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12v battery issues and best practice

Peelz

Budding EV fanatic.
Apr 10, 2021
332
454
SE Iowa, the armpit.
I've been reading a few instances where cars lose the 12v battery on these things sans warning. Im not excited about that lol but Id like to add a note, that our nissan pathfinder did this once too and stranded us in the cold.. but it was 3 yrs old, not totally unexpected-I guess.

My thoughts on this, without any real knowledge of the wiring-I imagine the 12v, being that its charged by the main pack, is only being charged while in use....just like a regular car. As in having a spinning alternator. I don't imagine its good for it to sponge fully off the main if it is just sitting any more than is to run to zero then charge all the time... Someone maybe can correct me?

Coming from an ICE world, It is not great for a battery to sit then only run for short times. Very important with my motorcycles... It is good if its charged for longer periods.

I'm curious. Outside of the usage I'm talking about and preemptively replacing at XX date or going to Li-ion - is there a system to check/maintain them to avoid an incident?

Iv'e got no problem stickin a meter on it or a charger occasionally(provided I have the right details). just curious what to watch for, if anything?
 

jcanoe

Active Member
Oct 2, 2020
3,746
3,817
Maryland
The Tesla vehicle is supposed to notify you when the 12V battery soon needs to be changed. (Not sure if the Tesla vehicle monitors the voltage of the 12V when not being powered or charged from the main Tesla battery or the frequency that the 12V battery needs to be charged when the Tesla is parked.) This method is not fool proof. You can change the battery at NN months if you wish but Tesla's warranty on the vehicle is 4 years, this includes the 12V battery.

Some prefer to keep a small 12V lithium battery jump starter pack (less than $50) at home or work for the day that the Tesla vehicle will not unlock, power on due to a failed 12V battery. If you store the jump starter battery in the Tesla vehicle and the vehicle is locked you won't be able to get to the jump starter pack.

The Tesla 12V battery looks like an AGM type battery but it is a sealed flooded lead acid battery (sometimes called a maintenance free (MF) battery. There is no starter motor in an EV so the 12V battery never has to delivery high current or peak power as when starting an ICE vehicle.

The things that cause a 12V lead acid battery to fail prematurely are load, heat, vibration and time. You can monitor the voltage of the 12V system from the 12V accessory port with a voltmeter or a USB adapter with a built in voltmeter (under $10.) At some point Tesla reprogrammed the charging routine for the 12V battery as they were probably overcharging the 12V batteries and causing the 12V batteries to fail. This change may have helped extend the life of the 12V battery.

If you take a trip, bring the jump starter pack and watch a video on how to unlock, start the Tesla vehicle should the 12V fail while on a trip.

Tesla will send a Mobile Service technician (where available) to your home or work location to replace the 12V battery if it fails. Compared to some other vehicles (including the Tesla Model S, X) changing the 12V battery on the Tesla Model 3 and Model Y is not difficult. If you wish to change the 12V battery yourself you can order the 12V battery from the Tesla Service Center and pick this up at the Service Center. There is no substitute replacement (deep cycle) flooded lead acid battery for the Tesla 12V battery other than from Tesla so you would want to use the Tesla 12V (the price is quite reasonable, even with installation.) Some prefer to replace the Tesla 12V battery with a lithium battery that should last longer than the Tesla 12V battery.
 
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Peelz

Budding EV fanatic.
Apr 10, 2021
332
454
SE Iowa, the armpit.
The Tesla vehicle is supposed to notify you when the 12V battery soon needs to be changed. (Not sure if the Tesla vehicle monitors the voltage of the 12V when not being powered or charged from the main Tesla battery or the frequency that the 12V battery needs to be charged when the Tesla is parked.) This method is not fool proof. You can change the battery at NN months if you wish but Tesla's warranty on the vehicle is 4 years, this includes the 12V battery.

Some prefer to keep a small 12V lithium battery jump starter pack (less than $50) at home or work for the day that the Tesla vehicle will not unlock, power on due to a failed 12V battery. If you store the jump starter battery in the Tesla vehicle and the vehicle is locked you won't be able to get to the jump starter pack.

The Tesla 12V battery looks like an AGM type battery but it is a sealed flooded lead acid battery (sometimes called a maintenance free (MF) battery. There is no starter motor in an EV so the 12V battery never has to delivery high current or peak power as when starting an ICE vehicle.

The things that cause a 12V lead acid battery to fail prematurely are load, heat, vibration and time. You can monitor the voltage of the 12V system from the 12V accessory port with a voltmeter or a USB adapter with a built in voltmeter (under $10.) At some point Tesla reprogrammed the charging routine for the 12V battery as they were probably overcharging the 12V batteries and causing the 12V batteries to fail. This change may have helped extend the life of the 12V battery.

