Near annual replacement of 12V battery is typical according to Tesla Service Tech

[scaesare]

I've worked on chips for smartphones and can tell you the hardware is absolutely designed to meet a strict power budget. So unless Tesla is making their own chips (extremely unlikely) the hardware hooks for low power operation are already there. I think this is a matter of the software architecture not taking advantage of the hooks because of the rush to market as has been stated earlier. I do hope we are able to see some improvements in the "classic" cars over time. I'm afraid the Model X and Model 3 are going to prevent that from happening. I just hope the software team has this problem under control for the first Model X.

I have enjoyed this discussion. Thanks to everyone for posting.

I have one more question. When the car is out of warranty and/or Tesla decides to stop replacing 12V batteries out of good will, would it make sense to put a 12V charger in the frunk and just plug it into a 120V AC outlet when parked in the garage? Short of Tesla addressing the vampire drain issue through software changes or the charger change mentioned up-thread, this could be a quick and dirty way to prolong the life of the 12V battery.

I suspect there's a lot to this.

From the "Made a connection on the internal Ethernet" thread, it appears that both the dash and the console screen communicate with at least one other internal computer. We also know that all the basic car platform "driving functions" continue to operate while the dash and center console are being rebooted... so I assume that's what that third internal system is: the "chassis computer".

If I had to venture a guess, I'd bet its some sort of industrial platform running a real-time OS. I'm not sure how optimized those are for low power. I'd wonder if it doesn't just run all the time at full power.

I wonder what is throttled down on the center screen. As I mentioned earlier, the Tegra3 has a fifth low-power core than can be used to for background tasks, such as to monitor a radio for connection requests to wake up the other cores for a 3g session l etc. We know both the 3g an WiFi radios are part of that center console, and even with the car off the car can communicate on the networks, HVAC systems can be triggered, etc... So it would be interesting to know how much of that system can go to sleep and to what extent...

 

[Lloyd]

Thanks Cottonwood, and you are correct. I neglected to compensate the number of cycles as they are 50% depth (my crude assumption), and that means the car would need 6 x 50% cycles per day to feed the 1.2 kWh vampire load. That shortens the battery life to ~300 days. The small correction you applied for the available capacity at this level of discharge is appropriate, so I concur on 6.6 x 50% cycles per day to feed 1.2kWh, nominal lifetime of 272 days (~9 months), if the discharge / charge cycles are really 50% capacity.

I read somewhere else on TMC that someone had tracked the duty cycle of the 12V, and it seemed that when the car is idle, the recharge comes on every 3 hours -- about 8 times per day. Assuming 8 recharge cycles per day is correct, then the depth of discharge is ~38%. The curve of cycles to wear out versus depth of discharge is strongly dependent on the depth of discharge and not linear. Eyeballing the cycle curve in the data sheet, I estimate the wear out at 38% depth of discharge is ~3,200 cycles. Using that, would mean 3,200 / 8 = ~400 days, or ~13-14 months.

In any event, we agree that the vampire is sucking the life out of the 12V battery, and I'm not surprised to see 12V battery replacements typically around 18 months. As you say, the vampire load could be less at times, and with tolerances on every other thing, these are at best educated WAGs (wild-ass guesses) of theoretical battery lifetime.

I believe you are wrong on the energy consumption. I was recently gone for 14 days. Parked the car with 137 miles, and retuned with 114 miles. Average temperature 45 degrees. Thats 1.64 miles per day, not 1.2 Kwh per day. That would equate to about 3.8 miles per day. My 12 V battery is about 9 months old. and replaced once.

 

[AMPUP]

Message popped up on my dash two weeks ago, I have an appt. February 2nd to replace. Haven't had any issues running it so far and the car has sat for 3-4 days at a time without starting. My S is about a year old.

 

[AoneOne]

I believe you are wrong on the energy consumption. I was recently gone for 14 days. Parked the car with 137 miles, and retuned with 114 miles. Average temperature 45 degrees. Thats 1.64 miles per day, not 1.2 Kwh per day. That would equate to about 3.8 miles per day. My 12 V battery is about 9 months old. and replaced once.

I understand that the car can further power down when it hasn't been used for some time. Perhaps that explains the lower average drain over a long shutdown.

