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What really happens if you keep the car plugged in after charging complete?

Probably a noob question, but when charging is complete does the car cut off power reception from the charger? Or does car communicate this to the charger which stops sending in more power? Or does it keep charging slowly to maintain the full battery? Does this depend on chargers? :confused:

Finally got the charger set up after more than a week, only to realize that I know so little about it.
My garage uses a clipper creek charger, I forgot which model but it has 40A output. I just set up the charging 30 minutes ago and it should finish at 80% in an hour. I really want to know what will happen after that and if I need to do anything at all (I know probably not but just want to be sure).
 

brantse

Member
Supporting Member
May 18, 2015
641
3,175
Somerset, PA
I can only speak based on the evidence I've seen from having a meter on my 1450 outlet, but it appears to me that the car draws absolutely no power from the wall outlet except during times that it's either charging or if you either turn the climate control on or wake the car up by opening a door. Depending on what the climate control is doing, it appears to draw between about 600 and 1500 watts. I'm sure that heat will draw substantially more, I just haven't witnessed it yet.
 
I'm pretty sure the current is completely disconnected after charging completes. You can hear the relays clicking. It will reactivate if you restart charging or engage climate control.

As for how it works, the J1772 standard (used by L2 chargers, including Tesla's, although Tesla uses a physically distinct plug) allows for two-way communication between the car and the charger. The charger informs the car how much current is available, and the car informs the charger when it wants to have power supplied. The available current is signaled by pulsing a control pin rapidly on and off (PWM), with different duty cycles representing different available currents. For example, if the signal is on 40% of the time that means 24A is available, and 50% means 30A is available. The car informs the charger by just putting a resistor on the line, with different values for "I want to charge" and "I am here but don't want any power."

As a computer programmer, I find it all to be weirdly and charmingly analog, but it is nice and simple.
 

Rocky_H

Well-Known Member
Feb 19, 2015
8,357
10,751
Boise, ID
It does not continually sip a low level of power from the wall connection. Once it has finished charging, the car will slowly use up a minor amount of battery power until it goes down by some allowed amount (about 3%, I think?), at which it will turn on charging again for a little while to get it back up to the charge limit it was set for.
 

efusco

Moderator - Model S & X forums
Moderator
It does not continually sip a low level of power from the wall connection. Once it has finished charging, the car will slowly use up a minor amount of battery power until it goes down by some allowed amount (about 3%, I think?), at which it will turn on charging again for a little while to get it back up to the charge limit it was set for.

That's an interesting point. While I think that the percentage discharge is a more likely trigger, in reality it seems to be an every other day top up. Now, it may be that it just takes more than 24 hours for the charge to drain enough to reach that percentage threshold, but in my multiple trips away it always seems to work out that the car tops up every other day.
 
That's an interesting point. While I think that the percentage discharge is a more likely trigger, in reality it seems to be an every other day top up. Now, it may be that it just takes more than 24 hours for the charge to drain enough to reach that percentage threshold, but in my multiple trips away it always seems to work out that the car tops up every other day.

The only way I can guarantee my car is charged to the full limit I set is to kick on the HVAC a few minutes before I want to leave. Doesn't matter of course, but it's fun.
 

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