GM Promises Full 40 Mile Range at the End of 10 Years

In a recent online chat, when asked by a member of the public, GM stated that the Volt will have its full 40 mile range for the warranty period of the battery (10 yrs/150,000 miles). How will they manage that?

The Volt’s battery is 16kWh, with 50% of the capacity (8kWh) used to propel the Volt the first 40 miles. So how does GM guarantee that it’ll last that 40 miles for the full life of the vehicle? Well, as the total capacity decreases, the Volt can still pull 8kWh of energy from the battery. There are a few issues with this approach however.

First is that the maximum amount of power you can draw from a battery at any given moment depends on the state of charge (SoC) expressed as a percentage of total capacity. Generally speaking, you have a higher maximum power when the battery is fully charged and as the SoC decreases, the maximum power you can draw goes down as well.

This plays into how battery deterioration works over time. A battery’s total capacity will drop over time (both calendar time and cycle count), and that 8kWh needed to power the Volt for 40 miles will go up from 50% of the total capacity. As that percentage goes up, the Volt will need to expand its 50% depth of discharge to get 40 miles. Out of the factory, the battery will discharge between 85% and 35%. However if total battery capacity would degrade over 10 years from 16kWh to 13kWh (roughly 20%), then the depth of discharge would be 61% instead of 50%. We can assume the pack would go from 90% to 30% SoC, so as the battery charge state goes below 35% the pack will be able to produce less power. The issue is how much.

Could this mean that over time, battery only mode (aka Charge Depletion mode, or CD) will have decreased performance? Will 0-60 times, top speeds, etc remain constant over the life of the vehicle? GM would need to build these margins into the battery pack out of the factory, which is currently a large set of unknowns (though GM’s battery testing facility will certainly help answer these questions).

The other factor that plays into this is the cycle life. If the battery is limited to a 50% depth of discharge (DoD), the cycle life will improve dramatically over the 100% depth of discharge bench test. If a battery can go 750-1000 cycles at 100% DoD before losing 20% of the original capacity, the battery can likely take 3 times as many cycles at a 50% DoD (Motorola states that Li-Ion battery cycle counts increase exponentially as DoD decreases from 100%). GM will likely need a maximum of 3,750 cycles (40 miles each) to reach 150,000 miles, though its likely actual battery cycle counts will be closer to 3,000 in real world use. Anyone recharging the battery twice a day (recharging at work for another 40 mile drive home) will likely run up against those cycle counts much sooner.

As a risk to the program and to GM, I think the risk is fairly small. By the end of 2010, GM will have had its battery facility open for over a year. Their ability to test batteries in the worst of environments and to test cells, modules and packs and rack up the cycle counts quickly. Even if GM were forced to replace batteries after 7-8 years for those vehicles in the harshest climates (desert southwest, cold northern climates), the prices of batteries by the time 2018 or 2019 rolls around will be much cheaper (as much as 70% less than the 2010 price) and GM could even monetize the replacement if they offer a discounted upgrade battery (though I don’t know if that’s kosher/legal).

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