- - - Updated - - -

On the topic of wasted energy, have you noticed that the cabin temperature, as reported by the app, is often 10-15F above the ambient. Something is heating the sensor.

 

[scaesare]

On the topic of wasted energy, have you noticed that the cabin temperature, as reported by the app, is often 10-15F above the ambient. Something is heating the sensor.

I suspect that's why the car was revised with a small set of vent holes to draw air across the sensor located in the lower arm rest...

 

[CalDreamin]

If you assume 1800 cycles per battery that the spec sheet claims, the net result is 1800/6.6 or 272 days battery life. Because the vampire is sometimes less thirsty than 1.2 kW-hr/day, the vampire stops drinking when the car is on, and the batteries probably do better than spec'd, we usually see more than a year from these batteries. If you want your lead-acid battery to last longer, then use all the power savings modes to make the vampire less thirsty.

A year ago I did a 34-day vampire test using Firmware 5.8 and found 0.9 kWh/day.

 

[scottf200]

My almost 4 year old Volt still has the same Absorbed Glass Mat (AGM) battery. It is charged DC-to-DC obviously too. Curious thread. Maybe I'd get on of these for my Model X <grin>. Likely just the difference in how the Volt vs the Model S charge that 12v. My 2011 only charges the 12v when the main traction battery is also charging (by flaw or design). If I left if for a several weeks at a time, I'd probably need/want a trickle charger on it.

 

[SteveS0353]

I just ordered this voltage data logger to see when the 12V battery is getting charged. Then I will put my 12V trickle charger on it and see what differences I see

That will be a very interesting experiment. As a refinement to this experiment, may I suggest that you track the vampire loss (range loss in miles or loss of capacity in kWh) from the main HV traction battery while you're measuring the 12V with and without trickle charging. That will provide a data set that will help us understand better what's going on.

- - - Updated - - -

I believe you are wrong on the energy consumption. I was recently gone for 14 days. Parked the car with 137 miles, and retuned with 114 miles. Average temperature 45 degrees. Thats 1.64 miles per day, not 1.2 Kwh per day. That would equate to about 3.8 miles per day. My 12 V battery is about 9 months old. and replaced once.

My vampire loss varies, but is typically 5-6 miles per day. There seems that there is a wide variation in vampire drain, and it could be dependent on a number of factors. It seems this would be perfect use of the initialism YMMV (your mileage may vary...)

 

[santana338]

That will be a very interesting experiment. As a refinement to this experiment, may I suggest that you track the vampire loss (range loss in miles or loss of capacity in kWh) from the main HV traction battery while you're measuring the 12V with and without trickle charging. That will provide a data set that will help us understand better what's going on.

Good idea. I'll make note of that when I do the test.

 

[KurtR]

Am I correct in thinking that, during time periods when the 85kW (or 60kW) battery is either operating the car while driving for example, or recharging from a wall outlet, that the 12V battery in front is no longer discharging to meet any of the car's power requirements? My assumption is that, while the 12V battery is getting juice from the big battery, any power needed to operate radios, CPUs, etc. is simply passed through from the big battery while the 12V battery either recharges or just sits at its present charge state while the big battery does all the discharging.

If this is correct, perhaps a solution to vampire drain on the 12v battery is to adjust the overnight charge rate downwards such that the car does not sit for any extended period of time without pulling power from the house. In other words, if I drive 70 miles a day, which would ordinarily require 2 hours of overnight charging on a Nema 14-50, I instead charge at 120V, 4A or whatever numbers are needed, so that the full overnight period is required to recharge the big battery. During that time, the 12V battery will not be discharging at all.

 

[SFOTurtle]

I'm curious how much Tesla will charge for this out of warranty. At $100 a pop just for the battery alone each year, that's more than I paid for oil changes/filters for my ICE. Then add in the labor to have it replaced, that's probably another $75 for a 1/2 hour labor.

I keep hoping the claim that EVs are less expensive to maintain will pan out, but the data points so far don't seem to be supporting it.

I suspect Tesla's warranty costs so far have been within the set aside amounts (according to ER calls) not because there isn't as much work as an ICE, but primarily because they do the work in house by their own salaried employees rather than pay dealers exorbitant rates for the repairs.

