120V Charging Battery Temperature Target

[GZDongles]

I noticed some strange charging behavior recently. Because I am a terrible procrastinator, I have not yet installed my 6-20 outlet in my garage and am relying on a 5-15 (which gives me 5 rated mph charging).

It's my first winter with the car, so this might be normal behavior. I have charged several times at temperatures in the 20-30°F range. Until today, I got the full 5 mph charge with no battery heating required, even with the blue snowflake icon present. For some reason, my battery has been heating for the last hour when I plugged in this morning (and I've been getting zero miles per hour charge). When I noticed the battery preheating (apparently at a slow rate because I was not losing any rated miles), I preconditioned the cabin manually to try to heat faster. I did this for about 20 minutes. However, the charge rate is still 0mph after over an hour of manual and automatic preheating.

I checked the overnight temperatures, and they were in the mid 20s all night. What gives? Does anyone know what the battery target temperature is for trickle charging? It seems that SMT says the battery target temperature is 50°F right now! I have seen the target temperature jump around a ton in SMT, but does anyone have any good rules of thumb? Heating the battery to 50°F to charge at 12A seems like a giant waste, but maybe I'm missing something. Also, from reading other threads it seems like most people are able to charge at 120V 12A with little to no preheating down to 0-10°F.

I'm not particularly worried about it, but I am curious! I find that my car almost always preheats the battery to charge at 5kW and above, but the preheating for 120V 12A (1.4 kW) charging is new behavior.

 

[UncleCreepy]

That's normal, and it's also the reason why using a 5-15 outlet is not feasible in colder climates: a huge chunk if not all of the abailable energy is being used just to heat the battery.

Your best option is probably to get a HPWC (up to 11.5 kW) or at least a NEMA 14-50 (up to 7.68 kW) outlet installed asap.

 

[GZDongles]

That's normal, and it's also the reason why using a 5-15 outlet is not feasible in colder climates: a huge chunk if not all of the abailable energy is being used just to heat the battery.

Your best option is probably to get a HPWC (up to 11.5 kW) or at least a NEMA 14-50 (up to 7.68 kW) outlet installed asap.

Do you know why this is a sudden change though? I've charged the car when it's been cold soaking in 20-30°F weather before at the same SoC with no preheating required, and preheating at this temperature range doesn't seem to match the experience of others on this forum and elsewhere...

 

[Brock-WI]

Yes get at least that 6-20 installed asap, it will only get colder for a month or two here. Also leave it plugged in as much as you can. Are you in the UP or down under the bridge?

I believe what happens is once you hit a certain temp it starts to try to warm up the battery. These temps are a wild guess but lets say the battery is at 31F, it might be happy to charge at up to 3kw without heating, but then once it hits (again a guess) 30F it decides to try to warm the battery to 50F before doing anything. You would think that once it is 35F it would be fine, but it seems to keep trying to warm it up further. Basically once it starts it wants to reach a "warm" temp instead of cutting off at an "ok" to charge temp. I believe there is more involved in this calculation to heat the pack than just the temp pack but haven't figured it out yet.

Oh and you may have done it this way but to heat the battery you have to turn climate on via the app and then I believe it only heats the battery if your above 20% or so. But being plugged in to 120vac and the car knowing that might also tell it to not heat the battery? If you have SMT you can see the stator running pulling about 3.5 kw and you can also hear the motor whining or whirring as it is heating up.

 

[UncleCreepy]

Do you know why this is a sudden change though? I've charged the car when it's been cold soaking in 20-30°F weather before at the same SoC with no preheating required, and preheating at this temperature range doesn't seem to match the experience of others on this forum and elsewhere...

I have only tried it once when a destination charger was unavailable and a 5-15 was all I could get. The first hour or so I got 0 km/h which indicates battery heating. Unfortunately I don't remember the temperature.

It's nothing but a wild guess, but if others have reported a different behaviour, it may have been at higher temperatures or perhaps a software upgrade changed things.

Either way, since the car doesn't sleep while charging, thus drawing ~250W, it is pretty wasteful to charge at low power. Even without preheating, more than 17% of the energy is used just to keep the car awake. On a 240 V/48 A charger the loss is only about 2%.

 

[GZDongles]

Yes get at least that 6-20 installed asap, it will only get colder for a month or two here. Also leave it plugged in as much as you can. Are you in the UP or down under the bridge?

