I bet you didn't. I'll bet you're thinking right now, "no, that's nonsense, that can't be right, let me get in this thread and tell this idiot off". Or maybe "what makes you think that, where'd you get that data?". Perhaps "no, that's just because DC charging causes the battery to heat up"? Maybe even "Tesla knows best, you're thinking too hard about this". Finally, "what's it matter?". Well, those are all reasonable thoughts from reasonable people that once tread here.
Let me break down these thoughts to help make this long post more digestable.
**First, an update**
Since writing this, a few points have been made. Yes, this is Scan My Tesla. I have seen cases of information being hidden to "Solve" the problem via obscurity, so not saying how I got the info is to protect the access to that info. Second, it doesn't heat constantly - it can eventually decide to stop heating - but there is a very hard-to-reach threshold of heating that it finally reached after a long period of heating (often the entire session). Third, it still stands, especially at slower DC charging stations and arguably at 72kW SCs, that this heating does more harm than good by wasting energy and becoming less efficient than a gasser (in extreme slow DC charging cases).
**We now resume the original post, unedited**
No, that's nonsense, that can't be right
Sadly, it appears that Tesla's battery heating logic is quite simple - "if DC charging, run the battery heater". At least, from the sessions I've run at various existing battery temps and with both CHAdeMO and Supercharging, at 150kW, 72kW, and at 40kW (CdM), while logging data and observing.
What makes you think that, where'd you get this data?
How'd I find this? I hesitate to reveal my methods (though they are publicly available), by the fact that this complaint/observation isn't already documented (I tried searching, believe me), and just like white-hat hacking, revealing the method can expose undesired side-effects - like disabling that access completely, or (in the case of hacking) letting n'er-do-well's do bad things. The data I see, though, correlates closely with one another with multiple sources (e.g. motor power -> cooling loop mode -> inlet temperature -> cell temperatures), such that I can say with fair certainty that this is what's going on, though admittedly I don't have a LARGE number of samples; I'd be interested if anyone with the logging tools to also correlate or refute my findings.
So, the battery heater in the Model 3 is achieved through the motor/controller, by stalling it and making it emit waste energy but not turning. Clever trick. When the battery heating function is active, I can see the motor gets quite hot (181F is the hottest I found it, I think), and that follows into the battery inlet coolant temperature (being hotter than the hottest cell by a few degrees, meaning it's heating the pack), and in turn, the cell temperature rises.
That's just because DC charging causes the battery to heat up
You can tell for absolute certain that it's heating the pack, not cooling it, by the inlet temperature always being higher than the cells. Also, the motor heats up (quite significantly) while parked and "off", which means the heater is active. You can also see this happening while "conditioning battery for Supercharging" while navigating to a Supercharger - it starts the heater "early" and continuously heats from the moment you start navigation until you're done charging. We're talking about heating a 110 degree (F) pack, not a 40 degree pack.
Shockingly, this heating consumes 4kW of energy, constantly. In a 40kW (100A limit x 400V max batt V) CHAdeMO station, that means you only get ~34kW of input to the battery, because the motor is consuming the rest of it, needlessly heating the battery. You effectively get a 10% tax on your energy/time bill for owning a Tesla and heating the battery, endlessly (possibly until 55C, a number found in a Tesla patent during research on this).
It's also quite possible that this discrepancy - the battery heating - is partly responsible for the discrepancy between the Supercharger vs. the in-car billing that was noted earlier, that Tesla later refunded for many owners in bulk. The SC dispenses a certain amount of energy, but the battery logs less than that full amount of energy entering the battery, so the in-car display is based on "kWh into pack" while the station measures "kWh into car".
Tesla knows best, you're thinking too hard about this
There's something to be said for "Tesla knows best", and "the battery is probably happier to charge warm", but not at these extremes, with no control logic thinking "well... maybe we ought to not heat the pack when it's not peaking cell voltages", or "well... maybe it doesn't need heating if it's within human-happy temperatures". But again, I'm seeing it heating the battery well past 110 F, and yet it keeps the heater running. On a Leaf, this would be darn near 11 or 12 temp bars, if I recall correctly. Except the car's *actively* doing this to itself.