Not sure where you live, but an oil and filter change for my BMW (at an independent shop) was never less than $150, if not more. For my old 190D, I do one oil and filter change a year (I only put on 2500 miles a year on that car on WVO) and that is now well more than $150. A MBZ dealer in my area would probably charge me more than $200. I also had to change out the battery twice on my BMW in 5 years and twice in 3 years on my 190D. I was surprised the battery life was so short and was told by my tech that he sees a wide range of battery life with no rhyme or reason as to why. He said it is very hit or miss, almost random, how long a normal car battery will last. Could be many, many years, or in my case it wasn't nuts that it lasted less than two years. So anyone assuming that their battery will last for many years based on past experience has probably been very lucky.

 

[jerry33]

f this is correct, perhaps a solution to vampire drain on the 12v battery is to adjust the overnight charge rate downwards such that the car does not sit for any extended period of time without pulling power from the house. In other words, if I drive 70 miles a day, which would ordinarily require 2 hours of overnight charging on a Nema 14-50, I instead charge at 120V, 4A or whatever numbers are needed, so that the full overnight period is required to recharge the big battery. During that time, the 12V battery will not be discharging at all.

The vampire drain will still happen, it will just hidden during charging. It really has nothing to do with the 12V battery other than that it will make a weak battery weaker. The battery will still be in the circuit. Charging at a very low rate (below ~30 amps) is highly inefficient. If it does, you're just going to be trading battery life for additional electricity cost.

 

[Moonwick]

Charging at a very low rate (below ~30 amps) is highly inefficient. If it does, you're just going to be trading battery life for additional electricity cost.

Sure, but how much of that inefficiency is due to the vampire drain?.

 

[ecarfan]

My understanding is that charging inefficiency is not related to the "vampire drain" issue. Those are two separate issues.

Charging inefficiency at lower amperages relates to the resistance of the charging circuit to current flow.

The vampire drain is due to some of the cars onboard systems operating even when the you perceive the car to be powered off; it is never completely "off" during normal use.

Two different things. If I've stated anything inaccurately I welcome corrections from those more knowledgeable Thani.

 

[AustinPowers]

Not sure where you live, but an oil and filter change for my BMW (at an independent shop) was never less than $150, if not more. For my old 190D, I do one oil and filter change a year (I only put on 2500 miles a year on that car on WVO) and that is now well more than $150. A MBZ dealer in my area would probably charge me more than $200. I also had to change out the battery twice on my BMW in 5 years and twice in 3 years on my 190D. I was surprised the battery life was so short and was told by my tech that he sees a wide range of battery life with no rhyme or reason as to why. He said it is very hit or miss, almost random, how long a normal car battery will last. Could be many, many years, or in my case it wasn't nuts that it lasted less than two years. So anyone assuming that their battery will last for many years based on past experience has probably been very lucky.

I have said it in another thread, but it fits here as well. What is it with those many oil changes in the US? Do you have bad quality oil? Do you drive loads of miles a year? Over here most cars have to change oil every 20K or 30K kilometres, some modern cars can do even better - or decide when an oil change will be necessary based on the individual driving style. Of course if you drive a lot in one year, then that means annual oil changes. But otherwise?

And as for the battery, I can't remember ever having to change one. My current car is eleven years old and still on its first Battery (Varta, don't ask me what model). My former car was ten years old when I sold it, again still with the original battery. My father had a Merc for 15 years (250K kilometres), after which he had to change the battery for the first time. Pity he sold the car two months later ;-)
And his current '07 E-class is also still on its first battery, with no signs up to now that hint at reduced capacity.
Perhaps we were just lucky in that respect...

But as someone said earlier on, the argument "EVs require less maintenance/are less costly to run" doesn't really apply yet, at least not from what ICE's I can compare to.

 

[JRP3]

If you want to talk about cycles, don't forget that in order for the DC/DC to recharge the 12V, the main contactor has to close too. Those cycles aren't infinite either.

I would think the DC/DC has it's own, smaller contactor or relay connecting it to the main pack.

