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Discussion Starter #1
I'm visiting a friend who had never used a DCFC for his Bolt. We tried last night and were disappointed by the slow charging rate. Here are the basic parameters:
  • Charger: ABB 50-500V, 0-125A, 50kW.
  • Ambient temperature: 48 degrees F (battery was likely a bit warmer - we'd only driven 10 minutes to get there)
  • Battery charge at start: 15%
Charge rate began at around 22.5kW. It increased slightly until we reached 20% SoC, and then a bit more rapidly to reach 33kW at 23%. We ended the test at 25%. We thought it might be a bad charger, so we switched to another unit at 18% SoC, but it didn't seem to make a difference. Unfortunately I didn't have my OBD-II reader to help troubleshooting.

Basically we were hoping/expecting 36-38kW from the beginning. This is a 100A charger (thanks EvGo...), and I've seen my Bolt charge between 360 and 380V. SoC is well below any taper point.

The only explanation I could think of is that the battery temperature was too low, causing it to throttle. I'm trying to decipher this info from the link Vertiformed posted in the other thread. Does the table below means it will throttle to 30A below 10 degrees C? Thanks!

// DC CHARGING PROFILE

// DC charging profile, where the first row (T0...Tm) is the battery temperature, the first column (S0...Sn) is the SoC buckets, and
// the cells (Axy) are the corresponding current rates (A). The corner cell (TT) is the index of the default temperature column.
//
// TT T0 T1 T2 ... Tm
// S0 A00 A01 A02 ... A0m
// S1 A10 A11 A12 ... A1m
// S2 A20 A21 A22 ... A2m
// .............. ... ...
// Sn An0 An1 An2 ... Anm
//
"DC_Charging_Profile": [
[0x008, -30.0, -20.0, -10.0, 0.00, 10.0, 15.0, 22.50, 25.00, 40.00, 45.00, 50.00],
[67.00, 0.000, 3.000, 12.00, 30.0, 54.0, 93.0, 134.0, 150.0, 150.0, 75.00, 0.000],
[80.00, 0.000, 3.000, 12.00, 30.0, 54.0, 93.0, 100.0, 100.0, 100.0, 75.00, 0.000],
[90.00, 0.000, 3.000, 12.00, 30.0, 54.0, 60.0, 60.00, 60.00, 60.00, 60.00, 0.000],
[96.00, 0.000, 3.000, 12.00, 30.0, 40.0, 40.0, 40.00, 40.00, 40.00, 40.00, 0.000],
[100.0, 0.000, 3.000, 12.00, 25.0, 25.0, 25.0, 25.00, 25.00, 25.00, 25.00, 0.000]
],
 

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Yes. Battery temperature. I was wondering what was wrong with a charge too. Started lower than in the summer. Then before any taper, it picked up and got to the summer rate of around 44kw. Then the normal tapers came in.
I can only assume the charging itself heats the battery and then it allows more to flow in. I dunno. But that's my observation.
 

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Discussion Starter #3
Yes. Battery temperature. I was wondering what was wrong with a charge too. Started lower than in the summer. Then before any taper, it picked up and got to the summer rate of around 44kw. Then the normal tapers came in.
I can only assume the charging itself heats the battery and then it allows more to flow in. I dunno. But that's my observation.
Right. I guess in my view, 48 degrees is not especially cold, so a taper of 50% seems very aggressive.

But after a bit more searching I see another forum member saw similar behavior charging with a temperature in the mid-40s (that was also at an EvGo station).
 

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But after a bit more searching I see another forum member saw similar behavior charging with a temperature in the mid-40s (that was also at an EvGo station).
Hi @dhazeghi, sorry I missed your initial post (for the future, if you mention other forum members with an “@” sign, it notifies them, which can help sometimes). In your original scenario, the battery charging voltage would have been around 336 volts, so a 22.5 kW charging rate means 67A charging, and so the battery was probably 55 ℉.

Overall, the lesson is don't take a Bolt that has been parked up in cool temperatures and take it to charge at a DCFC station. Instead, precondition (twice if possible) before you set out.

