Effects of Fast charging and efficiency (miles/kwH)

[TomK]

I just bought a 2015 Spark EV with about 50k miles on it. So far I'm pretty happy with it but this morning i used a DC fast charger (Charge Point) for the first time on my way into work. I started the day at about 9 bars on the battery gauge with 64 miles of range (middle number of the 3 number estimator). Drive cycle showed 22.86 miles @ 4.2mi/ kwH for that trip (which is on par with what i have been getting (4.0-4.8 mi/kwh). Estimator showed 37 miles left in the middle and 48 miles on the top end range. so just about right. Battery level was at 5 bars. The energy details screen showed total energy used was 42.7 kWh since i last fully charged.

I then charged to about 85% - which turned out to be about 4.065 kWh - in about 10 minutes. The middle number on the estimator showed about 64 miles range after charging.

I had 6 miles to get to work from here. I noticed that the range dropped almost immediately and battery level went down rapidly. When i got to work, the Drive cycle showed 6.58 miles @ 2.4 mi/kWh and the energy details showed 45.4 kwH used since last full charge. The estimator was now at 42 miles (middle number) and the battery level was on 7 bars.

I'm a little worried now that the Fast charging caused some damage to the battery resulting in much faster discharge. 2.7 kWH for 6.5 miles just seems out of this world, even with my relative inexperience when it comes to EVs.

Has anyone noticed similar issues previously?

Another concern is overall battery health and how this can be judged. The fast charger indicated that the battery was at 50% and charged to 85% using 4.065 kWh - this would mean the battery capacity is down to 11.6 kWh. When i bought the vehicle, it looked like capacity was in the upper 15 kwH (maybe low 16), based on usage and charge cycles.


Here are all the numbers in condensed form:




Thanks for any help or advise, much appreciated. Thomas

 

[TomK]

i ended up charging on a L1 - 12A charger at work for a few hours and then taking a 58 mile round trip. I fully expected having to charge on the way back but managed with just around 2 bars left showing 13 miles on the range estimator. What is strange is that this was mostly highway driving but at around 60 mph and on the way out i got the best mi/kWh yet (at 5.93 mi/kWh).

So at least for now, the battery seems to be more or less back to what I have been experiencing over the last week.

I still don't know why the efficiency after leaving the DC Fast charger was so horrendous. Does that mean Fast Charging is not advised and more detrimental to the battery longevity?




I really wish that there would be better resolution on the actual battery SoC. On a car with this little range, it seems quite important to know exactly what you are working with. Most of the screens seem to be fairly useless. Realtime SoC, battery health and efficiency would be way more useful.

 

[sachan0101]

Fast charging is relatively taxing on the main battery. I also see the extremely fast battery drain after a DC fast charge. I think it may have something to do with the main battery pack getting hot.

For the health of the battery, I think the main concern should be the depth of discharging of the Li-ion battery.
https://www.recurrentauto.com/research/depth-of-discharge-ev

 

[Randy960]

I've never ran into a situation where the car seems to get lower efficiency after DC charging, but if I had to guess I would say that your battery may have already been pretty warm, got hot while charging, and then used a lot of power to cool it down.

The bar graph is terrible resolution, to the point that it's difficult to work from with such a small battery. I found that the number of bars sort of implies a higher SOC than reality. For example, "5 bars" which implies 50% is actually 40-50%, 6 bars is 50-60, etc. I would recommend getting a bluetooth OBDII reader and Torque or a similar app. Then you can see an actual battery percentage, along with keeping an eye on temperatures.

 

[Infinion]

DC fast charging has losses. You may have paid for 4 kWh at the station's meter but less than that will have been responsible for moving your lithium from the cathode and electrolyte into the anode to intercalate with the graphite structure. More voltage than necessary is required to overcome the internal resistance of the ions at high charge rates, and more power must be diverted to the coolant pump, radiator fan, and possibly AC compressor to remove that heat and keep the cells within their ideal temperature window. In Summer with high rates of charging and discharging, your cells may have still been heat-soaked after a DCFC session, requiring prolonged active cooling as you continue on your journey. More cooling than just running your coolant pump to the radiator. This by itself could account for higher energy use.

https://en-us.topographic-map.com/map-r751/Saint-Paul/
This looks like your area. On your way to work did you travel from a low elevation to a high elevation? Did you go up and down hills more than once using regen braking? The round-trip efficiency of regen braking is not perfect, between 70% and 80% to get into and back out of your batteries to the wheels compared to simply coasting with your momentum. Gaining elevation on your way to work would contribute to a lower driving efficiency, while descending on your return trip could appear more efficient. There could also be a headwind or tailwind that day that further lowers or increases your air drag at 60 mph.

To understand how the energy was used, the cell temperatures and cooling power, your path to work and speed, and the elevation changes need to be factored into the equation. You can account for hills and cross winds with ABRP and an OBD reader for EVs that can stream live data https://abetterrouteplanner.com/

If you want to find out what's going on, it would be good to collect more data across more of the same trips. I think a headwind plus hot cells would be the culprit of your high energy use. You should also keep in mind that li-ion discharge efficiency goes down with discharge rate depending on cell chemistry and pouch construction. This is why the Spark EV's dash has a visualization with a green ball with leaves that changes color when you accelerate or regen too quickly. Essentially there is an unintuitive efficiency vs power curve.

If you discharge a 15kWh battery at less than 15kW, you will maximize your electrochemical energy conversion.
If you discharge faster than 1C (say 30kW-110kW), the sluggish chemistry will lag behind and your cell voltages will start to droop, meaning you are losing cell potential and efficiency, and will realize a smaller fraction of your stored chemical energy.

