Batt charge efficiency vs. SOC

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Echofive

New member
Joined
Jun 21, 2015
Messages
4
Have driven the Spark just over a thousand miles (so, fairly new to the 'experience').

Like a lot of the people on here, I have been playing the game. That one where you watch to see how well you are driving based on the MPGe scores. (Since I have not paid for the electricity, it shouldn't matter, but somehow I still have to get a better score).

The question I have is; Has anybody else noticed that the Lifetime MPGe (based on the OnStar app) somewhat depends on the SOC (battery level) point that you have decided to "fill the tank" again is? Meaning; when I charge (always L2) the car when the battery has been more depleted the Lifetime average goes up more, when I charge when the battery is mostly full then the Lifetime avg will drop slightly.

It seems like it may have something to do with how the batteries charge slower (less efficient) the closer they are to 100% and the app seemingly adjusting the MPGe based on the charge efficiency.

I'm guessing/hoping that Mr.DRMorgan (and perhaps others) have data that can validate charge efficiency changes based on the battery state of charge.
 
I suspect the math behind the calculation is relatively simple and that it always includes the additional miles but doesn't include the additional kWh until the vehicle is actually recharged. So the drop after recharging a mostly full battery only decrements the Lifetime MPGe a little, as compared to a complete recharge. Over time, as the total miles accumulate, the differences seen before and after a charge should decrease.
 
The MPGe is based on how electricity is used and totally ignores the power input (charge efficiency). MPGe (and your mi/kWh) displayed is simply battery energy out vs. distance driven. The guessometer (range estimate) is using the same battery energy out vs. distance driven, with some weighting based on previous usage history (more recent has more weighting). The battery will discharge more efficiently at higher SOC and higher temperatures, but that is an extremely small variance in efficiency compared to your driving habits, weather patterns, tire pressure, etc. so I'd be extremely impressed if you could single that statistic out compared to the others.

Bryce
 
I was mistaken about the MPGe calculation. Was led to that conclusion from the OnStar app.

Before charging: Lifetime: 142 MPGe, Since Full Charge: 169 MPGe
After charging: Lifetime: 134 MPGe

Any ideas why this happens consistently? The reduction in MPGe is greater when charging from a higher SOC.
 
After 1500 miles my on board efficiency is 5.5miles/kWh ~186 mpge...BUT on my on star report it is listed as 136 mpge (4.0 miles/kWh) = charging loss of 50 mpge or 1.5mpkWh/5.5kWh or 27%...pretty steep. (or OnStar mistake??)
95% level one charging, overnight mainly. no pre heating or cooling.. Overall number of 136 mpge still better than the EPA est..but a hecuva lot less (27%) than 186 mpge.
I was pretty excited about the 'onboard efficiency until I got my On Star report.. I will start to pay more attention to SOC and try to stop at 95%. Perhaps that last 5% is costing me 15% efficiency loss in kWh. I thought the 1772 VOLTec EVSE to battery to drive trip was about 85% efficient...
Over 10,000 miles/yr of driving this type of 'charging inefficiency' would mean 700 kWh of 'charging' losses.


2014 Silver Spark EV..
formerly 2012 Volt...lease exp. 12/14

drsurd
 
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