EV charging with a home backup battery

Images: Screen captures from our Tesla Powerwall app, with yellow text added to provide further insights.


Since we have “net metering” with 1:1 “banking” of excess solar energy production, the kilowatt hours we have “banked” with our local utility, West Penn Power, will likely fully offset this current usage from the grid.


During winter months, cold temperatures limit the charge capacity of our 2019 Chevy Bolt battery (I’m told this occurs in other EV’s as well). That limitation in the Bolt is typically around 20%, which reduces the average range from a ‘window sticker’ 258 miles (for the 65kWh battery) to around 200 miles.

Now that Spring temperatures have warmed to over 80-degrees F on some days, a full charge on our Chevy Bolt this week produced a range of 305 miles, which is nearly 20% over the average range. So overall, the maximum range from cold to hot weather can vary by 100 miles.


That being said, Bolt owners learned through the GM recall (where original Bolt EV batteries were replaced with new ones and software has been modified numerous times) a recommended technique on charging: Charge more often.


And through my research and discussions with others regarding lithium-ion batteries (which I also have in my iPhone, Nikon camera, Toro rechargeable power tools, etc) it’s best to keep them between a 30% to 80% charge level.

Doing that with batteries decades ago, could cause them to become “bracketed” and limited charge capacity – so most devices called for fully charging batteries, every time. Always read your instruction manual for specific manufacturer recommendations!


Apparently, the worst thing you can do with newer lithium-ion batteries, is keep them sitting around with a full 100% charge. I’ve been told doing that can encourage the formation of dendrites, and increase the possibility of a short circuit and subsequent fire. I was once again reminded of that advice as I read through the owner’s manual for my new Toro 16-inch rechargeable chainsaw, seen in this video.

Both the 2.5 and 7.5 amp hour Toro batteries have 4 lights on top that indicate the charge level of those batteries. Toro recommends that the batteries be kept between 2 and 3 lights (50% to 75% charge) if they are being stored for a long period of time.


I’ve also seen advice for other devices – not to leave rechargeable batteries (like cell phones) on “perpetual charge” meaning plugged in all the time, as well as the recommendation to unplug chargers – since they can draw electricity, even when not charging a device.

With all this in mind, I typically charge our Chevy Bolt often, to a maximum battery capacity of 80% to 90%, unless we are taking a longer trip than just normal everyday driving. With a Level 2 charger in the garage, I can add 100-miles of range in just 4 hours.


These warmer temperatures have also called for less use of the Bolt’s cabin heater, heated seats and steering wheel, so we are typically getting closer to 5 miles per 1 kWh, than 3 miles during past winter months.


We should begin to see big strides in battery technology, with so much focus on electric vehicles and grid storage.