, 2013

I am completely in agreement, also because I proposed this solution in an old blog more than six months ago
But then the blog is off. I'm glad that you have once again raised the issue that I think is very interesting

jat | April 10, 2013

@danielccc - but I'm not sure I want that much energy going into the battery that fast. Carrying around some thermal mass to use it more slowly seems like worse than just having a battery to send it to directly.