72 kW for a bone-dry S 70D?

[Missile Toad]

@David99 I've been wondering what the "sweet-spot" in ambient temps is for charging the batteries. By sweet spot, I'm taking on board your directive to drive the car for 60 minutes before charging, during which BMS or otherwise the battery is in an 'goldilocks' state at the start of the charge. Can you suggest a 'lowest ambient temp' (much lower than my bullets above), that your Delaware supercharging conditions 'force' the BMS into action at the earliest phases of charging (when tons of current is going in)?

I'm speculating that 65ºF is the highest ambient temperature that an already warmed pack would incur trivial current to cool the battery, based on past experience charging after a long trip. I note that Tesla internals start cooling the pack, itself, around 104ºF, which given the natural heating effects of charging, is nearly reached (my guess) after a 60 minute drive in such mild temperatures -- see Model S Overall Thermal Management for apparent temperature feedback loops. So can you elaborate on "very cold" you talk about above? And we are talking the ambient air temperatures, not the battery pack temperatures, OK?

Draft "sweet-spot":
<----------------------------------------------------------------------------------------------------------->
(lower, and substantial? heating? kicks in) 27ºF ............ 65ºF (higher, and substantial cooling kicks in)

 

[David99]

Interesting. I read the post about not heating the battery at low SoC here on TMC somewhere, I'll have to try and find it. I don't remember how they reached that conclusion.

I think that is true for the battery heater which will only kick in if the battery is below 10 Celsius (or maybe a little below that). The battery heater uses a lot of power. When the battery is full, the benefit of heating it is that you can actually get more energy out. So it makes sense to use some of the battery's energy to heat itself. If the battery level is low, there is not much energy left so there is no benefit of using energy to heat it. I think that's why the battery heater will probably not kick in if your state of charge is low.

 

[David99]

@David99 I've been wondering what the "sweet-spot" in ambient temps is for charging the batteries. By sweet spot, I'm taking on board your directive to drive the car for 60 minutes before charging, during which BMS or otherwise the battery is in an 'goldilocks' state at the start of the charge. Can you suggest a 'lowest ambient temp' (much lower than my bullets above), that your Delaware supercharging conditions 'force' the BMS into action at the earliest phases of charging (when tons of current is going in)?

I honestly don't have enough experience with cold weather to have been testing every scenario. I know that the batter is most efficient at close to 50 Celsius which is actually very hot. That's the threshold of feeling pain when you touch something. At that temperature the battery can deliver the most power and take charge really well. The chemical processes work great. If you go higher it causes damage to the battery, though.

At 0 Celsius a Lithium battery cannot be charged without causing damage. So anything between 50 and 0 is OK. The warmer the better in terms of performance. Based on my findings, the BMS tried to keep the battery at roughly 30 Celsius regardless of ambient temps. If you turn Range Mode on, the BMS will raise that temperature to about 40 Celsius. I found that even in very cold conditions, the BMS can maintain those temperatures during highway driving. In other words, if you are on a road trip, your battery will be nice and warm.

I'm not sure at what temperature the Supercharger will reduce it's power to protect the battery. I only have little data but I'd say around 20 Celsius or above there is no power restrictions. Between 0 C and 20 C the power is definitely reduced depending on the temperature.

Ambient temps really don't matter in that regard when it comes to charging. The BMS will warm and cool the battery either passive or active to stay within healthy working conditions. passive cooling is using ambient air. If that's not enough it will use the AC compressor. Same for warming. Passive is using waste heat from motor and inverter or AC chargers (when charging). Active heating is using the battery heater.

 

[jelloslug]

I have experienced the slow charging with a very low SoC. I got down to 8 miles in my 60D with an outside temp of just above freezing. The car was completely warmed up but it still took about 15 minutes to start charging at a normal rate.

 

[Saghost]

I think that is true for the battery heater which will only kick in if the battery is below 10 Celsius (or maybe a little below that). The battery heater uses a lot of power. When the battery is full, the benefit of heating it is that you can actually get more energy out. So it makes sense to use some of the battery's energy to heat itself. If the battery level is low, there is not much energy left so there is no benefit of using energy to heat it. I think that's why the battery heater will probably not kick in if your state of charge is low.

Hmm. So maybe the heater does get disabled at 10 or 15%, but the combination of the heat generated by discharging and the use of motor/inverter heat to warm the battery is enough to keep it from cooling off significantly in 27F weather?

 

[Missile Toad]

So in a few weeks, I'm going on a trip where I will HPWC charge the car overnight, outside at a hotel. I'm only 1/3 of my way into my route at this overnight stay, but I'm likely to see temps, overnight, of 32-40ºF. As a strategy, I want to top off for a range charge, but I understand range charge is something you want to burn off with an immediate trip. So I have a three step plan, to provide maximum range, but minimize time sitting at 100%:

1. charge to 90% overnight, turning range mode off;
2. as soon as my feet leave the bed, and hit the floor (or sooner), increase charge limit to 100% with the Tesla app;
3. gather my clan together, brush teeth, dress, eat... and some 60 minutes later, I should get between 95 and 99% of my charge, although the initial few percent will come in very slowly.

If the opportunity presents, I might trigger the final 10% charge 2 hours ahead of our departure, with the expectation that charge will occur at ~10 MPH if temps are around freezing (cabin heat will likely be turned on).

 

[WannabeOwner]

Here charging at 240V (16 AMPs I think) my P90D takes 1.25 hours to get from 90% to 100% so you might either want to allow a bit longer, or overnight charge to, say, 95% to be sure to be able to get to 100% first thing in the morning. (FWIW the exact same type of charger at work is a good 1 MPH faster than the one at home).

I have, just once, charged mine to 100% at a Supercharger. Generally in the 10% to 70% range it takes 5 minutes per 10% gained. From 90%-95% took 12 mins, 100% came up after a further 28 mins, and it did not stop charging (balancing cells I suppose, perhaps the duration of that is very variable) for another 24 mins - so 64 mins in total to get from 90% to 100%. I imagine it would be OK to leave the moment that 100% came up on the dash?, and maybe staying longer, until the charge finished, wouldn't gain much range?

Personally I wouldn't be bothered if the car sat at 100% for a couple of hours (and other threads I have read suggest there are people who have done same not-irregularly and have no noticeable range-loss as a result, but that's not proof of course). I figure that the car has to always sit for over an hour just to charge above 90%, of course, sometimes more if charging is slower, so, for me, another couple of hours sat at 100% before I leave I regard as "ok". The one time I charged to 100% and then my plans changed I drove to the shops and back just to get it back down again, rather than leaving it like that, but maybe that, once-in-a-blue-moon, would have been OK too.

All that said, I would like a Phone APP that could "Charge to 90% and then resume charging to 100% timed to finish at XXX and warm/cool the cabin". I think Visible Tesla is the only thing that comes close and, unless it has changed, my understanding is that its fiddly to set up.

 

[David99]

Hmm. So maybe the heater does get disabled at 10 or 15%, but the combination of the heat generated by discharging and the use of motor/inverter heat to warm the battery is enough to keep it from cooling off significantly in 27F weather?

Yes absolutely. I have been monitoring the the battery temperature in cold weather almost constantly and I found that even in very cold ambient temps, the car is always able to keep the battery temperature as a very good level. Normal driving produces enough waste heat to keep the battery temperature up.