I believe what happens is once you hit a certain temp it starts to try to warm up the battery. These temps are a wild guess but lets say the battery is at 31F, it might be happy to charge at up to 3kw without heating, but then once it hits (again a guess) 30F it decides to try to warm the battery to 50F before doing anything. You would think that once it is 35F it would be fine, but it seems to keep trying to warm it up further. Basically once it starts it wants to reach a "warm" temp instead of cutting off at an "ok" to charge temp. I believe there is more involved in this calculation to heat the pack than just the temp pack but haven't figured it out yet.

Oh and you may have done it this way but to heat the battery you have to turn climate on via the app and then I believe it only heats the battery if your above 20% or so. But being plugged in to 120vac and the car knowing that might also tell it to not heat the battery? If you have SMT you can see the stator running pulling about 3.5 kw and you can also hear the motor whining or whirring as it is heating up.

Thanks I think you're right. The battery heated up to 60°F(!) before directing all the energy to charging. Perhaps it sets the target so high because it's more efficient to heat in one go and let it cool over a matter of hours rather than heat slowly many times throughout the charging process.

As you all have said, I think I need to bite the bullet and get my 6-20 installed this weekend. Unfortunately I'm on a 100A panel so I don't have a lot of flexibility to install a 14-50 or a HPWC, but a 6-20 is perfect for the amount I drive. I'm down in Ann Arbor, so I don't have to deal with super cold temperatures that often!

 

[Gasaraki]

Thanks I think you're right. The battery heated up to 60°F(!) before directing all the energy to charging. Perhaps it sets the target so high because it's more efficient to heat in one go and let it cool over a matter of hours rather than heat slowly many times throughout the charging process.

As you all have said, I think I need to bite the bullet and get my 6-20 installed this weekend. Unfortunately I'm on a 100A panel so I don't have a lot of flexibility to install a 14-50 or a HPWC, but a 6-20 is perfect for the amount I drive. I'm down in Ann Arbor, so I don't have to deal with super cold temperatures that often!

60 F is not high when the car heats up the battery to 40 C to supercharge.

 

[GZDongles]

60 F is not high when the car heats up the battery to 40 C to supercharge.

True, it's just high compared to 30F where the battery charges at 1.4 kW with no preheating that I've experienced before. As others have pointed out though, 120V definitely isn't an efficient long term charging solution!

 

[Rocky_H]

When I noticed the battery preheating (apparently at a slow rate because I was not losing any rated miles), I preconditioned the cabin manually to try to heat faster.

D'oh! You're shooting yourself in the foot there and probably making it heat slower. If you're trying to charge, it's already trying to heat the battery. By turning on the cabin heating, that's just adding an extra thing it has to try to use energy for, which it already doesn't have enough of. I'm not sure if it would prioritize cabin heating energy over the battery heating and slow that down or not. But doing that certainly cannot help.

 

[Sophias_dad]

Is there some reason you wait for the battery to get cold-soaked before plugging in? If you plug in right after a drive you'll probably have a much better charging experience.

 

[Rocky_H]

Is there some reason you wait for the battery to get cold-soaked before plugging in? If you plug in right after a drive you'll probably have a much better charging experience.

There is a lot to be said for this. Plenty of people do just fine using only regular 120V outlets for charging, but not the way you are thinking of, @GZDongles. Plugging in a cold car on that low of a power source and then staring at the car for the next 5-10 minutes, tapping your foot, impatiently waiting for the car to charge is not going to work and will just frustrate you. If you're going to use 120V charging, you need to accept the idea that the car is going to spend half its life charging, and you should get that going as soon as you can and leave it going that way as long as you can.

 

[camalaio]

I really gotta finish my Tesla guide to the cold.

You might see a lot of people that plug in in at 10F and it starts charging right away, yes. That's because their battery was warm enough, probably from their drive - outside temperature and battery temperature can be very different things. Take most claims with a grain of salt, because a lot of context is missing. This might even be why you don't normally see it - perhaps due to driving/location habits, when charging off 120V, you happen to have a warmer battery.

The snowflake appears when 2.0% of your battery capacity is not extractable due to the cold. This effect varies with temperature though; at 15C on a high SoC battery (say, 90%) you might only be 0.5% down, but at the same temp at a lower SoC (say, 20%) it might be 2.0% down and show the snowflake. So the snowflake isn't actually a great temperature indicator, more of a somewhat-impacted-by-the-temperature-right-now indicator.

