Chevy Volt Charging Math doesn't add up


Chevy Volt Charging Math doesn't add up

Posted on Oct 2, 2008 by Paul White

When something sounds too good to be true it usually is.  The ChevyVolt has been getting lots of hype.  GM's press department has beencareful about what figures to release.  Knowing that any numericalfigures would be scrutinized by engineers and techies.  Well I workedsome numbers and have come to the conclusion that the Chevy Volt is notwhat it seems.

A few of the facts we know are the Chevy Volt has a 16 KWH battery.
The Chevy Volt is supposed to do 40 miles on the battery, before going into Gasoline mode.
The Gasoline mode kicks in when the battery reaches 30 percent charge remaining.

So from this we can assume the battery never gets below 30 percent.
This means that the 16 KWH battery never has any less than 4.8 KWH remaining.
This means that the Volt is able to go 40 miles on 11.2 KWH

Assuming 100% efficiency which doesn't happen a 11.2KWH battery would take 11.2 KWH to charge.
GM assumes household utility rates of 10 cents / KWH.    Meaning it would cost $1.12 to charge it at home.
Of course in Texas and other states, we don't get these cheep rates.  Try more like 16 cents / KWH.
This means at the average Texas rate it would cost $1.79 to charge your Volt.

But once again no charger is 100% efficient.  There is always lost efficiency due to heat.
A highly efficient charger still only makes 60-70%. We will give the Volt the benefit of the doubt and assume 70% charging efficiency.
This means means the 11.2 KWH you are trying to charge back into your Volt only represents 70% of the power you used to charge it.
In reality you used 14.56 KWH to charge your Volt.
Meaning your cost to charge the car in Texas is closer to $2.32.

One of the claims GM was making was that the cost to drive the Volt was 2 cents / mile.
If that was so then it would only cost 80 Cents to Charge your Volt for the 40 Miles in Electric only mode.
But from what we have found it costs almost 3X this much to fully charge your Volt in Texas.

The Volt is rumored to get 50 MPG in Gasoline Mode.  This would be slightly better than a prius.
But considering you can buy a new prius for $22K, and the Volt is rumored to be priced closer to $40K.  I don't see how the Volt will ever make it off the ground.

UPDATE
I have written an updated article comparing the 2011 Chevy Volt vs 2011 Toyota Prius





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Discussion

paul | Aug 18, 2009 5:47 AM
Guys, keep in mind that these figures were based on what was available to me at the time of writing this. GM has been reluctant to release any concrete figures, other than that cooked 230 MPG figure. Once I see some updated figures on the volt, I will update this blog entry. But for now, $40K plus for a car that maybe gets slightly better MPG than a prius, isn't logical. A loaded prius costs $30K+. Bare bones in the low $20K's Even with Tax Credits it still doesn't make up for this fact. This and GM's branding is at a handicap to Toyota in this market, meaning if anything they would need to sell their car for less than Toyota, and not more.
Alex | Aug 23, 2010 10:29 PM
The volt is only charged to 85%, and here you assume its charged to 100%. So it would cost less to charge than what you calculated.
Paul | Aug 24, 2010 7:15 AM
Alex,
That is news to me.  I knew that the volt would not tap into the bottom 30% of its battery to maintain long life of the cells, but to only charge the battery to 85% doesn't add up.  Of course none of us have been given access to a Volt where we could use a Kill-a-Watt meter to see how much juice it really sucks down during recharge.  Everything is just speculative from what little information GM has released.  Hopefully we will all find out more soon enough.
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