Charging the last 5-10 miles takes longer..

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KingUltra

Well-known member
Joined
May 3, 2015
Messages
65
I drive about 40 miles round trip for work.
I notice that on my Bosch 240V charger, when I recharge from 40-75 miles, it may take 2 hours.
But the last 5-10 miles take another hour.

Does anyone think that since it's sucking up power the whole time it's charging, I can save electricity by charging it up to 75 miles and unplug it.
 
All current generation EV's have this behaviour, including the Spark, Smart, ... and Tesla.

This is known as "taper". The last 10% of the charge takes the longest, as the battery management system will balance the Lithium Ion cells with much lower current (amps) than was used from 0-90% of the charge.

Don't worry about unplugging, the car isn't using more power, it is using less power actually, and that is why charging gets slower as the battery gets more full.

10000km+ on my Smart ED, charged to 100% every night.
 
Yeah, basically charging a battery can be compared to filling a bucket. Initially you can ramp up the water flow to max but when you are almost full you have to reduce the flow in order to fill to capacity.
 
The OP is correct in that it's more energy efficient to stop charging when the charge rate begins to taper strongly. This is due to the more or less fixed power overhead that goes to charger and power electronics cooling regardless of the charge rate. I don't have exact figures for the Spark, but an EE measured the charging efficiency of a 2011 LEAF at L1 (120V/12A) and L2 (240V/16A) and found it to be about 75% and 85% respectively, as the car used virtually the same power for cooling at both rates.

However, as others have pointed out, the cells undergo top balancing as they get close to a maximum charge, and it's a good idea to let the battery fully charge every so often (at least once a month, if not once a week or more) to allow this to occur.
 
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