By Bill Destler
With summer drawing to an end and the cold days of winter just a few months away, I thought it might be a good time to discuss how lower temperatures and winter weather in general affect Volt range and efficiency.
All gas-powered cars get lower gas mileage in cold weather. My Toyota Prius, for example, gets about 48 mpg in the summer but only 38-40 mpg in the cold Rochester, NY, winters. The reasons gas-powered cars get lower gas mileage in cold weather include increased friction from thicker motor oil, transmission fluid, power steering fluid, and differential gear oil and increased rolling resistance as we slog through snow and sleet, especially if we forget to maintain tire pressure between 35-40 psi, since without adding air, tire pressure goes down by 1-2 psi for every 10 degrees the outside temperature goes down. Finally, gasoline engines operate optimally at a specific temperature, and during warm up periods they are simply not as efficient.
Now the Volt is subject to many of these effects, especially when operating in charge-sustaining (gas consuming) mode. But in all-electric mode, several other factors are at play. First, let’s consider battery performance as a function of temperature. If you charge any battery in a heated garage and then take it outside into freezing weather, the temperature of the battery will quickly be reduced to the ambient outside temperature and the electrochemical reactions that provide the current will occur at a slower rate. Thus the battery will seem to have lost capacity since as it is used the current will decline from a lower starting point and the battery will quickly fail to provide enough current at a given voltage to perform its function. Interestingly, if you then move the same battery back into the heated garage, the battery will recover as it warms and you can ultimately draw almost all of the original charge power out of the battery before you have to recharge it. So cold weather doesn’t really reduce battery efficiency in the sense that you put 10 kwh in and get only 6 kwh back. If you warm the battery back up to room temperature, you can get the missing 4 kwh from the battery again, or when you recharge it you will only need to provide 6 kwh to reach full charge again.
Efficiency aside, however, in an electric vehicle a cold battery will deliver less range than a room-temperature battery because of the lower electrochemical reaction rate. GM has addressed this issue in the Volt by using a sophisticated battery temperature control system aimed at keeping the battery at a good operating temperature. I do not know what the acceptable temperature range for the Volt’s battery has been set at, except that it has to be above 32 degrees F and below 100 degrees F to adequately protect any lithium-based battery and extend its life. But everything has a price, and the energy consumed to maintain the battery temperature in cold weather has to come from somewhere, so winter range is reduced. This effect is most severe if you don’t keep your Volt in a garage and don’t plug in the charger when you are not driving the car, since the battery temperature control system needs power from somewhere and if it took the power from the battery then all-electric range would be reduced even further just to keep the battery warm when the car is not being driven. In fact, it appears that GM has chosen to not drain the battery for battery temperature maintenance in cold weather when the car is idle and not connected to a charging station, which means that the system has to consume even more energy once you start driving to get the battery temperature up to a good operating point. At very low temperatures, the Volt will start the gasoline engine to warm up the battery more quickly, but this consumes energy of a different sort.
In fact, many of us have seen all-electric range of between 45-50 miles in the spring and summer reduced to 25-30 miles in the winter. Is that reduction all due to the battery temperature control system? No. A majority of the energy is lost in the human temperature control system, i.e. the cabin heater. In a gas-powered car, much of the energy in the gasoline that could be used to drive the wheels is lost as heat radiating from the engine, so providing heat for passengers is a simple matter of diverting some of that heat into the passenger cabin. An electric drivetrain car is much more efficient than one using a gas-powered drivetrain, so very little heat is generated by the electric motor and heat for passenger comfort has to be provided by passing electrical current from the battery through a heating element as you would in an electric stove. This is not a disadvantage of an electric vehicle, but rather a result of its higher operating efficiency. If we want cabin heat, we must use some of our battery energy to provide it.
These two winter energy consumers, the battery temperature management system and the cabin heater, are the primary reasons for the observed reduced range in cold weather. So is there anything we can do about it? For some of us, the answer is yes.
If we keep our car in a heated garage and keep it plugged into the charger when we are not using it both at home and at work, we will be using home heating systems or grid power to maintain the battery temperature and the cabin temperature while we are not driving. Starting both systems off at the right temperature doesn’t mean we won’t consume battery power to maintain them while we are driving, but it will save battery energy and extend our winter all-electric range. In an unheated garage, you can pre-heat the cabin using grid power by remotely starting the Volt (and the heater) before you leave for work (or from work if you can plug in there). One strategy to consider would be to preheat the cabin while connected to the charging station by setting the climate to “Comfort”, turning off “Auto” temperature control and turning up both the temperature and the fan. Then when you start driving you can turn the system back to “Eco” and “auto” to conserve energy and range. But remember, there is no free lunch. The extended winter range has to be paid for in additional electricity consumption while you are plugged in.
It’s probably a good idea to also limit your use of Sport mode in the winter, especially if your Volt has been standing idle in the cold and not connected to a charging station. And the use of L mode while driving is also a good idea, since it provides added braking under slippery conditions and regenerates power more effectively than D mode.
Volt engineers have found that heated seats use up less energy than does heating the cabin, so using the seats (if you have them) in cold weather and limiting cabin heating to the extent to which you are comfortable is another good idea.
In charge-sustaining mode, the use of a low-viscosity synthetic motor oil, such as Mobil 1, in the Volt’s gasoline-powered generator would probably improve gas mileage because such oils flow much more easily at low temperatures. Happily, the Volt will divert heat from the gas engine to the cabin in this mode, so passenger heating is not as much of an issue in this mode.
It might be interesting to see if Mountain mode, which operates the gas generator more and therefore makes some gas engine heat available for cabin heating, might also be used intermittently during the winter to extend all-electric range.
I would be interested to hear of other strategies for cold-weather Volt operation. It would also be helpful to get reports from users on what all-electric range they achieve at below-freezing temperatures.
This entry was posted on Wednesday, August 24th, 2011 at 5:55 am and is filed under General. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.