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When does 12V battery actually get charged

hun

Member
Jan 14, 2014
88
12
Budapest, Hungary
I couldn't plug the car in for a couple days. It's been very cold here (few degrees below freezing), and I've been driving short distances in the city with lights, heat, music, etc. The regen limit line has not disappeared from the dashboard for days.

Today, I got a "12V Power Low - Car May Shut Down Unexpectedly" later turning into "Car Needs Service - Car May Not Restart". I called roadside assistance (Tilburg). I was told they don't see a problem on their end, but I should make a service appointment. I came home, and the charge port turned red immediately when opening. I don't have enough juice to get to the closest service center, and I can't charge the car.

This made me think. When does the 12V battery get charged? I assume it gets charged when the car is plugged in. How about when driving around? Does it always get charged, or only when the main battery temperature reaches a certain level?

I'm basically trying to figure out if I can charge the 12V battery by driving around for an hour or two (assuming the battery is simply depleted, and nothing is wrong with it or with the charger).
 

tezco

Sig P85
Nov 9, 2012
819
4
Colorado
The 12V charges from the main pack battery from time-to-time as needed, under software control. Driving around or plugging in won't change this. Most folks only get about a year out of the 12V before it needs replacement.
 

HankLloydRight

No Roads
Jan 18, 2014
12,951
10,973
Connecticut
I wonder why Tesla hasn't incorporated something like this in the software/hardware of the car to report back to HQ when the 12V is going bad.

http://www.pulsetech.net/390PT-PulseTech-Battery-Analyzer-6V-and-12V-Battery-Tester-7332.aspx

12months is very short for a battery, I deal with MANY batteries that are on/off chargers and I usually get 24-30months out of all of them.

They do. I got a call one day from Tesla telling me that my 12V battery needed to be replaced. Tesla came to my house and replaced the battery.
 

spottyq

Member
May 7, 2015
278
49
Belgium
To answer OP's question, the 12V battery is being charged anytime the contactors of the HV battery are closed.
That is, when charging, when the car is on, when pre-conditionning, or automatically (can happen anytime) to prevent the 12V from getting discharged.
 

hun

Member
Jan 14, 2014
88
12
Budapest, Hungary
To answer OP's question, the 12V battery is being charged anytime the contactors of the HV battery are closed.
That is, when charging, when the car is on, when pre-conditionning, or automatically (can happen anytime) to prevent the 12V from getting discharged.

I still suspect the 12V battery may not get charged while the regen limit is in place. I drove the car for long enough for the regen limit line to disappear. Then I parked in a warm garage, and after a couple hours the error message disappeared. I was able to charge the car, and all has been well since. I'll ask for confirmation when taking the car for service.
 

GlmnAlyAirCar

Active Member
Mar 19, 2015
1,445
1,205
Sanatoga PA
I still suspect the 12V battery may not get charged while the regen limit is in place. I drove the car for long enough for the regen limit line to disappear. Then I parked in a warm garage, and after a couple hours the error message disappeared. I was able to charge the car, and all has been well since. I'll ask for confirmation when taking the car for service.
Take this with a grain of salt, but when I got my 12V battery replaced a few months back, the SC gave me different information. The 12V battery ONLY charges when it gets below a certain voltage, regardless of whether you are parked, driving, preconditioning, etc. All of the electronics and other 12V systems are running off the 12V battery continuously. The 12V battery is deep cycling even while driving.
 

Soolim

Member
Jun 11, 2015
852
38
Vancouver, BC, Canada
Take this with a grain of salt, but when I got my 12V battery replaced a few months back, the SC gave me different information. The 12V battery ONLY charges when it gets below a certain voltage, regardless of whether you are parked, driving, preconditioning, etc. All of the electronics and other 12V systems are running off the 12V battery continuously. The 12V battery is deep cycling even while driving.
I believe this explanation, and it make sense. My question is: Can Tesla not minimize the deep cycling of the 12V battery (to extend the life of that small battery) since we have a BIG battery?
 

GlmnAlyAirCar

Active Member
Mar 19, 2015
1,445
1,205
Sanatoga PA
I believe this explanation, and it make sense. My question is: Can Tesla not minimize the deep cycling of the 12V battery (to extend the life of that small battery) since we have a BIG battery?
I fully agree. I know this has been discussed in great detail in other threads and it's a more complicated problem that it appears on the surface. But I'm sure it can be solved.
 

Lloyd

Well-Known Member
Jan 12, 2011
6,276
2,082
San Luis Obispo, CA
I wonder why Tesla hasn't incorporated something like this in the software/hardware of the car to report back to HQ when the 12V is going bad.

http://www.pulsetech.net/390PT-PulseTech-Battery-Analyzer-6V-and-12V-Battery-Tester-7332.aspx

12months is very short for a battery, I deal with MANY batteries that are on/off chargers and I usually get 24-30months out of all of them.

They do this. I was called after 6 months of ownership and told that they had telemetry that indicated that my battery was going to fail. It was replaced the following week with no outage or loss of use.

Another time I called to ask a question, and the person indicated that I had several instances in the previous week where the tire pressure indicator had gone off. So they are able to monitor that also. They sent a ranger with a tire and rim if necessary. We found a screw in the inside sidewall, and the tire was replaced at a local tire shop by the ranger.

Great service.!!!
 

ecarfan

Well-Known Member
Sep 21, 2013
19,220
13,869
West Vancouver, British Columbia
They do this. I was called after 6 months of ownership and told that they had telemetry that indicated that my battery was going to fail. It was replaced the following week with no outage or loss of use.
Same thing happened to me after about a year of ownership. Great service! Now at just over two years of ownership. Will be interesting to see how long the replacement 12V battery lasts.
 

miimura

Well-Known Member
Aug 21, 2013
6,252
5,865
Los Altos, CA
Take this with a grain of salt, but when I got my 12V battery replaced a few months back, the SC gave me different information. The 12V battery ONLY charges when it gets below a certain voltage, regardless of whether you are parked, driving, preconditioning, etc. All of the electronics and other 12V systems are running off the 12V battery continuously. The 12V battery is deep cycling even while driving.
I don't believe this is true. When the HV system is active, the DC/DC should be floating the 12V battery and entire 12V system at about 14V. The HV system is active when driving and charging. The car will cycle the 12V battery when the traction battery is otherwise not needed.
 

GlmnAlyAirCar

Active Member
Mar 19, 2015
1,445
1,205
Sanatoga PA
I don't believe this is true. When the HV system is active, the DC/DC should be floating the 12V battery and entire 12V system at about 14V. The HV system is active when driving and charging. The car will cycle the 12V battery when the traction battery is otherwise not needed.
That would make sense, but it's contrary to the information I was given.

Correction: 12V battery needs to be operational to hold the HV contactors closed. By continually holding the LV bus high, cutting the 12V battery cables would NOT de-energize the contactors, preventing first responders from isolating HV DC.
 
Last edited:

miimura

Well-Known Member
Aug 21, 2013
6,252
5,865
Los Altos, CA
That would make sense, but it's contrary to the information I was given.

Correction: 12V battery needs to be operational to hold the HV contactors closed. By continually holding the LV bus high, cutting the 12V battery cables would NOT de-energize the contactors, preventing first responders from isolating HV DC.
As long as the 12V line to the contactor goes directly to the battery instead of being tapped in somewhere else, you could still cut it and the high voltage would be de-energized. The DC/DC can still float the 12V system high whenever it has HV power.
 

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