When heating is required for 120V charging, it's a losing battle. About 1.1kW is available for battery heating. That's less than a little plug in space heater in your house, but the car needs to heat a 1000lb battery with a lot of thermal mass instead of just air and some furniture. And if it's doing that in a poorly insulated area (or outside), it may never get warm enough to switch to charging. This is why 240V charging is effective: raise the temp faster, switch to charging faster (and charge quickly while it's still warm so it doesn't have to heat again!), and be done with it so the temps can settle back down. It's a win in every way.

As for the SMT numbers, those are a bit misleading. The target battery temperature is sort of, sometimes, the coolant inlet temperature target. It's hard to know via SMT what the "actual" temperature targets are, because you're only able to see the "how" target and not the actual target.

---

If you want to see "giant waste" regarding battery heating, look through my post history on low-power (10kW, 25kW) CHAdeMO charging. I'm still mad about that.


EDIT: So many typos.

 

[GZDongles]

I really gotta finish my Tesla guide to the cold.

You might see a lot of people that plug in in at 10F and it starts charging right away, yes. That's because their battery was warm enough, probably from their drive - outside temperature and battery temperature can be very different things. Take most claims with a grain of salt, because a lot of context is missing. This might even be why you don't normally see it - perhaps due to driving/location habits, when charging off 120V, you happen to have a warmer battery.

The snowflake appears when 2.0% of your battery capacity is not extractable due to the cold. This effect varies with temperature though; at 15C on a high SoC battery (say, 90%) you might only be 0.5% down, but at the same temp at a lower SoC (say, 20%) it might be 2.0% down and show the snowflake. So the snowflake isn't actually a great temperature indicator, more of a somewhat-impacted-by-the-temperature-right-now indicator.

When heating is required for 120V charging, it's a losing battle. About 1.1kW is available for battery heating. That's less than a little plug in space heater in your house, but the car needs to heat a 1000lb battery with a lot of thermal mass instead of just air and some furniture. And if it's doing that in a poorly insulated area (or outside), it may never get warm enough to switch to charging. This is why 240V charging is effective: raise the temp faster, switch to charging faster (and charge quickly while it's still warm so it doesn't have to heat again!), and be done with it so the temps can settle back down. It's a win in every way.

As for the SMT numbers, those are a bit misleading. The target battery temperature is sort of, sometimes, the coolant inlet temperature target. It's hard to know via SMT what the "actual" temperature targets are, because you're only able to see the "how" target and not the actual target.

---

If you want to see "giant waste" regarding battery heating, look through my post history on low-power (10kW, 25kW) CHAdeMO charging. I'm still mad about that.


EDIT: So many typos.

Thank you for taking the time to type out this informative reply! Clearly there are a lot of nuances with both the SMT data as well as the charging restrictions that vary based on temperature SOC, and maybe other factors. This is probably making it a fool's errand to try to compare two different charging sessions or go off of the experience of others.

I drive so little that I was hoping I might get away with using a 5-15 for the long term, but it's become clear that I need to get at the very least a 6-20. My driving is sporadic enough that I might drive 15 miles one week and 500 the next, so I should bias my charging setup to handle more of the edge cases.

 

[nayr14]

I'm not sure what you situation is, but if you're creating a new run for a 6-20, I'd try to go for 240v charging. 5-15 to 6-20 is only a small step forward. Not only will it charge faster but it will be more efficient since charging has electrical overhead (car is on, pumps are running, etc), so less time charging is typically better. It will also make your garage more EV future proof (especially if it's a two car garage).

Most folks recommend the 14-50, but you're wasting $$$ and conduit space running a neutral. Go with a 6-50 (two hots, ground) and in the future you can upgrade to the Tesla HPWC or another EV charger. None of them require a neutral now, and they won't require one in the future.

 

[camalaio]

Thank you for taking the time to type out this informative reply! Clearly there are a lot of nuances with both the SMT data as well as the charging restrictions that vary based on temperature SOC, and maybe other factors. This is probably making it a fool's errand to try to compare two different charging sessions or go off of the experience of others.

