Cold pack energy mystery?

[hobbit]

Winter in the Northeast is a ways off yet, but I was thinking about
range reduction from cold battery packs this morning. For modern
LiPo cell chemistry, what should a typical driver expect from a pack
setup that can't keep itself warm, and gets down to subfreezing
temperatures? And more importantly, where does the "lost" energy
actually go, *besides* that used to heat a cabin? If we push X
amount of charge into a pack like always but get less distance
from it assuming it's all used for propulsion, where's the
effective waste?

Sorry if I'm asking the wrong question. I know that pack currents
get limited in thermal extremes for safety of the cells, but it's
the *amount* of stored energy I'm wondering about here and what
actually becomes of it in cold temps when it doesn't get to the wheels.

[Edit: I did find the infographic & discussion at
http://www.insideevsforum.com/community/index.php?threads/cold-weather.3888/page-2
and related, but that is likely not the whole story]

_H*

 

[Thevenin]

There are a number of effects that stack up to create the reduced range phenomenon.

First, when cold, a cold lithium ion cell's internal resistance increases. As shown in the graph below, this can be a pretty significant effect, depending on battery chemistry. So when drawing high current, cold batteries can waste a lot of energy.

[Source: https://www.researchgate.net/public...tudies_of_Batteries_used_in_Electric_Vehicles ]

Second, when cold, a lithium ion cell has its capacity "locked." As depicted in the graph below, a fully charged cold battery contains less energy than a fully charged warm battery, even if they are initially charged at the same voltage (here, 4V).

[Cell type: Panasonic NRC18650PD, 2.8Ah nominal, LiNiCoAlO2 (NCA)]
[Source: https://batteryuniversity.com/learn/article/discharging_at_high_and_low_temperatures ]

Third, when cold, a battery management system will typically expend power to keep the battery warm. I don't have a good graph for this (it's stuck in google docs), but when I charge in the winter, a small amount of power is used to keep the battery above a certain temperature. This gives the illusion that the battery is accepting summer-levels of energy, when it's actually wasting around 10% of that as heat.

 

[Pushmi-Pullyu]

...where does the "lost" energy actually go, *besides* that used to heat a cabin?

1. Energy is used by the battery heater to heat the battery pack up to operating temperature.

2. Li-ion batteries below about 40° F, temperature varying slightly by the exact chemistry, are significantly less energy efficient when discharging. (See the graph posted by Thevenin, in his post above. And thanks, Thevenin!) The good news here is that since they're significantly less efficient at low temps, a lot more of the energy when discharging (powering the car) gets converted to heat; so that should help the battery heater in heating up the pack to operating temperature fairly quickly.

3. There will be slight reduction in the drivertrain's mechanical efficiency, due to lubricant fluids being cold and viscous, until the powertrain warms up. That probably isn't very important, since the effect should last for only a few minutes.

Note that if you drive somewhere, leave the car out in the cold for some time, and then drive it again, the car will lose just as much energy from warming up as it did when you first set out that day. Repeatedly leaving the car out in the cold for an hour or more, and then driving it again the same day, can have a big impact on range, reportedly up to as much as 50% in extreme outlier cases.

 

[bwilson4web]

I have a different attitude because my EV cars are normally plugged in when parked on the driveway. The Tesla Model 3 is a small shed on wheels with electric heating and air conditioner. I have recorded EVSE data showing this effect.

Now away from home, all bets are ‘depends.’ On a 110-120 VAC, 15 A (12 A), the available ~1.2 kW barely covers the temperature management. You really need NEMA 14-50 rates at ~31 A to also cover overnight charging.

So my rule of thumb:

1. L2 at home (32 A) - no problem.
2. NEMA 14-50 or L2 (32 A) - no problem.
3. 1.2 kW - marginal temperature control in Dixie but no overnight range charge.

So I’m fairly sanguine about temperature effects. With the grid, no problem. But at ~1.2 kW, in Dixie, marginal. But there is one exception.

This past week, I noticed that at 100 kW, the Model 3 SR+ A/C fan reached a peak rate on a 69F night. I will have to wait for colder weather in Dixie to see if ambient temperatures have an effect.

Bob Wilson

ps. I welcome good Prius friend hobbit to our community. He is a good and faithful friend.