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Tuesday, December 13, 2016

Revised charging strategy for the 2017 Volt

I am still discovering differences between the 2012 Chevy Volt and the new 2017 model that I leased a few weeks ago.  Previously, I wrote a detailed comparison of the two vehicles, but now I want to delve into the difference in the way these two vehicles utilize power from the charging station.

The first difference I noticed is that the new one draws 500 W more power in order to charge the 15% larger capacity battery more quickly from my level II charging station.

I am currently using a 240 V JuiceBox charging station that I installed myself and it can fully charge the new Volt in about 4.5 hours.  This is convenient when I have to make multiple long trips within any given day.  Years ago, I installed a live-to-the web energy monitoring system that show charts of the last 24 hours of energy drawn from the charging station on my web page.  

With the 2012 model, I noticed that the vehicle would draw power in brief bursts even after the vehicle was fully charged.  It would only do this during the winter after temperatures got below freezing.  In fact the further the temperature dropped below 20°F the more often it would draw power.  My assumption is that this was all about keeping the battery warm.  As I understand it if a lithium polymer chemistry battery is below freezing when you draw significant power from it, the battery can be damaged.  So GM engineers have implemented systems to ensure that the battery does not freeze.  A warm battery is a happy battery!  The combined chart above shows this clear correlation between temperature and these short energy draws.


click the image to see current charts at the bottom of my Chevy Volt web page
The 2017 model seems to draw power on a consistent basis in short bursts irrespectful of temperature.  The charts above (from mid December) show that the temperature dropped down to around 7°F and peaked around 35°F while the maintenance charge intervals remain the same.  Note that there is a full charging cycle at the left edge of the chart and two short vehicle charging cycles to the right.

The resolution of my data monitor is not fine enough to resolve details of the brief energy draws because it only takes a reading every 10 minutes, so I hooked up a data logger to do a deep dive on exactly what is going on.  The chart above shows a 12 hour period sampling power every second with a fully charged vehicle and temperatures hovering around freezing.


Zooming in on the left side of the chart above I discovered that the 2017 Volt often starts out by drawing 4000 W and then tapers off.


Zooming in even more to the center of the top chart you can see a single cycle that starts at 4 kW, drops out briefly and then holds at about 4 kW for just over nine minutes, eventually tapering off a little.  This dropout and tapering off varies from cycle to cycle for reasons not yet apparent to me.


I am seeing around 9 of these 9+ minute cycles every 12 hours, so a fully charged vehicle is drawing 4 kW for a combined total of around 180 minutes every 24 hours.  That adds up to around 81 kWh per month!  (For reference, the 2012 Volt needs about 13 kWh for a full charge, and the 2017 needs around 15 kWh.)

Looking at a 4 day data log of charging Watts vs temperature, there is no clear correlation between temperature and charging power used for battery maintenance.  In fact energy draws seem less frequent as temperature increases which is odd.


The average cost per kilowatt hour in the US is $.10, so this battery maintenance energy is costing around $8 per month if it remains very cold.  Here in Maine I pay around $.13 per kilowatt hour for energy that is 30% renewable so I'm paying about $10.50 per month to maintain the battery.  (Actually for most of the year my electrical energy comes entirely from my 5 kW solar array, so there is no cost to me).

Another way GM engineers are squeezing more range out of the new, 15% larger battery is that they are using more of its capacity.  Based on readings taken from my DashDAQ-XL performance monitor I learned that the 2012 utilized from 22% to 87% state of charge, while the 2017 uses from 14% to 90% state of charge.  So the new Volt is using about 11% more capacity from the battery.  When I met the GM battery engineers back in 2011, they sounded quite paranoid about not abusing the battery in order to give it plenty of life.  I'm guess that they learned a lot from the 2012 battery and applied those lessons to a different battery management strategy for the 2017 model.

The main lesson to be learned for those of us driving this amazing vehicle is that it is essential to leave it plugged into the charging station if you live in a cold climate to protect the battery.  If it is not plugged into the charging station it will draw down battery energy, and even resort to starting the gasoline engine in order to utilize heat from the coolant to maintain a safe battery temperature.