Depth of Discharge, or DOD. Its one of the few remaining mysteries of the LEAF.
As a primer to the term, DOD is basically the amount of usable battery available to drive the car. Conventional wisdom on lithium ion batteries is that the use of 50% of the pack will give the greatest longevity and the most full cycles over time.
However, leaving 50% of the pack as a buffer would be pretty impractical in a full battery electric vehicle, as you would be leaving half the range as well. In a vehicle like the Chevrolet Volt, GM does indeed only use 8 of the 16kWh available to propel the car because the benefit of the gas extended range allows this to be feasible.
By the same token, using 100% of the pack is also a nightmare scenario, as lithium ion (unlike nickel-cadmium) batteries are very sensitive to a full cycle discharge. It would result in many fewer cycle counts, and performance would degrade much quicker.
Therefore 80% has become a magic number of sorts. The perfect level between automobile performance and battery management. Mitsubishi employs the 80% rule in the i-MiEV’s 16 kWh pack, giving the car aprox 80 miles of range, and the 2012 Ford Focus Electric will have a similar 17kWh of 23kWh total in use.
So what of the LEAF’s battery usage? Nissan has never said. Or have they? We had a article here yesterday about a test drive from plugincars.com, which reported among other things (really high miles per kWh) that Nissan had given away this number to them.
“The biggest revelation from Mark (Mark Perry-Nissan exec) was that Nissan engineers are allowing 95 percent of the LEAF’s energy storage to be used…Mark believes that it will be a rare occurrence for a LEAF driver to dip so far into the state-of-charge. Regardless, Nissan is showing a great degree of confidence in the capability and durability of its battery technology to allow so much of its capacity to be used in those rare times…Combine the big number for miles-per-kilowatt with the 95% battery usage figure to get a picture of a robust well-managed battery that—at least for in-town driving—could mean high real-world numbers for driving range.”
…et le wow! 95%
I actually started to write a piece how battery developers at AESC and engineers at Nissan where going to win a Nobel prize for this breakthrough, but about a paragraph in, the pessimist in me took over. Then the Nobel prize story turned into a ‘Nissan engineers have gone mad’ piece. ..then I remembered I’d like the good people at Nissan to keep picking up the phone when I call . So I decided to get some confirmation before running with one of the two choices that presented themselves.
I talked to Mark Perry, who is Nissan’s Director of Product Planning & Development in North America, and who had also apparently given plugincars.com this scoop. With a simple, “Never said 95%,” Mr. Perry quickly assured me that basically a lot of things happen on these test drives, there is lots of back and forth Q&A, and quite often in the quest to break new information, some facts sometimes get confused. Unfortunately, that was the case here…and my piece about the Nobel prize (and/or the mad scientists running the ship) for Nissan died. No problem. It happens, we move on.
However, as is often the case, in shooting down one thing, we learn another. Mark took the time to confirm that the DOD was indeed at least over 75%, and that Nissan has taken steps to inform the driver when their range is getting low, and that at very low levels Nissan has solved the problem of ‘how does the car know how I am going to drive when I am almost out of electricity?’
“Software set up so first warning lamp (like low fuel lamp on ICE) illuminates at 4kw to go. After that we manage energy usage down to 2kw then go into a (power limited) mode designed to get you safely the final couple of miles (won’t have 90 mph top speed) to the charging station. You’re NAVI automatically showed reachable area, (and) all charging stations within reach and fastest route .”
So the DOD question was not solved, (even though it strongly appears Nissan will be using the 80%ish rule), but the question of whether of not the car will be accurate peg the amount of range left ‘in the tank’ when it gets low has.
Nissan will in a sense force you into that 5 miles/kWh sweet sport for the last 10 miles or so to ensure you arrive at your destination, rather than coming up short on the side of the road. /sounds good enough to me