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So Why Does the 12V Battery Die?

Discussion in 'Model 3' started by snakeopus121, Feb 14, 2018.

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  1. snakeopus121

    snakeopus121 Banned

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    In an ICE car - the 12V battery has two functions: a) power the vehicle control system/audio/etc; b) Physically turn the engine crankshaft at startup. The latter by far takes the most energy, albeit just for a brief period each trip --- however 99% of the times an ICE needs to be "jumped," the systems associated to "function a)" still operate ---- the radio and lights still work but not enough juice to turn the engine.

    In a Tesla/EV --- the 12V battery only performs "function a)" ---- it only powers the computer systems/radio/etc, and obviously has a huge and constant source to charge off of......

    I think it is safe to assume the draw on the 12V system is greater than its recharge rate ---- maybe all the OTA updates, uploading/downloading, etc --- basically the energy consumed computing "stuff" is draining the battery faster than it can be recharged ---- perhaps only is recharged when the car is actually in motion/driving?

    I'm puzzled to say the least ----- Tesla/EVs have two batteries: the huge traction battery and the typical 12V ---- and the latter can die while the former is fine.....
     
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  2. dhanson865

    dhanson865 Active Member

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    It's technically hard to tell the true health and charge level of a 12v battery. If you over charge it or let it get too hot it shortens it's life (and may vent gases that aren't safe to breathe). If you undercharge it, it shortens it's life.

    Engineers pride themselves of doing better than the last guy on monitoring, charging, desulfating, and such on 12v systems but it's essentially a losing battle. It's a consumable that is nearly impossible to take care of perfectly and it will fail sooner or later.

    EVs aren't really much harder on the 12v than ICE, you just don't have the same early warning as it starts to die since you don't have starter drag on an EV.
     
  3. animorph

    animorph Active Member

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    Tesla used to put a lot of cycles on the 12V battery. They charged it from the HV battery then let it run down and then charged it back up, over and over. With a fairly significant load even when the car was "off". That doesn't seem easy on the battery.

    The newer cars have something I guess is closer to a trickle charger and should be easier on the 12V battery.
     
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  4. snakeopus121

    snakeopus121 Banned

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    #4 snakeopus121, Feb 14, 2018
    Last edited: Feb 14, 2018
    You say Tesla put a lot of cycles on the 12V battery --- what is drawing the current, necessitating the cycles???

    ICE systems have to be harsher on the 12V battery with the physical cranking of the engine --- these 12V batteries are robust, their designed has lasted the test of time.

    Not sure of the newest cars (late 2010's) ---- but I know at some point before the 80's --- they figured out a simple yet incredibly effective 12V battery charging system --- attached is the diagram for a 1982 Honda Helix scooter ---- no different functionally than an ICE

    The rectifier controls the battery charging --- the alternator is literally copper coils mounted to the engine driveshaft --- when the engine is running it is generating alternating electrical current --- the rectifier, which is completely electrostatic - no programming, controls the direct electrical current charging of the 12V battery.

    I can personally report that the 12V battery on my 1982 Honda Helix lasted well over 10 years --- and it didn't fail - at some point the engine sounded like it was struggling a little bit to start so I rode to Autozone to get analyzed -- they suggested replacing, which I did.

    These battery driven 12V systems have existed on ICE engines(cars, boats, motorcycles, high end lawn mowers, etc) for many, many decades
     

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  5. snakeopus121

    snakeopus121 Banned

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    Just to clarify -- I'm only discussing this issue bc it obviously has carried into the Model 3, and is an immobilizing problem. Just want to understand the nature of the problem.

    I'm a mainstream car buyer --- only have owned one car at time, and have never bought a new car in my life. These things matter. My car is directly linked to my ability to earn money; I have to be 99.9% sure it will take me to work every weekday morning.

