Given the massive surface area on the bottom of the car, I'd be surprised if it were well insulated.
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Regen limited due to cold today
- Thread starter cinergi
- Start date
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- Driving Dynamics Model S
Given the massive surface area on the bottom of the car, I'd be surprised if it were well insulated.
Why? Insulation is cheap and it wouldn't have to be very thick to do a lot of good.
Insulated or not, it still doesn't make sense to me that regen would be just limited (as opposed to completely on/off) when cold. And what really doesn't make sense is that it's limited at just over 40 deg F! This makes it very impractical. Roadster behavior is much better. I'm not doubting Tesla's engineering just yet. I'm just puzzled. Maybe they're making some tweaks for a future software update.
It's only 4" thick, and there's a lot of surface area. There isn't a lot of room for insulation. It's bound to lose some heat.
I hope Tesla does not decide to warm the battery pack whenever the car is plugged in, that would waste lots of energy. Making it easy to charge just before driving is a much better solution. This doesn't help if the car isn't plugged in, but in that case spending energy to improve regen makes no sense either, IMO.
strider
Active Member
Keep in mind that there are other reasons for regen to be limited like traction for instance. My regen is limited (no light on the dash but not getting full regen) on chilly mornings headed down my hill. The car is garaged and pack is around 20C when I leave so it's not due to a cold pack. Having a heater (the best way to dump a bunch of electricity) to provide a dummy load just adds unnecessary complexity and won't solve the problem of reduced regen due to reduced traction. It's just not that big of a deal.
Now hsharp's complaint that Tesla is being too conservative in regen'ing into a cold battery is different and we can talk about the pros and cons of that.
Now hsharp's complaint that Tesla is being too conservative in regen'ing into a cold battery is different and we can talk about the pros and cons of that.
strider
Active Member
It absolutely does. That was my point. The Roadster automatically limits regen when traction is limited and so you cannot always count on full regen all the time as it can be limited for a variety of reasons. When you're coming up to a stop sign or whatever you lift off the accelerator and cover the brake. If the car doesn't slow as quickly as you'd like under regen then you press the brake pedal. It's really intuitive when you're doing it.
I take it for granted that this is good engineering by Tesla given it probably preserves energy and more importantly battery life (most critical given battery costs).
My biggest issue is the lack of communication/education on how this works. My wife was very freaked out by the non-trivial change in driving dynamics as she abruptly lost regen one cold morning and then just as abruptly gained it back as the pack warmed. You want a car to drive consistently and if it can't, give you a clear indication when it changes (we eventually figured out what the dotted line means, but it wasn't immediately obvious given no previous communication on the topic or experience with a roadster).
Needs to be messaged clearly by the delivery specialists and probably should give a clearer message at start-up on one of the screens when regen is limited.
My biggest issue is the lack of communication/education on how this works. My wife was very freaked out by the non-trivial change in driving dynamics as she abruptly lost regen one cold morning and then just as abruptly gained it back as the pack warmed. You want a car to drive consistently and if it can't, give you a clear indication when it changes (we eventually figured out what the dotted line means, but it wasn't immediately obvious given no previous communication on the topic or experience with a roadster).
Needs to be messaged clearly by the delivery specialists and probably should give a clearer message at start-up on one of the screens when regen is limited.
JRP3
Hyperactive Member
I think that actually only happens below freezing. However there is another reason to limit regen at temps above freezing, cell voltage. Colder temps mean the internal cell resistance is higher, so current going into the cell will drive the voltage higher than when it's warm, and the higher voltage can cause electrolyte solvent breakdown.
strider
Active Member
That's a good point. On the Roadster there's a light on the dash that's very obvious. When I say it for the first time I stopped, pulled out the manual, and read about it. After that it wasn't a surprise. Seems like it's much more subtle in Model S.
I think this is an extremely important real and perceived safety issue and should be addressed. Tesla Engineers just need to fix it.
mknox
Well-Known Member
While probably wishful thinking, it seems an ideal solution would be to switch the regen from batteries to some sort of resistive heating element in the HVAC system when the batteries can't take the charge due to cold. It seems a waste to throw the energy away as brake heat, and it would resolve the issue of regen braking characteristics changing.
mnx
2018 M3P
60kW is a lot of energy just to dump somewhere other than the battery charging circuit... Don't think of it as wasted energy on a cold morning, thinking of it as giving your brakes the necessary exercise they need to prevent glazing.
JRP3
Hyperactive Member
Seriously this is not going to be "engineered" away. Better education and warnings are what is needed until a better battery chemistry is used that can take the regen when cold.
No kidding. To put this in perspective, the average electric baseboard heater is one kilowatt. You could dissipate it in a resistive heater but it would need massive heat sinking or it would go up in flames. You'd probably have to distribute it around the battery pack to take advantage of its cooling. Wouldn't be worth all the extra weight and complexity.
I don't think it is that bad: after all, the amount of heat is exactly the same as the heat to be dissipated in the friction brakes if regen is disabled. So a resistance heater in the wheel-well (cooled by the air after it has passed through the radiators) would leave the heat distribution exactly as it is now. Involving the battery cooling is probably unhelpful - to get rid of that power, the output air temperature is going to have to be much higher than anything in the battery system wants to see.
Also worth noting that "rheostatic braking" is widely used in other electric traction applications such as railway locomotives (where the objective is to save the wear on the brake pads) with much higher power ratings involved - but of course those applications are less constrained by weight and space.
So, it's perfectly feasible but as you say probably not worth it.
JRP3
Hyperactive Member
Possibly they could program the brakes to come on when lifting off the A pedal if regen is limited to keep the same feel. Might be complicated but certainly possible.
