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The Nissan Leaf Energy Efficiency Meter Would Be More Useful If It Was Accurate

The Nissan Leaf is a great car, and a great early entry in defining what an electric car can be. The acceleration is quick, smooth, and effortless. The drive is pleasantly comfortable and astonishingly quiet. The short range is more than adequate for my daily driving needs, and I can charge it up every night in my garage. I never have to think about going to a gas station for a fill up.

It's nice. In fact, it's so nice that I couldn't imagine going back to an ICE car for my commuter car. The replacement for the Leaf, when I need to get one, will definitely be another EV. If we can get a bigger, affordable EV with a 200+ mile range to replace our Prius when the time comes, we'll probably do that, too.

Not everything is perfect, of course. Having a realistic range of 70-80 miles can be limiting on busy days, and I haven't gone far outside the city since I got it home. It takes some adjustment to live with a car with that kind of range. If you're like me, and you don't charge everyday and only charge to 80%, you can't jump in and drive all over town without a little planning first. It takes some getting used to, but for the most part it's not a hindrance at all.

However, there is one issue with the Leaf that I have not gotten used to because I cannot understand why Nissan got it wrong or why they haven't fixed it. That nuisance is the grossly inaccurate energy efficiency meter. It really bugs me that Nissan reports inflated numbers for this measurement. It would be so much more useful to know the actual miles/kWh the car was getting so that I could know how much it cost to drive without an external power meter, and the GOM (Guess-O-Meter) would also be more accurate.

How Do I Know The Energy Efficiency Meter Is Wrong?

Before I get too far into discussing the consequences of the efficiency meter, I should show exactly why I think it's off. I talked about this briefly in my in-depth report on a year and a half of mileage data, but I've learned a few things since then so this should make more sense.

Let's look at a typical couple days of commute in the summer. Even though the numbers tend to vary because of changing temperature and traffic patterns from day to day, the trends that these numbers show are very representative of what I see on my daily commute. I'll start the morning with an 80% charge and the GOM reads 84 miles-to-empty in ECO mode. The temperature outside is about 70℉. This GOM reading agrees with the efficiency meter, which reads 5.0 miles/kWh from the previous day's drive. If you assume that you have 21 kWh of useable battery energy, multiply by 5.0 miles/kWh, and take 80% of that result, you get ... 84 miles.

Why 21 kWh and not the 24 kWh that the battery is rated as? Well, there's a certain amount of reserve charge that the car will never allow you to use to protect the battery from damage, and the charger will never completely charge the battery for the same reason. The amount of useable energy ends up being about 21 kWh in practice.

Now clearly, the GOM is assuming that I'll get 5.0 miles/kWh of driving efficiency on this charge, and if I achieve that, I should be able to go 84. In reality? Not. A. Chance. After going to and from work, I've traveled 22.5 miles and the GOM reads 54 miles. It appears that the GOM lost about 8 extra miles. The next day I go another 22.5 miles and the GOM reads 21 miles. I've lost another 10 miles on the GOM. If I extrapolate from the original 84 estimated miles at the rate that the GOM is losing miles, I'll probably be able to go another 15 miles on the leftover charge. Here's the extrapolation as a chart:

Chart of Estimated Range from the GOM for 80% Charge

In total, I could go 60 miles on an 80% charge. That equates to 3.57 miles/kWh, but what does my efficiency meter read? 4.9 miles/kWh, and the next day I get another 82 miles on the GOM. This is clearly wrong, unless I really only used 9.2 kWh out of an available 16.8 kWh  on the 80% charge. That does not sound reasonable, so the only explanation is that the efficiency meter is lying to me.

Why Is My Leaf Trying To Give Me a Snow Job?

I'm really confused here. With a little simple arithmetic, it's blatantly obvious that the efficiency meter is reporting bogus numbers, and those numbers are affecting the GOM's accuracy as well. It doesn't even matter that I'm driving in ECO mode at 55 mph and not really getting the efficiency benefits of ECO mode. The car should still be able to measure the battery power being used and miles being driven and do a simple division. Heck, I can do it myself after the fact without knowing for sure what the kWh levels are and come up with a much more reasonable efficiency number anyway.

The point is, the car should be able to calculate this number very accurately. All it needs is a circuit that measures the voltage and current coming out of the battery, accumulate the product of those values over time to get energy consumed, and then divide the miles traveled in that time by the accumulated energy value. Why isn't Nissan doing this? Is it to make me feel better about how great my EV's efficiency is? Because it's not working, and I'm not fooled.

Look, I understand the desire to report the best numbers possible to put the car in the best possible light. As an engineer, I get that it's very tempting to measure performance with rose-colored glasses, and there's a lot of pressure from marketing and management to inflate metrics. But this efficiency number doesn't lend itself well to optimistic reporting. It's too obvious when it's wrong, and it's not at all useful to the driver of the car if it's not accurate.

In fact, reporting an inflated efficiency number is worse than useless because I'm constantly reminded that this great car, with all of its high-tech monitoring and control systems, is incapable of giving me a transparent and honest account of its energy usage. I know that the real measurements are being done and are available within the ECU. Somehow the battery management system and motor control system need to know the amount of power coming from or going to the battery to properly control acceleration and regenerative braking, and the odometer must be within a certain tolerance as well. Why can't the car report energy efficiency accurately on the dashboard?

Nissan Could Do Much Better

Correcting the energy efficiency meter would be a great start. With an accurate efficiency number, the GOM would improve dramatically, and I could more easily calculate the cost per mile for my driving. I know the cost is low, but I'd like to know what it actually is instead of some optimistic, sugar-coated nonsense.

I think Nissan could do even better, though. There are also power losses in the on-board charger and the battery when charging, and I'm sure the Leaf knows the power going into the charger already because it has to control the process. The Leaf could report a charging efficiency number as well. It could also combine the charging and driving efficiency measurements by taking the number of miles driven since the last charge and dividing it by the amount of energy used to recharge the battery, similar to the way you calculate mpg with an ICE car.

With that overall efficiency number I could easily calculate the real cost of driving the Leaf. Actually, the Leaf could do it for me if I could enter what I pay for electricity, the same way I can enter the price of gas in my Prius so it can calculate cost for me. Wouldn't that be nice. I'm sure plenty of Leaf owners would love a feature like this, if it was accurate. In the meantime, I can do this measurement manually if I buy a $30 electricity usage monitor and measure the electricity used to charge my Leaf at the outlet. I'm planning on doing just that because, well, inquiring minds want to know.

The Rest of the Leaf Series:
Part 1: The Acquisition 
Part 2: The Summer Drive 
Part 3: The Winter Drive
Part 4: Frills and Maintenance
Part 5: The Data
Part 6: The Future
Part 7: The Energy Efficiency Meter