Several Curious Charging Questions

Jim McCall said:

1) If you have hilltop reserve on to limit your max charge to 90%, how does cold weather change the equation? For instance San Francisco is having some cold weather, and the mileage estimate is going down from around 212 to 180. If my charging session is complete in the morning, is the battery still charged to 90%? I was wondering whether the lower number suggests that maybe I might disable Hilltop. In truly cold climates much more extreme than San Francisco, I wonder how much more the maximum range is reduced.

2) I have the feeling that my 30 amp home charger slows down the number of miles per hour of charge, but I haven't actually tested it. I was wondering if this generally happens and if so, what exactly happening? Either the rate is slowing and it is actually not using as much power, or it is using the same amount of power but I am getting fewer miles per KWH. If the latter is true, then one might have incentive to plug the nozzle earlier.

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Don't confuse State of Charge (SoC) and range estimates. Charging to 90% (HTR) results in a 90% SoC regardless of temps. The range estimate is based on recent driving efficiency which drops in colder weather due to a variety of factors. So range estimates will tend to drop in colder times, rise in warmer times. The "sweet spot" is roughly human temperature preferences, 60-80F.

Charging times in colder weather is mainly impacted when it gets cold enough (<30F or so) to require battery conditioning which consumes some of the energy to heat (or in summer, cool) the pack. Otherwise, the pack will charge to full or target levels in roughly the same time. But again, the miles per hour charge rate is a factor of recent efficiency. So, it may feel like fewer miles per hour.

To illustrate, consider recent efficiency of 4 vs 5 mi/kWh efficiency. At 30A charging speeds (7.2kW), you will add roughly 7 kWh of energy per hour or 7*4=28 miles if recent efficiency was 4mi/kWh. If recent efficiency was 5 mi/kWh, that same hour of charging would tend to boost range by 7*5 or 35 miles. It is not that precise as the range estimates are based on a variety of factors which seem to include the EPA rated efficiency of ~4mi/kWh and average efficiency over hundreds or thousands of miles.