2011 Chevrolet Volt: Changing Battery Performance Over Time
January 13, 2012
I found a few interesting tidbits while looking over our 2011 Chevrolet Volt's charging records.
On average, it took 12.2 kilowatt-hours (kWh) to fully charge the battery using the 240-volt charger in our Santa Monica parking garage. That figure rose to 13.0 kWh when using the 120V home charge cord that comes in the Volt's trunk.
Why? Charging losses. Any laptop user can tell you that a certain amount of charging energy is wasted as heat. Charging losses are a fact of life with plug-in hybrid and EV recharging to the tune of about 10 to 20 percent. It seems the Volt's 120-volt charge cord resides closer to the high end of that range.
On-board vehicle systems continuously monitor the proceedings, throttling the charge rate and battery cooling systems throughout. Because 120V charges take about twice as long, these systems operate for far longer periods (albiet somewhat less energetically, we assume). Also, the Volt's 120V charge cord is notably skinnier and may impart more resistance.
Whatever the reason, the difference amounts to 6.6 percent more electricity purchased for a given recharge, which works out to $25 if we apply this offset to all the kilowatt-hours our Volt consumed this past year.
That's not nearly enough to cover the cost and hassle of installing a fancy-pants 240V home charger. The only reason to buy one of those is reduced charge time. For the Volt, at least, that wasn't really an issue for us.
But wait, there's more...
Our 240V charger dispensed an average of 12.3 kWh to fill our Volt in the first two months we had it, but that fell to 11.9 kWh in the last two months -- a drop of 3.5 percent.
Huh? Is our Volt's battery losing it? Is this a sign of battery degradation?
In a word, no. It's seasonal temperature variation. That became very clear when we compared our charging records to Santa Monica's mean daily temperature.
The overall trend line matches up, but it even tracks at the detail level.
A spike in charge amounts in May corresponds to a period of warmer weather. One particularly high data point in October that looks like an outlier actually occured on a specific day that was 10 degrees warmer than those before and after it.
So there's nothing wrong with our Volt's battery.
This data does suggest that charging your Volt, Nissan Leaf or other plug-in when the temperature is lower reduces charging losses and saves you money. The fact that we can see this effect here in Santa Monica, where temperture swings are of the kiddie variety, means that battery charge sensitivity to ambient temperature is rather high.
However, this probably means that 120-volt charging is even more inefficient than we observed because almost all of our 120V charging was done at home overnight, when temperatures were at their lowest. If we saw an average of 13.0 kWh then, this data suggests even higher 120V charge amounts in the warm part of the day.
Dan Edmunds, Director of Vehicle Testing