For anyone else who doesn't click the like for my earlier post, here's the key content quoted, including a cool graph, which I'll copy and paste for extra visibility:



Although the car is capable of charging at 150A, but it will limit current based on battery temperature (and state of charge). We actually know a little about how it does that (source).
  • -30.0 ℃ / -22 ℉ — 0 amps, car will not charge at all until battery has warmed
  • -20.0 ℃ / -4 ℉ — 3 amps, very slow (1 kW, slower than home charging from a 120 volt outlet)
  • -10.0 ℃ / 14 ℉ — 12 amps, slow (4.3 kW, slower than typical level-2 home charging)
  • -0.0 ℃ / 32 ℉ — 30 amps (10.8 kW, finally faster than home charging)
  • 10.0 ℃ / 50 ℉ — 54 amps (19.4 kW)
  • 15.0 ℃ / 59 ℉ — 93 amps (33.4 kW)
  • 22.5 ℃ / 72.5 ℉ — 134 amps (48.2 kW)
  • 25.0 ℃ / 77 ℉ — 150 amps (54 kW, faster than almost all DCFC chargers can supply)
  • 40.0 ℃ / 104 ℉ — 150 amps (54 kW, faster than almost all DCFC chargers can supply)
  • 45.0 ℃ / 113 ℉ — 75 amps (27 kW)
  • 50.0 ℃ / 122 ℉ — 0 amps, car will not charge at all until battery has cooled
We can visualize this as the following graph:



Note, however, that (unlike the Leaf and the American edition of the upcoming Hyundai Kona, the Bolt can also heat its battery, but there is a lot of thermal mass to the battery, so it takes a while for the heater to raise the battery temperature). Thus, if you'd stayed longer, you'd have seen charging speed pick up a bit as the heater got the battery warmed up.

If we assume that the interpolation line I drew between the known points mirrors reality, at battery temperature of 45 ℉ you would have only charged at 46 amps (16.6 kW) initially. If your car had been sitting in the cold (e.g., overnight without being plugged in), it's possible that your battery was even colder.

On the other hand, driving the car warms the battery a little, and the battery has a significant thermal mass, so it's very possible for it to be 45 ℉ outside and have the battery be significantly warmer.

From your description, it seemed like you were using a DCFC charge as an experiment to test it out (and were disappointed).

In a more typical cold winter trip scenario, it'd go like this. You'd charge the car overnight while you slept. As part of the charging process after the battery is charged (which generates some heat all by itself), the car will automatically run the battery heater to raise the temperature closer to a good temperature. Before setting out, as you ate breakfast and got ready to leave, you'd use the preconditioning feature to warm the car up with wall power before setting out. One 30 minute session is great, two would ensure the car is as warmed up as can be. Preconditioning will further make sure that the battery is brought up to a good temperature.

During the trip, a mix of the thermal mass of the battery retaining the heat from preconditioning and the heat generated from using the battery will keep it at a good temperature so that when you arrive at your first charging stop you'll be well placed to charge at a good rate. The charging process will further raise the battery temperature so that further driving and charging will always see good speed.

Put simply, EV batteries don't like the cold. There are things you can do to help, but it's also possible to be caught out. It's true for all EVs. There are frustrated folks on Tesla forums, too, for example.
 

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My experiences match up nicely with Vertiformed's handy chart. Also, the battery on the Bolt has a pathetic little 2KW heater that kicks on when you start a DCFC session if the battery is cold, and it then kicks off right around the temperature where the battery can accept 100 amps... I assume the software was written before higher current DCFC stations were common. From the 100 amp point on you are relying on charging current to warm the battery.

I have seriously considered adding a manual relay with a toggle switch to activate the battery heating system, it would be great to be able to pre-warm before a DCFC session like a Tesla... but I would need to be out of warranty and do a lot more study of the system before making the changes :)

Keith
 

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Also, the battery on the Bolt has a pathetic little 2KW heater that kicks on when you start a DCFC session if the battery is cold, and it then kicks off right around the temperature where the battery can accept 100 amps... I assume the software was written before higher current DCFC stations were common. From the 100 amp point on you are relying on charging current to warm the battery.
It's worth remembering that the heater will also come on at times outside of DC fast charging. When plugged in, it'll come on about twice a day to keep the battery at around 60 ℉, and when unplugged it'll try to avoid the battery being really cold (I forget the threshold).

If you precondition, it'll turn on the “pathetic” battery heater (which according to ODB-II draws a little over 2 kW) and the cabin heater (which draws 7.5 kW). Although I just preconditioned and although it ran the heat, it didn't seem like it warmed the battery by very much. I'll check the data more closely a little later today.
 

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So, here's some graphs from preconditioning and driving, in cool-but-not-really-cold conditions (45–50 ℉).

First, let's look at preconditioning…

27799


We can see that the cabin heater maxes out at 7.5 kW and the battery heater maxes out at 2.35 kW, but the cabin heater doesn't run at full tilt for long. After the initial blast of heat, it settled down to drawing an average of 2.1 kW. What's more interesting, perhaps, is that after 15 minutes of heating the battery, it quit, whereas it heated the cabin for 20 minutes.

But what does all this power expended do for the battery?

