How Charged Up Is Your Battery?


Better Place plugged in 1Electric car geek out alert!

Yesterday I received an update to the Better Place Oscar in car navigation and range prediction system. We’ve been waiting a long time for this and over the next few days I’ll review it thoroughly. I’m most keen to see if they’ve improved the system’s ability to cope with predicting the effect of large changes in altitude on battery use.

With this upgrade, however, we’ve lost one major cosmetic feature: a 3D view that used to show pretty graphic versions of the buildings. They’ve done this because a new range prediction system is much more computationally intensive and the unit in the car couldn’t walk and chew gum.

I’ll only know if the range prediction has improved after a few trips when the car has learned my driving style (pedal to the metal usually).

There is one other hidden feature we’ve lost but perhaps with some lobbying we’ll get that one back.

Typically today’s electric car battery capacities are measured in kilo-watt hours: kWh. 1 kWh is the amount of energy used by a 1000 W room heater if you run it for 1 hour. My car uses around 16 to 18 kWh to drive 100km and the specification of the battery is a maximum of 22.0 kWh. The most expensive Tesla Model S has an 85 kWh battery (so you can see why it can drive so much further).

The old Oscar system used to allow us to view the battery’s state of charge as either a percentage or as a raw kWh number. Even displaying an accurate percentage figure puts Better Place ahead. The Nissan Leaf and the Tesla don’t do this and their owners rely on a “guess-o-meter” that tells them the expected range they have left. That, however, is a complex number derived from a calculation involving both the battery’s state of charge and the most recent driving style. So it varies wildly.

By comparison, Oscar’s percentage or kWh readout drops in a predictable linear way depending on the terrain and the speed and most owners get used to this. Oscar even takes this further and predicts your state of charge at a destination including taking up-hill, down-hill and speed limits into account.

The advantage of the kWh display was two fold:

  1. It gave a figure to one decimal place (0.0 to 22.0) making it much more graduated (220 graduations vs 100) than the 0% to 100% available with percentage and;
  2. The system reports all values of 20.4 kWh or above as 100% and 0.3 kWh and below is 0%

Point 2 is important because there is some variation in the state of charge when coming to the car in the morning. I’m not sure what this is linked to but it isn’t always battery age. Remember, I switch batteries fairly frequently but sometimes go a week or two with the same one. I see a range from about 20.8 kWh to 22.0 kWh. All these are reported as 100% but it does make a difference in how far I drive before the percentage starts dropping.

Before we lost the system I made a little graph and it clearly shows this hysteresis and a linear relationship between battery percentage and kWh in between.

kWh vs Displayed % for battery state of charge on Better Place Renault Fluence ZE

If you want the raw numbers:

  • Lower cut off (at or below which 0% is reported) is 0.3 kWh
  • Upper cut off (at or above which 100% is reported) is 20.4 kWh
  • Each kWh represents 4.9495% of the Renault Fluence ZE’s battery.
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