    The moderators can attest (if they have access to this data) --- I have been a daily visitor to this forum for over 4 years now, I'm a huge fan --- the Model 3 is the first Tesla I could conceivably purchase (hence why I've never posted until recently)

    Not trying to tarnish Telsa's image, and trust me - I am not a stock market "player"
     
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  6. Xenoilphobe

    Xenoilphobe Active Member

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    Sounds like a poor supplier. Tesla is in the battery business, why don’t they just make their own LiPO 12 volt baattry like this one.

    element3batteries
     
  7. TEG

    TEG Teslafanatic

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    For safety reasons, the "traction battery" (main pack) tends to be disconnected unless really needed to drive the car or for charging.
    So the "contactors" are frequently not engaged unless driving or charging.
    So there are times when the car will just be doing "vampire drain" on the 12V system because the main pack is disconnected, there is no charging happening and the DC/DC isn't active.
    If the main pack is full, and doesn't need a charge, and the charge cable is plugged in, will it wake up periodically to run the DC/DC to "top off" the 12V?
    I am not sure exactly how they do it, but thought to mention that there is a bit of a juggling act for them to decide which systems should be on and at what time.
     
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  8. ohmman

    ohmman Maximum Plaid Member

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    I personally attest to nothing, other than your final sentence certainly provides evidence that you've been around here plenty.
     
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  9. FlatSix911

    FlatSix911 Porsche 918 Hybrid

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  10. TEG

    TEG Teslafanatic

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    Some reasons why they have a 12V system and a 12V battery:
    #1: Many standard accessories that Tesla can buy run off of 12V so it is helpful to have a 12V system to power things like lights, power window motors, wiper motors, etc.
    #2: For safety reasons, it is helpful (required?) to have an aux battery that can run critical things like headlights and ABS braking system if the main traction pack failed or became disconnected.
     
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  11. gregincal

    gregincal Active Member

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    Just to be clear, there are three separate things being tossed around here:

    Can the 12v battery get drained because it isn't able to be recharged in time: No, the car will always charge the 12v battery from the main battery when it detects that it is low. This is actually a huge advantage in an EV. My wife has had here 12v battery drain twice in the last couple years by spending an hour doing stuff with accessory mode turned on when the engine is off. You can play music and do whatever you want parked all day long and the 12v battery will never die. Even if you are not charging and not driving, a Tesla will re-engage the main battery to charge the 12v if it's getting low (if the 12v has totally died, it is unable to re-engage the main battery, because that requires a functioning computer and relay that run off the 12v battery).

    Is it possible to have a bad 12v battery in a new car: Sure, I had a 12v battery replaced in a quite new BMW because it was faulty, and presuming this thread is based on one incident of a car failing to turn on, we so far don't even know if that was the 12v battery. Are there really any actual confirmed cases of a failing 12v battery in a Model 3?

    Do batteries in Teslas run out sooner than in ICE vehicles: In the Model S yes (and I assume the X), although we have no idea whether this will be the case in the Model 3. The best theory is that it is a difference in charging pattern that caused this. An ICE uses a lot of charge very quickly to start the engine, but then the alternator provides charge the whole time the battery is running. Using the battery when the engine isn't running is hard on the battery (see my wife above). A Model S/X lets the battery run down quite a bit before recharging it, in a totally different pattern than what happens in an ICE. I went through 3 batteries in the 5 years I owned my Model S.
     
  12. ohmman

    ohmman Maximum Plaid Member

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  13. Cosmacelf

    Cosmacelf Well-Known Member

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    I've always thought that Tesla should spend some time coming up with a better system than what they have with the S/X. Unless they've done something different with the Model 3, it's been 5 1/2 years and no significant change. My 2 year old Model X and my 5 year old Model S both recently popped up with a 12V battery replacement message. Annoying. On the other hand, I haven't been stranded due to a 12V failure ... yet.
     
  14. gregd

    gregd Active Member

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    I think it's actually a little more complicated than that.