If you want good efficiency, be mindful of the discharge power you're requesting from the battery pack.

If you feel like your cells have degraded, take a look at this thread for suggestions based on a research article that may limitedly allow lithium-ion cell reconditioning.
Partially Reverse Battery Degradation in NMC Li-ion Batteries

 

[TomK]

Thanks for all the great info.

My driving style is more akin to Grandpa Jo than Michael Schumacher. The trip from the Fast Charger to Work was all city roads, no highway and I'm pretty easy on the accelerator for the most part.

Still figuring out how to get the most out of information available without a OBD reader. Probably will get one soon though. As of now, what i have noticed is that the car seems to discharge at a faster rate (regardless of driving conditions*) when in the upper range of the battery (so between 80-100% charge level).

*the last leg of the trip was through stop and go traffic and much slower so might account for the bigger jump in mi/kWh up to 8)

I did a test this morning where i started of a full charge so i can actually use the data to get Battery capacity. I noticed that the efficiency was much less than the average at the beginning of the trip. I took a picture of both the HUD and the center console display every time i noticed the battery gauge drop one level - this coincided fairly accurately with the percentage on the center console increasing by 10%.



Does this match what others are seeing on longer trips?

I guess my biggest questions/concern right now is, how bad in shape is the battery and calculated ~15.8 kWh capacity - so about 83%? What does that mean for the future of the vehicle, considering my daily round trip is 52-60 miles - charging in between.

 

[boatbum11]

Having a 2014 Spark with 90,000+ miles and frequently fast charging my battery capacity is around 13 Kwh. I'm still able to get around 64 miles of range. I've found that a headwind ( even very minor) significantly decreases efficiency, while a tailwind increases it. Also road surface and weather affect it too. Wet pavement eats much more power than dry and old rough pavement also eats more. Using the heater sucks where you can watch the mileage drop. AC is not that big a drain ( depending on outside temp and inside settings).

I have had the battery meter drop rapidly a couple times ( like dupe, dupe, dupe ). Never could figure out what caused it. Also the faster you drive the faster the guage drops. If you set your instrument cluster to show power draw, I've found 10-11 KW to be maximum optimal draw for highest range ( depending on conditions 55-62 mph, YMMV).

 

[TomK]

Ok, so i think it probably has to do with the battery management system drawing some power after fast charging. I had another adventure over the weekend that took me on a 105 miles round trip, spread out over 6 hours, so lot’s of stops and running around stores in between. I used the fast charger twice during that time (temps around 78F). First time I only got 30 miles out of it (went from 82% to 15% in 30 miles – I would have expected at least 50 miles). Got to the charger with 14 miles left. (i have another table but won't bore you guys with more data here).

When I finished the second time at the fast charger, I noticed that the power draw was hovering between 0-1 kw, indicating that something was using power (AC was off and car was still in Park). The battery conditioning indicator still showed 0% (usage was at 80 kWh since last full charge).

I think next time I have the opportunity; I will try and only fast charge to 78% and then connect the L2 charger and try to run pre-conditioning while connected. Maybe that will help cool the battery initially without sacrificing range. Anyone ever try that? Do I just turn on the car while on the charger then? Is there a way to force the battery to cool while charging?

 

[Infinion]

When I finished the second time at the fast charger, I noticed that the power draw was hovering between 0-1 kw, indicating that something was using power (AC was off and car was still in Park). The battery conditioning indicator still showed 0% (usage was at 80 kWh since last full charge).

I think next time I have the opportunity; I will try and only fast charge to 78% and then connect the L2 charger and try to run pre-conditioning while connected. Maybe that will help cool the battery initially without sacrificing range. Anyone ever try that? Do I just turn on the car while on the charger then? Is there a way to force the battery to cool while charging?

I'm not sure if battery conditioning includes cooling. I've only seen it count up in very cold weather. Also with 80 kWh since your last full charge the resolution on battery conditioning is only 0 kWh or 8 kWh at 0%/1%, very poor resolution to make heads or tails of it, but if there were other DCFC sessions over the 80 kWh, surely it hasn't accumulated anything significant under "battery conditioning". I think it only counts for heating. You might want to try using no A/C and monitor the energy use for "Climate Settings".



So from what I've gathered, the ONLY place heat can be rejected from the battery coolant is through the AC coolant chiller that transfers heat to the refrigerant. See the video above for a breakdown of the parts and functions. There is a mini evaporator/coolant heat exchanger that is called the battery coolant chiller. This mini evaporator is in PARALLEL with the cabin air conditioning evaporator. Each of these evaporators have their own TXV's (thermostatic expansion valve) to regulate respective refrigerant flows.

I bet you'll feel cold air after a DCFC even without turning your cabin AC settings on, since it's all passive TXV's.

You can run the compressor hard to cool the cabin, but it could just result in you using more power to maintain very high discharge pressures which get thrown away at the TXV as a pressure drop. The TXV will restrict the flow according to how much heat load it is tuned to stay within, and the compressor speed and cabin fan speed modulate the heat load to meet the automatic climate temperature you set.

Just like the cabin fan modulates cooling by blowing air on the cabin evaporator at different speeds, the battery pack coolant pump can adjust its flow rate through the battery coolant chiller.

If you run pre-conditioning that involves cooling the cabin with AC / AUTO or dehumidifying the windows with defogger enabled, the AC compressor will run at some capacity and will pressurize the system, pushing fluid through the TXV's, which will do their job restricting flow according to the evaporator temperatures they sense.

Simply L1/L2 charging will activate the compressor on its own too. You can do a deeper dive with TorquePro and the Bolt EV PID's. You can see battery and coolant temps, refrigerant high side and low side pressure, compressor speed and power use, etc.