General Motors announced today the 2013 Chevrolet Volt will see a slight bump in its rated all-electric range, addition of a “Hold mode,” improved battery robustness, and an improved energy consumption display, among other tweaks as the car anticipates its third model year run.

In a conference call today, GM explained the existing LG Chem battery pack was slightly upgraded without changing dimensions or weight to yield 16.5 kilowatt hours compared to 16.0 kwh for the 2011 and 2012 model years. The U.S. Environmental Protection Agency’s rating system – which has not changed from prior model years for the Volt – will now show 38 miles on the window sticker compared to 35 miles electric vehicle range for 2012s.

This said, on good weather days and with careful driving technique, Volt owners have been known to get in excess of an actual 50 miles EV range – and half that under worst conditions), so time will tell whether the EPA’s testing proves conservative, and real world reports show this “three mile” increase may in cases be worth somewhat more or less.

While GM has milked an incremental gain in energy density, it also reports “tests have revealed less battery degradation,” not that there were known problems before. The company’s engineers have also expanded the state-of-charge window to use 10.8 kwh of the total battery energy – up from 10.3 kwh used in previous models. The battery system maintains a buffer to ensure battery life, but that buffer has been reduced.

Charge times will increase slightly and a full recharge could take 10.5 hours using 120-volt house current, or 4.25 hours using a 240-volt level 2 charger.

The officially rated miles per gallon equivalent (MPGe) was also increased for a slight game of one-upmanship over the 95 MPGe Toyota Prius Plug-In Hybrid Vehicle. The new Volt will show 98 MPGe, vindicating for now the 94 MPGe 2012 Volt that was previously bested by Toyota’s 2012 PHV.

In all, GM said the tweaks are considered a total win and based on battery chemistry experimentation begun prior to the 2011 model year’s launch and now brought to market due to customer demand for additional electric range. The chemistry is subtly modified on the cathode side, but GM is still using manganese spinel chemistry as its foundation.

“The best way to explain what we’ve done at the cell level is to compare it to a cake batter recipe. Sometimes if you use more sugar and less vanilla you get a better tasting cake. We’ve done some work at the cell level to modify the ‘ingredients’ to make a better end result,” said Bill Wallace, GM director of Global Battery Systems Engineering. “This attention to detail will allow our customers to experience more pure EV range, which is the true benefit of owning a Volt.”

Additional 2013 changes are a Hold mode which allows the driver to run on gas only and save the battery for later. This feature is already on 2012 European Volts and Opel/Vauxhall Amperas.

Also, energy flow data showing how much propulsion power is via gasoline or battery and more will be displayed on a larger info screen. This also was done due to popular demand.

Other changes include an optional rear arm rest for the four seater. GM is now mulling whether to give this its own part number for owners of 2011 and 2012 Volts to retrofit if they wish. New wheels, which we’ve not yet seen are reportedly also available, and said to be quite eye catching and increase the car’s generally perceived curb appeal.

The 2013 Volt is likely to be available at dealers some time in August. We’d not be surprised to see them possibly erode sales from 2012 models in the next couple months, or perhaps even dealers may be more willing to negotiate prices on existing models?

Prices for the improved 2013 Volt are not expected to change from those suggested for the 2012 model year.