Why does the battery discharge while plugged in?

[ZeeDoktor]

With charge target set to 80%, my car while plugged into the wall at home goes through several discharge to 78% then charge back to 80% cycles a day. Is there a smart reason it is doing that, or is that a BMS planning oversight? I would have thought that minimising any charge cycles while plugged in would be best for battery health. But perhaps it's better to keep a little bit of load on them while sitting? Any learned battery experts with an opinion on this?

 

[ucmndd]

It's working as designed.

Powering the car electronics continuously from the wall would require the charging circuitry and DC/DC converter to be active at all times, which is a terribly inefficient way to provide a few hundred watts. So instead the low voltage power needs are provided by the 12v battery, and the HV battery contactors are closed occasionally to power up the DC/DC system and recharge the 12v as necessary.

The shallow cycling is really inconsequential to any measure of battery health.

Though I will say that losing 2% "several times a day" is pretty excessive. Are you using sentry mode, cabin overheat, or other power draining features at home?

 

[ATPMSD]

Though I will say that losing 2% "several times a day" is pretty excessive

Maybe not, 2% in an MS with a 100 kWh battery is 2 kWh. If Senrty mode is enabled it draws around 250 wH, so cycling ever 8-hours world seem normal.

 

[ZeeDoktor]

It's working as designed.

Powering the car electronics continuously from the wall would require the charging circuitry and DC/DC converter to be active at all times, which is a terribly inefficient way to provide a few hundred watts. So instead the low voltage power needs are provided by the 12v battery, and the HV battery contactors are closed occasionally to power up the DC/DC system and recharge the 12v as necessary.

The shallow cycling is really inconsequential to any measure of battery health.

Though I will say that losing 2% "several times a day" is pretty excessive. Are you using sentry mode, cabin overheat, or other power draining features at home?

No sentry mode, this is at home. And you're right, looking at teslafi it's only about 1% per 24h when the car is not in use at all. Good explanation, thank you!

 

[Tomilett]

Older cars engage the battery coolant pumps at anything above 78% (or 80% - something around there) so it'll drop to 78% pretty sharpish but below that won't move as much. But it looks like yours is a 2020 car so unlikely to be that.
If you're bothered by it, set a scheduled charge time (e.g. midnight) and it'll only do one of these little charges per day.

 

[OrbiterTI4]

Older cars engage the battery coolant pumps at anything above 78% (or 80% - something around there) so it'll drop to 78% pretty sharpish but below that won't move as much. But it looks like yours is a 2020 car so unlikely to be that.
If you're bothered by it, set a scheduled charge time (e.g. midnight) and it'll only do one of these little charges per day.

The threshold varies both car-to-car and over time.

When it first got implemented, my vehicle's pump would come on at 77%+; I was ok charging to 75%.
Now, however-many months later, it's kicked on 74%, so my safe "don't run pump" level is 70%.

For me, is has also been independent of ambient temperature. There was an overnight I had to charge to 90%, so you could hear the pump running in the morning, but the car also said it needed to warm-up before full regen braking was available (50°F outdoor temp).

 

[lgli]

The threshold varies both car-to-car and over time.

When it first got implemented, my vehicle's pump would come on at 77%+; I was ok charging to 75%.
Now, however-many months later, it's kicked on 74%, so my safe "don't run pump" level is 70%.

For me, is has also been independent of ambient temperature. There was an overnight I had to charge to 90%, so you could hear the pump running in the morning, but the car also said it needed to warm-up before full regen braking was available (50°F outdoor temp).

Do you know when it got implemented?

 

[Tomilett]

Do you know when it got implemented?

I think 2019.16 update - the one that caused batterygate, or 2019.20 - the one that caused chargegate

 

[capcave]

I have found the vampire drops considerably if you unplug after charging. I was getting about 2% a day leaving it plugged in, it's under 1% if I unplug it.