What really happens to the State of Charge (SOC) when charging an EV?

[Art Hunter]

For EV owners and those planning on making the switch, here is a supercharger 17 minute time record of a Tesla Model 3

At 9:29 p.m. the Tesla Model 3 vehicle connected to a Tesla supercharger in Trenton, Ontario when the SOC was depleted down to about 17% full.   Immediately the 250 kW Nametag Charger provided its full design capacity of 250 kW.   However this rapidly dropped as the battery SOC started to climb in a near linear fashion up to about 65%.  By 9:46 p.m. or 17 minutes later the driver disconnected, (after adding 304 km to his range) to continue his journey as the in-car software told him he had plenty of range (360.5 km) to make it  to his destination.   There was no point in continuing to charge at $0.59/kWh at the supercharger when he can do it at home for about $0.026/kWh at the overnight ultra low rate.   Do also notice the Rate of charge started at 250 kW and dropped to about 80 kW during the 17 minutes and was continuing to slow down at an exponential rate while the SOC was also flattening out (slowing down).   

Without a graph, it is nearly impossible to explain this to a non-EV owner.    I also expect many EV owners don't realize that the charge rate is NOT constant but it is controlled by smart software within the vehicle.   This software was designed by the folks who worry about the impact of charge rates on battery health and life.

While we are talking about rate of charge, a recent article measured Tesla battery range loss with an interesting and unexpected result.   After a couple of years for those that almost constantly charged at superchargers (very fast at 250 kW) and those who charged at home (very slow at 1.3 kW), the battery range degradation was about 5% in both cases.   In other words, Slow charging or fast charging made no difference in battery degradation and the range loss was minimal.   EV owners need not concern themselves about the rate of charge on battery health.   I charge at home  at the very low rate of charge and the ultra-low charge price after 11 p.m.  Most of the time my battery is at my setpoint of 75% SOC at 7 a.m. but even if it is not there yet, I disconnect and get about my business.   It will be made up the following night.   

I hope you find this interesting.

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Art Hunter- Canadian Association for the Club of Rome (canadiancor.com)

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