"Proactive" 12v battery replacement - good idea or overkill?

[gearchruncher]

I'd much rather not bother with a 12v at all and just use the main battery like they did in the roadster.

Because of the way a 1500W, 400V to 12V DC-DC converter works, combined with HV safeties like the contactor, having this converter on burns about 200W just to have it running.

200W, 24/7 is 1,750 kWh per year. That's the same as driving an additional 7,000 miles a year. Not very environmentally or financially friendly (neither is sentry which does the same thing)

It also means that in any case where the car needs to disable HV, like after a crash, all 12V is lost immediately.

There's a reason every EV learned from the roadster and also has a ~12V battery. Even Tesla learned right away, only v1 Roadsters don't have the 12V.

 

[gearchruncher]

This is what I've been using since August of last year to monitor mine

What are you monitoring with that? What information does that give you to start telling you the 12V is about to die? Are you sure given Tesla's advanced and nothing like an alternator management of the battery that it's accurate? If you believe this is a good measure of the battery health, why don't you trust Tesla's monitoring which is much more advanced?

 

[gearchruncher]

For all of you worried about your battery failing on a trip, just turn on sentry. This forces the car to run off the HV pack and basically stops using the 12V at all. It does this at the expense of 1 mile per hour range loss like sentry always does, but it does work.

Also know that driving the car more is easier on the 12v battery, not harder, unlike an ICE car.

 

[flyingowl]

For all of you worried about your battery failing on a trip, just turn on sentry. This forces the car to run off the HV pack and basically stops using the 12V at all. It does this at the expense of 1 mile per hour range loss like sentry always does, but it does work.

Also know that driving the car more is easier on the 12v battery, not harder, unlike an ICE car.

this a shitty solution to the problem.

 

[RayK]

I'll be at 4 years this July and currently at 23.5K miles with the original battery. Going on a trip up to Ashland, OR next month so I'm seriously considering proactively replacing the 12V battery even though I've had absolutely no problems with it.

So I scheduled service yesterday (6/1) for 6/6 to proactively replace the 12V battery. Got a message this morning saying they had some cancellations and that there were several openings tomorrow (6/3) so now mobile service is coming out 12n-4p. Cost estimate is $127.26 ($119.50 for the battery and $7.76 taxes). I also asked if they could bring out a 1537264-00-B but I'm not hopeful they'll find one in time.

Question: For anybody who has done the battery swap using mobile service, were you able to keep the old battery? There's nothing on the service invoice saying that a core charge is involved or that I need to relinquish the old one.

 

[texas_star_TM3]

this a shitty solution to the problem.

not at all. We mostly stay at airbnbs when traveling and i always plug the car into a household outlet. so sentry range loss isn't a thing overnight for me.

 

[gearchruncher]

this a shitty solution to the problem.

It's not, given then "problem" is one of human anxiety against a problem that doesn't exist anymore, and people thinking they should proactively replace the battery before a trip.

The actual solution is better built in testing of batteries and then changing how the car behaves when it detects a bad battery to prevent someone getting stranded, which Tesla has already implemented, but that hasn't solved the anxiety problem from people reading threads from years ago.

Throwing sentry on for a week is a more environmentally sensitive solution to your anxiety than replacing a hunk of lead way before it needs to be (yes, most batteries are recycled, that still takes a lot of power).

 

[android04]

I didn't think sentry and all the smart features used the 12v, I just assumed it was stuff like a/c that used it although I've not looked into it in much detail. I'd much rather not bother with a 12v at all and just use the main battery like they did in the roadster.

It's the opposite of what you think. Everything in the car uses 12v, except for the drive units and A/C compressor/cabin heater (both are one unit in the newer cars with the heat pump).

 

[android04]

What are you monitoring with that? What information does that give you to start telling you the 12V is about to die? Are you sure given Tesla's advanced and nothing like an alternator management of the battery that it's accurate? If you believe this is a good measure of the battery health, why don't you trust Tesla's monitoring which is much more advanced?

To be clear I am not one of the people that's paranoid or concerned about my 12v battery going out, so I am not using the battery monitor to alert me if the 12v battery is about to die. I mostly hold similar views and opinions about the 12v battery and system on my Model 3 as you are expressing on this thread. In 50 months and 79,000 miles of use my 12v battery is still going strong and a load tester that I used to check on it a few days ago still indicates it has 87% SoH (state of health). I have observed the changes that Tesla has made to the charging/discharging behavior over time for the better, and have read the experiences that people have had with the improved behavior in saving them from coming back to a dead 12v battery.