I recently read a bit about lithium battery degradation - the mechanisms of degradation, or at least the primary ones for voltage abuse, are electro-plating (ripping apart the copper materials) at low voltages, and acidic reaction (ripping apart the separator material) at high voltages and high temperatures. There's also cycle life, as lithium ion migration punches little molecular holes in the material with each cycle. Knowing these mechanisms, it seems terribly counterproductive to punch the voltage higher (causing voltage/temperature-based acidic decay) while charging, though it does indeed move molecules around faster (thus allowing faster charging rates) for high-current Supercharging.
There just seems to be no need to do this at lower speed charging, like 72kW charging or ultra-sluggish CHAdeMO rates. The battery is basically just being cooked that way.
Oddly, right after you're done charging and unplug it, the temp parameters go back to normal, the heater turns off, and the active cooling kicks in (if it's overheated the pack by its blind heating, you'll hear the radiator fan turn into a vacuum cleaner). So it cools right back off after it's done in the oven, throwing away all that energy.
What's it matter?
Well, if Tesla's mission is sustainable energy, then an unnecessary energy tax is, at bare minimum, undesirable and avoidable waste. Despite their best efforts to the contrary, many people still can't have home charging - particularly those in apartments or other shared housing (#bayarea). In particular, I've never had home charging in my one-car-one-EV-household since 2014 (briefly had it from 2013-2014, though). So, many people use Supercharging regularly. In all but the fastest V3 stations, this unwanted battery heating takes away from peak energy, and even at the fastest charging (250kW, but let's say 200kW for realism), 2% of that being "loss" ends up being 4kW, which is exactly the same as what's being input as battery heater energy - so why would it even be necessary? It's a bit self-defeating, and has a narrow band of utility that's disputable from both directions - useful or not useful - but seems to bias toward "not useful" to me.
It's also potentially harmful to the batteries, given the nature of temperature-based high-voltage degradation (a fully charged, hot battery - especially under charging load where voltage distribution may be internally uneven).
So why do it? Is this just an overlooked side-effect of "welp it worked once, ship it" and then forgetting about filling in additional logic for other cases (e.g. the heater was originally 2.5kW, I think I read elsewhere)? Other thoughts?
Let me break down these thoughts to help make this long post more digestable.
**First, an update**
Since writing this, a few points have been made. Yes, this is Scan My Tesla. I have seen cases of information being hidden to "Solve" the problem via obscurity, so not saying how I got the info is to protect the access to that info. Second, it doesn't heat constantly - it can eventually decide to stop heating - but there is a very hard-to-reach threshold of heating that it finally reached after a long period of heating (often the entire session). Third, it still stands, especially at slower DC charging stations and arguably at 72kW SCs, that this heating does more harm than good by wasting energy and becoming less efficient than a gasser (in extreme slow DC charging cases).
**We now resume the original post, unedited**
No, that's nonsense, that can't be right
Sadly, it appears that Tesla's battery heating logic is quite simple - "if DC charging, run the battery heater". At least, from the sessions I've run at various existing battery temps and with both CHAdeMO and Supercharging, at 150kW, 72kW, and at 40kW (CdM), while logging data and observing.
What makes you think that, where'd you get this data?
How'd I find this? I hesitate to reveal my methods (though they are publicly available), by the fact that this complaint/observation isn't already documented (I tried searching, believe me), and just like white-hat hacking, revealing the method can expose undesired side-effects - like disabling that access completely, or (in the case of hacking) letting n'er-do-well's do bad things. The data I see, though, correlates closely with one another with multiple sources (e.g. motor power -> cooling loop mode -> inlet temperature -> cell temperatures), such that I can say with fair certainty that this is what's going on, though admittedly I don't have a LARGE number of samples; I'd be interested if anyone with the logging tools to also correlate or refute my findings.
So, the battery heater in the Model 3 is achieved through the motor/controller, by stalling it and making it emit waste energy but not turning. Clever trick. When the battery heating function is active, I can see the motor gets quite hot (181F is the hottest I found it, I think), and that follows into the battery inlet coolant temperature (being hotter than the hottest cell by a few degrees, meaning it's heating the pack), and in turn, the cell temperature rises.