BatteryTemp.jpg


We can see that it quite quickly raised the coolant temperature to 25 ℃ (77 ℉) and then it looks like it started circulating the coolant, which brought the temperature back down. We end up with the coolant at 21 ℃ (70 ℉). The battery, on the other hand, has a large thermal mass and was warmer to begin with (due to being kept somewhat warm already by being plugged in), but only gains 1 ℃ over the preconditioning process.

It would have been interesting to see what happens if I'd preconditioned a second time, but I didn't (sorry). Instead I set out to actually get some stuff done today (crazy, right?). Here's the full trip, which had some freeway driving, but about twice as much street driving.

27802


We can see that battery temperature steadily climbed, ending up at 19 ℃ (66 ℉). This isn't totally fantastic for fast charging, but it could be worse.
 

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2.35KW is just about the capacity of the OEM EVSE on 240v. Guess those in extreme colder climates may really need to get a 240v 32A EVSE or even a DC medium charger.
 

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2.35KW is just about the capacity of the OEM EVSE on 240v. Guess those in extreme colder climates may really need to get a 240v 32A EVSE or even a DC medium charger.
A 32A charger can deliver 7.68 kW, which is enough to do some significant heating, even if it isn't the full 9.85 kW the Bolt will draw with everything on full tilt.

Last time I was posting about this stuff, 10 months ago, I said:
The specific heat capacity of a lithium battery is about 0.950 J/(g K), and the Bolt's battery weighs 960 lbs, so that works out to 0.115 kWh for every K the battery raises in temperature.
From the theory, it seems like it takes about 3.5 minutes for the heater (once it gets going) to raise the battery pack temperature by 1 ℃ (or about 2 minutes to raise by 1 ℉) . That means that warming the battery a lot takes a considerable time; for example, it'd take 40 minutes to get from 40 ℉ to 60 ℉.

The overall lesson is that if your Bolt has been sitting in the cold and you're going to need to do some fast charging, precondition the heck out of it.

Edit: Actually, based on this post, also from ten months ago, my theory was too optimistic compared to practice. @Fivedoor found that it can actually take three minutes to raise the temperature by 1 ℉, possibly because the heating system was fighting against cooling effects of low exterior temperatures.
 

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Discussion Starter #10
My experiences match up nicely with Vertiformed's handy chart. Also, the battery on the Bolt has a pathetic little 2KW heater that kicks on when you start a DCFC session if the battery is cold, and it then kicks off right around the temperature where the battery can accept 100 amps... I assume the software was written before higher current DCFC stations were common. From the 100 amp point on you are relying on charging current to warm the battery.

I have seriously considered adding a manual relay with a toggle switch to activate the battery heating system, it would be great to be able to pre-warm before a DCFC session like a Tesla... but I would need to be out of warranty and do a lot more study of the system before making the changes
Thanks for the tips and confirmation Fivedoor and Vertiformed. Today I had to fast-charge a relatively cold battery (12C). I could manually confirm the ramp up in current as the battery warmed. I will precondition next time.

How are you measuring the power draw of the battery and cabin heaters? Are there PIDs for those? I guess what I'd like to figure out is how much of a charge budget I should leave myself if I anticipate needing to precondition. Thanks!
 

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How are you measuring the power draw of the battery and cabin heaters? Are there PIDs for those? I guess what I'd like to figure out is how much of a charge budget I should leave myself if I anticipate needing to precondition.
Yes, there are PIDs for the heaters, and for various temperature sensors. (I'm somewhat disappointed that the temperature sensors are fairly coarse, only measuring in whole Celsius increments.)
 

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Discussion Starter #13
I just returned from a fairly lengthy trip in California and Oregon (I'll post a report in the trip subform shortly). Temperatures were mostly in the 30s and 40s. One thing that surprised me was the wide variation of battery temperatures, even after the car had been driving for an extended period.

I charged at two different 350kW EA stations. One I managed to pull >50kW (first time ever!) within a few minutes of plugging in while the other started around 24kW and very slowly ramped up to 37kW. Of course when I double-checked the battery temperatures, the first was in the mid 20s while the second was in the low teens. In both cases I'd been cruising along at around 60mph, heater on, for at least 30 minutes before. Outside temperatures were a bit colder the second time, and the road did have more downhill. On another stop I managed 43kW from a ChargePoint station that I'd never gotten more than 37kW from before, even on warm fall days. Again, I suspect battery temperature is the explanation (25C).

I'm wondering if there are any tips/recommendations for getting the battery temperature up while driving? The only thing I've been able to come up with is trying to increase the load on the motor by going faster, but obviously that's limited by the roads/speed limits.
 

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Yes, there are PIDs for the heaters, and for various temperature sensors. (I'm somewhat disappointed that the temperature sensors are fairly coarse, only measuring in whole Celsius increments.)
Can't you go into the setup, and change the number of decimal places? You can do that for most things, I found.
 
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