My plan is to run the 12v until it dies (or I get the battery replacement warning) and then replace it myself with a locally-sourced group size 51R battery. The battery monitor I've been using has to be calibrated to the battery when it's fully charged and uses a shunt on the negative terminal to measure ALL current going through it in both directions. So it displays the battery voltage, current +/-, charge level in %, and capacity in Ah. The charge level in % and capacity in Ah are set when the battery is fully charged so that the monitor can track the usage over time. It is powered by the 12V battery. The only thing I doesn't have that I wish it did was some sort of logging or memory of all the metrics over time. I have resorted to leaving a spare smartphone in the car set up to take periodic pictures of the battery monitor screen to see changes over time. My long-term solution is to use an Arduino with a microSD card module and RTC module to log the changes over time, but haven't had time to finalize that yet.

I have been using the battery monitor to observe the behavior of the 12v system on my Model 3 and how it changes over time. My 12v battery was manufactured in January 2018 and has been in use in my car since March 2018. I don't plan on replacing it until it dies because I can pop open my frunk very easily and use the LiFePO4 jump starter I carry in there to jump start myself. I carry a tiny A23 battery in a Ziplock bag behind the tow hook cover to use for popping open the frunk. I always carry the jump starter to help other cars if they need it (and have used it 6 times), but can use it on myself. If I had to replace my 12v battery because I couldn't jump start it (in case of multiple dry cells or shorted cells), then I can walk or get a ride the short distance to my O'Reilly Auto Parts or Walmart and buy a replacement group size 51R battery. I carry the only tool needed for replacing it (a 10mm wrench) in the pouch with my jump starter. I'm pretty confident that in an emergency where I am not near a store that carries a replacement battery that I can use my jump starter in bypass mode to act as my 12v battery (I've done that a few times on ICE cars with a dead 12v battery and broken alternators to limp them somewhere).

I also understand very well how the Tesla charging system works and how to force the car to stay on with Sentry Mode or Keep Climate On in case I get the "replace battery soon" warning and need to get out of the car and park it. Although that doesn't seem to be necessary for the most part now because Tesla now keeps the car awake automatically when they detect a weak battery. I also understand that it's impossible to predict a battery failure 100% of the time because there are many different failure modes and they manifest themselves differently. So there will never be 0 incidences of the 12v battery being dead when people come up to open their Tesla doors, but they have been reduced with software updates.

 

[gearchruncher]

The charge level in % and capacity in Ah are set when the battery is fully charged so that the monitor can track the usage over time.

I still don't get it- fully charging a battery doesn't tell you the Ah capacity, only that the SoC is 100%. Unless you are running a battery model and monitoring how fast SoC changes as you put in/take out energy, you don't know the SoH. But that itself is super complex, as battery temperature and impedance changes can really mess that up. All you can really know from coulomb counting is the amount of Ah you are using, not what the battery actually supports. If the charger always drains about 10Ah then recharges, that's all the external monitor will ever see. It can estimate SoC from that based off an assumed battery capacity, but it can't verify that estimate.

This is the whole reason Tesla needs a full cycle of the HV battery now and then to really estimate SoH of the pack. But Tesla never really allows the 12V through an arc like that, so estimating SoH with an external device like this is very advanced.

 

[android04]

I still don't get it- fully charging a battery doesn't tell you the Ah capacity, only that the SoC is 100%. Unless you are running a battery model and monitoring how fast SoC changes as you put in/take out energy, you don't know the SoH. But that itself is super complex, as battery temperature and impedance changes can really mess that up. All you can really know from coulomb counting is the amount of Ah you are using, not what the battery actually supports. If the charger always drains about 10Ah then recharges, that's all the external monitor will ever see. It can estimate SoC from that based off an assumed battery capacity, but it can't verify that estimate.

This is the whole reason Tesla needs a full cycle of the HV battery now and then to really estimate SoH of the pack. But Tesla never really allows the 12V through an arc like that, so estimating SoH with an external device like this is very advanced.