I drive so little that I was hoping I might get away with using a 5-15 for the long term, but it's become clear that I need to get at the very least a 6-20. My driving is sporadic enough that I might drive 15 miles one week and 500 the next, so I should bias my charging setup to handle more of the edge cases.

Nuances indeed! Turns out cars are complicated.

When my wife had a 120km (75mi) daily commute, the standard 120V NEMA 5-15 was actually (barely) enough for us in summer. We got a NEMA 14-50 setup before our first winter so I don't have this experience first-hand, but there is no way we would've made it through the winter on the 120V setup even with the car in the garage. I found out the hard way that 120V wasn't even enough for a 5km daily commute in the cold.

As a result, I used to think 120V charging was almost universally sufficient. I now know it's not necessarily, and even if it is, it's very inefficient. You might genuinely save money in the long run by going to a 6-20 outlet. It's made for some awkward backtracking on discussion with friends and coworkers (especially since this is a Tesla problem more than a general EV problem).

I'm not sure what you situation is, but if you're creating a new run for a 6-20, I'd try to go for 240v charging. 5-15 to 6-20 is only a small step forward. Not only will it charge faster but it will be more efficient since charging has electrical overhead (car is on, pumps are running, etc), so less time charging is typically better. It will also make your garage more EV future proof (especially if it's a two car garage).

Most folks recommend the 14-50, but you're wasting $$$ and conduit space running a neutral. Go with a 6-50 (two hots, ground) and in the future you can upgrade to the Tesla HPWC or another EV charger. None of them require a neutral now, and they won't require one in the future.

6-20 is 240V. But going with 6-15, existing wires/conduit could be re-used I believe. I'm just not sure if that's a relevant thing to install in the year 2021, nor if that's up to code. If it is, it's probably the cheapest way to more than double the power going into the battery and greatly increases the efficiency (in terms of charging efficiency and less time spent keeping the battery warm).

 

[fiehlsport]

I'm not sure what you situation is, but if you're creating a new run for a 6-20, I'd try to go for 240v charging. 5-15 to 6-20 is only a small step forward.

The last reply covered this, but a 6-20 will charge at 16A @ 240V. It's a huge step up from a 5-15 or 5-20.

6-20 is nice if you have 12 gauge wiring in your garage, and it's the only receptacle on the circuit. Very easy to convert a 5-20 to 6-20 at the panel.

 

[GZDongles]

The last reply covered this, but a 6-20 will charge at 16A @ 240V. It's a huge step up from a 5-15 or 5-20.

6-20 is nice if you have 12 gauge wiring in your garage, and it's the only receptacle on the circuit. Very easy to convert a 5-20 to 6-20 at the panel.

That's exactly what I plan. I have plenty of space on the panel and the wiring already going to a detached garage. I've played with the idea of biting the bullet and upgrading my 100A panel now, but it's hard to justify when a 6-20 would be sufficient for my (and probably most driver's) daily needs. I also have several free L2 chargers and a supercharger within a few miles of my house, so I have a good plan B if I need to top up before a long trip.

 

[Rocky_H]

but it's hard to justify when a 6-20 would be sufficient for my (and probably most driver's) daily needs. I also have several free L2 chargers and a supercharger within a few miles of my house, so I have a good plan B if I need to top up before a long trip.

I'm glad you have considered that part of it. I see people sometimes rant a bit about how people NEED faster home charging because "What if [extremely unlikely use case] and you need to charge really fast, and your home charging won't cut it?"
Well, I've considered the same thing. There is a Supercharger in my city about 10 miles away that's never full, so that's a very serviceable plan B.

 

[UncleCreepy]

For some folks including me it's def more a want than a need issue. I could've kept my Prius but wanted a Tesla. Do I get anywhere with the Tesla where the Prius didn't take me? Probably not, but I still like the Tesla better.

Same with the home charging setup. A 6-20 may have been sufficient but I wanted the HPWC anyways.

Long story short: do whatever you're comfortable with. If it doesn't work, you can always upgrade later or find another plan B. Nobody but you needs to like it.

 

[JCtx]

I was expecting to pay to the wazoo for a 14-50 outlet, but it only cost me $80, and done by the same certified electrician who wired my house. It was made at the back of the electrical box, which is the optimal way, since there was maybe 2 feet of wiring. And he used 6AWG, which is good for 60A, and the car only draws 32A, so nothing gets even warm.