That's just because DC charging causes the battery to heat up
You can tell for absolute certain that it's heating the pack, not cooling it, by the inlet temperature always being higher than the cells. Also, the motor heats up (quite significantly) while parked and "off", which means the heater is active. You can also see this happening while "conditioning battery for Supercharging" while navigating to a Supercharger - it starts the heater "early" and continuously heats from the moment you start navigation until you're done charging. We're talking about heating a 110 degree (F) pack, not a 40 degree pack.
Shockingly, this heating consumes 4kW of energy, constantly. In a 40kW (100A limit x 400V max batt V) CHAdeMO station, that means you only get ~34kW of input to the battery, because the motor is consuming the rest of it, needlessly heating the battery. You effectively get a 10% tax on your energy/time bill for owning a Tesla and heating the battery, endlessly (possibly until 55C, a number found in a Tesla patent during research on this).
It's also quite possible that this discrepancy - the battery heating - is partly responsible for the discrepancy between the Supercharger vs. the in-car billing that was noted earlier, that Tesla later refunded for many owners in bulk. The SC dispenses a certain amount of energy, but the battery logs less than that full amount of energy entering the battery, so the in-car display is based on "kWh into pack" while the station measures "kWh into car".
Tesla knows best, you're thinking too hard about this
There's something to be said for "Tesla knows best", and "the battery is probably happier to charge warm", but not at these extremes, with no control logic thinking "well... maybe we ought to not heat the pack when it's not peaking cell voltages", or "well... maybe it doesn't need heating if it's within human-happy temperatures". But again, I'm seeing it heating the battery well past 110 F, and yet it keeps the heater running. On a Leaf, this would be darn near 11 or 12 temp bars, if I recall correctly. Except the car's *actively* doing this to itself.
I recently read a bit about lithium battery degradation - the mechanisms of degradation, or at least the primary ones for voltage abuse, are electro-plating (ripping apart the copper materials) at low voltages, and acidic reaction (ripping apart the separator material) at high voltages and high temperatures. There's also cycle life, as lithium ion migration punches little molecular holes in the material with each cycle. Knowing these mechanisms, it seems terribly counterproductive to punch the voltage higher (causing voltage/temperature-based acidic decay) while charging, though it does indeed move molecules around faster (thus allowing faster charging rates) for high-current Supercharging.
There just seems to be no need to do this at lower speed charging, like 72kW charging or ultra-sluggish CHAdeMO rates. The battery is basically just being cooked that way.
Oddly, right after you're done charging and unplug it, the temp parameters go back to normal, the heater turns off, and the active cooling kicks in (if it's overheated the pack by its blind heating, you'll hear the radiator fan turn into a vacuum cleaner). So it cools right back off after it's done in the oven, throwing away all that energy.
What's it matter?
Well, if Tesla's mission is sustainable energy, then an unnecessary energy tax is, at bare minimum, undesirable and avoidable waste. Despite their best efforts to the contrary, many people still can't have home charging - particularly those in apartments or other shared housing (#bayarea). In particular, I've never had home charging in my one-car-one-EV-household since 2014 (briefly had it from 2013-2014, though). So, many people use Supercharging regularly. In all but the fastest V3 stations, this unwanted battery heating takes away from peak energy, and even at the fastest charging (250kW, but let's say 200kW for realism), 2% of that being "loss" ends up being 4kW, which is exactly the same as what's being input as battery heater energy - so why would it even be necessary? It's a bit self-defeating, and has a narrow band of utility that's disputable from both directions - useful or not useful - but seems to bias toward "not useful" to me.
It's also potentially harmful to the batteries, given the nature of temperature-based high-voltage degradation (a fully charged, hot battery - especially under charging load where voltage distribution may be internally uneven).
So why do it? Is this just an overlooked side-effect of "welp it worked once, ship it" and then forgetting about filling in additional logic for other cases (e.g. the heater was originally 2.5kW, I think I read elsewhere)? Other thoughts?
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