 

[SteveS0353]

Am I correct in thinking that, during time periods when the 85kW (or 60kW) battery is either operating the car while driving for example, or recharging from a wall outlet, that the 12V battery in front is no longer discharging to meet any of the car's power requirements? My assumption is that, while the 12V battery is getting juice from the big battery, any power needed to operate radios, CPUs, etc. is simply passed through from the big battery while the 12V battery either recharges or just sits at its present charge state while the big battery does all the discharging.

If this is correct, perhaps a solution to vampire drain on the 12v battery is to adjust the overnight charge rate downwards such that the car does not sit for any extended period of time without pulling power from the house. In other words, if I drive 70 miles a day, which would ordinarily require 2 hours of overnight charging on a Nema 14-50, I instead charge at 120V, 4A or whatever numbers are needed, so that the full overnight period is required to recharge the big battery. During that time, the 12V battery will not be discharging at all.

Now we are getting into the details, and "the devil is in the details". The power to charge the 12V battery follows this path...

Shore power -> HV charger(s) -> HV battery (via contactors) -> DC-DC converter -> 12V battery.

We know that when the car is "plugged-in", and not charging, there is still vampire drain; shore power does not feed the vampire directly. When the HV battery is charging, the vampire drain might still be happening but is at the very least "masked" since the HV battery is accumulating charge, not losing it. When the car is in motion (HV contactors closed), all of the 12V systems are powered up and running. The detail we don't know is the control of the DC-DC converter / 12V charger and whether it's enabled all the time when the HV battery contactors are closed. Knowing that detail would answer your question. However, as others have pointed out, charging at low-power is less efficient overall, so what you could possibly gain on one hand, you'd potentially lose on the other.

Perhaps santana338's rather interesting experiment can shed some light on what's happening during driving and charging too.

 

[Moonwick]

I'm probably veering off topic just a bit, but this thread's given me a bit of an idea. It obviously depends on several variables (headlights on? heated seats operational?) but how much of the power drawn from the traction battery do you suppose goes towards powering the 12v system? Not just power directly consumed by the 12v system, but also losses from the DC->DC converter.

I wonder if we could eek out a kWh or two of extra driving from the traction pack if we augmented the 12v battery with a couple of kWh of Li-ion batteries, and some wizardry that only engaged the DC->DC convertor to recharge the 12v pack once it fell below a certain voltage or when the car's connected to a charger. This obviously wouldn't directly increase the capacity of the traction pack (since the DC converter only flows in one direction) but if (to just make an estimate) the auxiliary systems normally use 2 kWh over the time it takes to drive the ~250 miles an S85 is rated for, then by adding 2 kWh of battery to the 12v system you'd gain 6-7 miles of range.

I won't be the first to invalidate my warranty by trying this out, but it's an idea to ponder.

ETA: Thinking about it a bit further, the added complexity and circuitry this would require (something to monitor voltage and also to control the relay going to the DC convertor) means it's probably not worth it unless the capacity gain would be significant, perhaps 3 or more kWh.

 

[miimura]

I would think the DC/DC has it's own, smaller contactor or relay connecting it to the main pack.

That doesn't make any sense. The main contactor is the primary safety mechanism to isolate the high voltage inside the main battery pack. How many high voltage paths do you think there are coming out of the main battery? I think there is only ONE.

Regarding charging efficiency when reducing the current to prolong the charge cycle - yes, you would probably reduce the overnight cycles on the 12V battery by doing that but it would drastically increase the power consumption from the wall. Not only would you be running the on-board charger at a less efficient operating point, but you would be running the coolant pumps during the extra hours and you would be running the DC/DC converter continuously during that time, which has its own losses. You would be much better off putting a battery tender on the 12V directly.

 

[JRP3]

It wouldn't make any sense to keep opening and closing the high voltage contactors every time the DC/DC kicks on and off. The DC/DC is low current and could easily have it's own smaller contactor or relay. More likely the DC/DC is always connected to the pack unless there is an accident and an inertial switch opens its contactor or relay. It needs to stay powered up so it can sense the 12V battery voltage and kick on when needed. Obviously I don't know how Tesla does it but I can say I don't know anyone in the DIY world that uses the high voltage pack contactors to turn the DC/DC on and off.