If you take a trip, bring the jump starter pack and watch a video on how to unlock, start the Tesla vehicle should the 12V fail while on a trip.

Tesla will send a Mobile Service technician (where available) to your home or work location to replace the 12V battery if it fails. Compared to some other vehicles (including the Tesla Model S, X) changing the 12V battery on the Tesla Model 3 and Model Y is not difficult. If you wish to change the 12V battery yourself you can order the 12V battery from the Tesla Service Center and pick this up at the Service Center. There is no substitute replacement (deep cycle) flooded lead acid battery for the Tesla 12V battery other than from Tesla so you would want to use the Tesla 12V (the price is quite reasonable, even with installation.) Some prefer to replace the Tesla 12V battery with a lithium battery that should last longer than the Tesla 12V battery.
thank you for this. I was curious about going flat then overcharging just as much as I was just flat out undercharging.

I just wanted to ask the questions, apply new knowledge to the antiquated stuff in my head.

I do wonder if some folks maybe just had a loose connection, which I imagine would cause a much bigger headache than with a car that only needs the voltage to start.
 

jcanoe

Active Member
Oct 2, 2020
3,746
3,817
Maryland
It may be worthwhile to remove the shroud cover from the frunk at least once or twice a year so you can inspect the fluid levels (coolant, brake fluid) and also the connections on the 12V battery for signs of corrosion. My Model Y is now a year old and I just removed the shroud to inspect these items. Everything looked good.
 
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Peelz

Budding EV fanatic.
Apr 10, 2021
332
454
SE Iowa, the armpit.
There were some cases of Model Y vehicles with a loose/bad chassis ground but this is not common.

Figures. but if one bad thing happens to something a group doesnt like, the pile-on begins. All ev's catch fire etc.

I work on digital printers for a living. Its pretty amazing what just a tiny amount of lost voltage can do to ruin your day :p
 

jcanoe

Active Member
Oct 2, 2020
3,746
3,817
Maryland
The new Model S/X have a small lithium 12V battery. Let's hope they make that available for the 3/Y as a retrofit.
If you ever watch a video of what needs to be done to change the 12V battery on a Model X you'll understand why Tesla is switching to a lithium battery in the Model S, X vehicles. Hopefully the 12V lithium battery really does last longer in the Tesla vehicle than a flooded lead acid battery. It is much, much easier, faster to access and change out the 12V battery in the Model 3 and the Model Y so changing the 12V battery takes the technician less time.

I'm not convinced that a lithium battery is necessarily better in this application. Lithium batteries don't handle cold or heat well (without active cooling.and heating) The 12V battery in the Tesla never has to deliver high current (something a lithium battery excels at doing.)

It might make more sense to change to an absorbent (fiber) glass mat (AGM) type 12V battery as these have been proven in hybrid vehicles such as the Toyota Prius and in the Chevy Volt and Bolt vehicles. An AGM battery is ~20X more resistant to vibration and 8X more resistant to heat than a standard flooded lead acid battery. (AGM batteries were first used in construction equipment and off-road vehicles.) Also, although not impossible it would be unusual for an AGM battery to freeze even in extreme cold temperatures.

An AGM battery has different charging parameters than a flooded lead acid battery so it would not be a simple drop in replacement. An AGM battery costs more to manufacturer. Cost is probably the reason that Tesla originally decided to use a sealed flooded lead acid battery instead of either an AGM or a lithium battery.
 
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Oct 3, 2020
205
241
Seattle
Having gone through this before with my i3, I can confirm that it was quite scary with my entire family in the car. Unfortunately, until this happens to you, you won't understand just how impactful it is when the 12v battery fails in an EV, especially while driving and without any warning. The hydraulic brake system was basically offline when this happened and it was only using regenerative braking.. Power steering was also offline, which made steering the super light i3 extremely difficult at low speeds.

For this reason, going forward I will be changing these batteries every 5 years, just like we do in the life safety equipment industry. You just can't expect a lead acid or AGM chemistry battery to last longer than that, no matter how well it's cared for.
 
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Silicon Desert

Active Member
Oct 1, 2018
3,675
3,789
Sparks, / GF1
And to say that the algorithm for checking and charging the battery has changed so many times in the last 4 years that I stopped looking. 4 times the last I checked. At least the process improves each time.
 
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AlexHung

Member
Mar 13, 2021
351
340
Santa Cruz, CA
This is one of the biggest ironies of EVs. Awesome, massive high voltage Lithium-Ion batteries, yet the issues with maintaining the low voltage batteries is forever a challenge.
You can blame the entire auto industry for its reluctancy of moving away from 12V. It's been known for many decades that a higher voltage system (39V?) is more efficient and better in general but there's so much inertia in the OEM parts industry that only now there are some movement in changing, due to HV system in EV.
 