    First, referencing Snakeopus121's diagram, the regulator is significantly more than just a diode. There are also voltage controls in there that determine how much voltage the car's alternator puts out, in order to match the ideal charging cycle that lead-acid batteries like to see. My '72 Datsun 240Z used a simple set of relays to do this, with the contacts flitting up and down to control the output (*), but more modern cars use semiconductors. Briefly, you charge the battery with a constant voltage that starts out high (14v or so) until the current drops to some level, then drop the voltage to a "float" level (13.5-ish) where the battery charge is maintained. The car's accessories (radio, headlights, etc) are designed to run on whatever that charging system is putting out at the time; there is only the one 12v bus. Note that swapping the lead-acid battery for some other type (e.g. NiMH or Li-Ion) can be problematic unless the regulator is changed, since they have a much different charging pattern.

    As TEG notes, EVs don't keep the main contactors engaged all the time. But, some of the newer cars - I think from the Refresh forward - have a small DC-DC converter inside the main battery pack, where it can provide some part of the 12 v system's power. This limits the cycling stress on the 12v battery. So with this change, the car shouldn't be doing deep cycles of the 12v battery, and they should last a long time. At least, that's what I have come to understand from other threads in this forum. I presume the Model 3 will use the same design. What I am not clear on is what conditions would cause the 12v battery to be the sole source of power in the car, other than when the main pack is flat.

    One thing to note is that when a car is plugged into an EVSE (wall charger, et al), there is actually no power being delivered to the car unless the car is actively requesting it, for example, when charging. That's the big "thump" you hear at the start and end of the charging. So, the car can't use "shore power" as a trickle charger for the 12v battery, because that power is disconnected by the EVSE when charging completes.

    ----
    (*) It was really "fun" one evening while driving home at night in the rain, when the constant switching of the 240Z's voltage regulator relays had built up enough crud on the contacts that they shorted across, top to bottom. I watched in mild horror as the amp meter on the dash pegged over to the negative side (40 amps discharge), then slowly step back towards zero as, one by one, the diodes in the alternator burned out. Managed to make it home on the last diode. That was fun to troubleshoot... Really miss that car :) A lot of fun to drive, the the electrical system was a disaster.
     
  15. Cosmacelf

    Cosmacelf Well-Known Member

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    Yeah, that blog concludes that Tesla doesn't (didn't?) know what they were doing when it comes to looking after a 12V battery. I wouldn't be surprised if it was an accurate conclusion. In my field of expertise, software, it is obvious that they lacked basic competence in that area as well.

    Of course, the car is still an engineering marvel overall and I am glad that I have two. It's the silicon valley mentality. Ship betaware and fix it over time. They are still doing it. The Model 3 software is half baked, and AP2 is really half baked. No company is perfect it seems.
     
  16. TEG

    TEG Teslafanatic

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    Nissan put a small panel on the LEAFs which purpose is to keep the 12v battery topped off...

     
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  17. FlatSix911

    FlatSix911 Porsche 918 Hybrid

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    Previously posted ... another Enzo :cool:


     
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  18. Snerruc

    Snerruc Member

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    Tesla’s are never really off. They are always doing housekeeping. Ford among others has had problems with the 12 volt failing on their hybrids. Also, I know plenty of people with early 12 volt failure on ices. The vampire drain on some modern ices is significant.
     
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  19. Cosmacelf

    Cosmacelf Well-Known Member

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    True, our old QX56 Infiniti 12v battery would die every 2 years like clockwork. I don’t know if that was a peculiar parasitic loss with that particular car, or if all those Model year cars had the problem, regardless, it left us stranded a time or two.
     
  20. TEG

    TEG Teslafanatic

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    I had 12v battery issues with old MY2000 Ford Ranger EV truck.
    If you left the truck plugged into the charging station, but the traction battery was full, it would leave the DC/DC off but keep draining the aux battery.
    It was somewhat of a design flaw that it didn't trickle charge the aux battery when plugged in.
    So the procedure was to make sure to unplug the charge cable once the vehicle was full.
    These days we expect the car to be smart enough to do the right thing in all circumstances.
     

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