I set the Ah capacity to what's on the OEM battery label and manufacturer spec sheet -- 45Ah. That's only used to show me how that number changes over time and usage. The most I've ever seen it go down, even after letting my car sit for 3 days without use, is to about 40 or 41Ah. That just indicates to me that Tesla does not cycle the battery much anymore, or at least not on my car. I know well that the full capacity of a lead acid battery is not 100% useable and that trying to use or measure the full capacity will cause damage to it. I am also aware that my over 4 year old (and 78k mile) 12v battery does not have the full capacity that a new one does, but I am just using the Ah metric to gauge depth of discharge. The charge level % on the battery monitor also serves to show me the depth of discharge. Therefore, from what I've observed since August 2021 in my car, Tesla does not discharge the 12v battery more than 5% before charging it back up again. This is closer in behavior to a starting battery in an ICE vehicle where the battery is only used for starting the engine and the alternator then keeps it charged up. This makes sense to me because I don't believe that the OEM Tesla 12v battery is a deep cycle, nor AGM battery like some people seem to think it is. In my view, Tesla probably reduced the depth of discharge that it exposed the OEM battery to over time and reduced the amount of battery failures.

Edit to add that when my 12v battery gets replaced, I plan on dissecting it to see if it's really an AGM and how thick the lead plates are on it.

 

[Theone8181]

I thought the max voltage on the 12v gets (slightly) lower when the capacity reduces?

 

[mrau]

This is another battery monitor that uses bluetooth to send voltage info to your phone.

Amazon.com: QUICKLYNKS Auto Battery Monitor BM2 Bluetooth 4.0 12V Device Car Battery Tester : Automotive

Buy QUICKLYNKS Auto Battery Monitor BM2 Bluetooth 4.0 12V Device Car Battery Tester: Battery Testers - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

 

[TulsaCoker]

Question: For anybody who has done the battery swap using mobile service, were you able to keep the old battery? There's nothing on the service invoice saying that a core charge is involved or that I need to relinquish the old one.

nope they took it

 

[gearchruncher]

This is another battery monitor that uses bluetooth to send voltage info to your phone.

Voltage alone is not useful to estimate the health of a battery, particularly when some external charger is managing it.

 

[mrau]

The external battery monitor just monitors the voltage of the battery, it is not a charger. It plots all the info on a daily graph when you synch monitor with your phone. It is handy for seeing when the car sleeps (since voltage slowly drops) and when it wakes up (voltage jumps and remains higher). Generally the car wakes up about once every 24 hours if it has not been driven. If voltage resting level is below 12.3 it may be indication of a weakening battery.

Here is what a non-driving day looks like when car awoke about 1800 hrs (6 PM) with a spike to 14.5 volts.

 

[gearchruncher]

If voltage resting level is below 12.3 it may be indication of a weakening battery.

Voltage is a measure of the state of charge of a battery, not the state of health.
12.3V is about 70% state of charge. 12.2 doesn't mean the battery is weak, it means it's at 60% SoC.

Unless you are monitoring this constantly and see how often your car is charging your battery, no one measurement means anything, and then all that data is irrelevant once Tesla does a SW update since we know they keep tweaking the behavior. That's a LOT of manual work, and for what? You don't think Tesla's own battery failure detection is doing exactly the same thing?

 

[brkaus]

Voltage is a measure of the state of charge of a battery, not the state of health.
12.3V is about 70% state of charge. 12.2 doesn't mean the battery is weak, it means it's at 60% SoC.

Unless you are monitoring this constantly and see how often your car is charging your battery, no one measurement means anything, and then all that data is irrelevant once Tesla does a SW update since we know they keep tweaking the behavior. That's a LOT of manual work, and for what? You don't think Tesla's own battery failure detection is doing exactly the same thing?

Having been stranded by a dead battery once, I have my doubts. But I’m optimistic that they are improving it.

 

[Theone8181]

How many people have been standed so ffar had an issue, I'd bet its less than 1%. Be interesting to see what the spread is, I'd expect the S to have more due to age, but then there must be more 3s on the road. I don't know what it's like over there but it kind of feels like the 3 is 19% of all the cars on the road , I occasionally see an S or a Y. X seems to be the rarest.

 

[RayK]

So I scheduled service yesterday (6/1) for 6/6 to proactively replace the 12V battery. Got a message this morning saying they had some cancellations and that there were several openings tomorrow (6/3) so now mobile service is coming out 12n-4p. Cost estimate is $127.26 ($119.50 for the battery and $7.76 taxes). I also asked if they could bring out a 1537264-00-B but I'm not hopeful they'll find one in time.

Question: For anybody who has done the battery swap using mobile service, were you able to keep the old battery? There's nothing on the service invoice saying that a core charge is involved or that I need to relinquish the old one.

Mobile service just left my house after replacing the 12V battery and leaving me the old one. Since it was effectively a parts sale for the battery and not a warranty issue, he said it shouldn't be any problem with me keeping it. Only took about 20 minutes to swap out the battery, install the air vent grille over the intake area that I've had for a couple of months and check the tires.

But that's not the best part: he also brought the -00-B ECU.