Pianewman

Active Member
Oct 28, 2020
1,541
1,110
Fort Worth
Having gone through this before with my i3, I can confirm that it was quite scary with my entire family in the car. Unfortunately, until this happens to you, you won't understand just how impactful it is when the 12v battery fails in an EV, especially while driving and without any warning. The hydraulic brake system was basically offline when this happened and it was only using regenerative braking.. Power steering was also offline, which made steering the super light i3 extremely difficult at low speeds.

For this reason, going forward I will be changing these batteries every 5 years, just like we do in the life safety equipment industry. You just can't expect a lead acid or AGM chemistry battery to last longer than that, no matter how well it's cared for.
Living in hot TX, I will consider replacement every 3 years. My experience is based on VAG and Ford products. In fact, at my local Ford dealer, they have a map on the wall at the parts counter, with "contour" lines, showing the various temperature zones of the US, and the life expectancy of 12V batteries. Where I live, expected lifespan for a 12v battery is something like 28-36 months. (I've blotted it from my memory, as BOTH the Fords I owned, 2013 CMax, 2018 Fusion Energi, depleted their 12V batteries every 12-14 months...HAHA!)

My 2015 Nissan Leaf 12V battery failed almost exactly at 3 years. Changed over to AGM, with zero charging issues.
 

AquaY

Supporting Member
Supporting Member
May 30, 2021
481
1,559
Long Island NY
Having gone through this before with my i3, I can confirm that it was quite scary with my entire family in the car. Unfortunately, until this happens to you, you won't understand just how impactful it is when the 12v battery fails in an EV, especially while driving and without any warning. The hydraulic brake system was basically offline when this happened and it was only using regenerative braking.. Power steering was also offline, which made steering the super light i3 extremely difficult at low speeds.

For this reason, going forward I will be changing these batteries every 5 years, just like we do in the life safety equipment industry. You just can't expect a lead acid or AGM chemistry battery to last longer than that, no matter how well it's cared for.
Have you considered upgrading the battery to lithium?
I heard some do it just to reduce the weight ( which seems like a waste of money to me) but if they are more reliable and last longer then when changing it it might be worth it.
Not that I know. I'm wondering about this myself
 
Oct 3, 2020
205
241
Seattle
Living in hot TX, I will consider replacement every 3 years. My experience is based on VAG and Ford products. In fact, at my local Ford dealer, they have a map on the wall at the parts counter, with "contour" lines, showing the various temperature zones of the US, and the life expectancy of 12V batteries. Where I live, expected lifespan for a 12v battery is something like 28-36 months. (I've blotted it from my memory, as BOTH the Fords I owned, 2013 CMax, 2018 Fusion Energi, depleted their 12V batteries every 12-14 months...HAHA!)

My 2015 Nissan Leaf 12V battery failed almost exactly at 3 years. Changed over to AGM, with zero charging issues.

Good call. Climate certainly plays a factor in how long they last.

The point is, change it as a preventative maintenance item at whatever interval makes sense for your usage/climate, do not wait for the EV to tell you that it needs replacement as it will always be too late when this occurs.
 
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Oct 3, 2020
205
241
Seattle
Have you considered upgrading the battery to lithium?
I heard some do it just to reduce the weight ( which seems like a waste of money to me) but if they are more reliable and last longer then when changing it it might be worth it.
Not that I know. I'm wondering about this myself

I‘ve definitely thought about swapping the 12v out for a lithium solution, but they’re still quite expensive and, as others have mentioned, are not yet refined enough to survive all of the extreme conditions that the car and lead acid battery can.

Lithium batteries have been available for motorcycle battery replacements for quite some time due to their lightweight characteristics, but they do not have the built in protection that modern lithium batteries should include (Such as a BMS, low temp sensing to stop the charge or activate a built in battery heater, etc.). For as much as they’re asking for a suitable lithium solution, I expect it to include all of these features.

EDIT: I just checked the ohmmu site and apparently their 12v battery for the MY does have a BMS built in, but it says nothing about below freezing conditions, only ’extreme situations.’ For almost $450, it should be able to charge at below freezing temps, which will destroy most Lithium batteries if not equipped with a built in heater.
 
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chnry

Member
Apr 8, 2021
36
29
Vancouver
When a mobile tech was installing homelink onto the MY. He was telling me all about the components under the frunk. He mentioned to me that the 12v battery does not get charged when the MY is plugged in. He said it only charges when the vehicle is on and being driven. I thought it was strange. He said some people put a trickle charger on the 12V when they park it for an extended period. Can anyone confirm that's actually the case?
 
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