12v info and charging hack

[LongRanger]

Looking for some info on the 12v battery - 2 main areas seeking help on

1) the pos/neg terminals - will they be suitable for something sized at M8 for eyelet/ring connections - and if I unscrew the terminal bolts do I need to consider any particular systems impact with 12v power off for a few minutes.

2) How good is the high voltage system at maintaining the charge and health of the 12v system - average/acceptable or excellent.

I’m considering adding a CTEK comfort connector to the frunk location to allow me to keep 12v in peak health (low amps charging only), but wondered if it’s even needed and I guess I should ask if this can confuse the car / cause any issues.

Many thanks

 

[Gasaraki]

1) I don't know. I'll let others respond to that.
2) I think it's acceptable right now. Some people have had their 12V batteries die within 2 years. Elon said that they will make changes in software so you have earlier warning before the 12V battery dies and take better care of the 12V battery.

 

[camalaio]

The stickied thread regarding powering things with the 12V has some related info.

It can keep the 12V battery charged, but only if the load is light enough. Heavy sustained loads, it cannot keep up with.

Keeping the 12V battery topped with a battery tender is apparently a great and effective way to maintain it for long term storage (this means less draw from the main pack as well).

 

[smatthew]

The stickied thread regarding powering things with the 12V has some related info.

It can keep the 12V battery charged, but only if the load is light enough. Heavy sustained loads, it cannot keep up with.

Keeping the 12V battery topped with a battery tender is apparently a great and effective way to maintain it for long term storage (this means less draw from the main pack as well).

When you say "It", do you mean a battery tender? Or the car? Because the car can handle heavy sustained loads of up to 200 amps on the 12V rail.

A battery tender is a joke on a Tesla. The car has one built in.

 

[LongRanger]

I guess that’s what I’m asking. How good is the car really at keeping a 12v battery in peak health, not just topped up but topped up in the right way to extend 12v life ?

I find my CTEK invaluable for ATV, sit-on mower and tow-behind mower electric start systems, and they all have batteries of a similar capacity and type to the model 3.

Is it of benefit to have the car charged by an external device like a CTEK once a month, couple of times a year or just no need because the BMS is measuring 12v health and giving “pulse float” to keep all the electrons nice and happy ?

 

[camalaio]

When you say "It", do you mean a battery tender? Or the car? Because the car can handle heavy sustained loads of up to 200 amps on the 12V rail.

A battery tender is a joke on a Tesla. The car has one built in.

The car via the battery specifically. I was summarizing the findings of the thread I mentioned - the roughly 200A the PCS can source is apparently not applied directly to the battery. The battery appears to be on a separate "charger", which comes off VCFront IIRC but that thread contains the info.

If OP had a heavy load to plug in we could point them in the direction of attaching it to the PCS and not the 12V battery, but that's not what they were asking about.

One thing I noticed that wasn't answered was if there's an issue disconnecting the battery for a few minutes. This is probably fine if the car is "off", but most will point you at the "HV disconnect" procedure as well. I don't have a link for that right now, sorry.

I guess that’s what I’m asking. How good is the car really at keeping a 12v battery in peak health, not just topped up but topped up in the right way to extend 12v life ?

I find my CTEK invaluable for ATV, sit-on mower and tow-behind mower electric start systems, and they all have batteries of a similar capacity and type to the model 3.

Is it of benefit to have the car charged by an external device like a CTEK once a month, couple of times a year or just no need because the BMS is measuring 12v health and giving “pulse float” to keep all the electrons nice and happy ?

Hmm, this is where @Gasaraki 's point of only "acceptable" comes in. The car will charge it if it gets "low" in standby, but we don't have a lot of data on where that threshold is (and it appears to be changing). Charging it once it a month externally would have little to no impact probably, unless you're driving about once a month as well. It will usually charge the battery during and briefly after a drive as well.

So it just depends how much you will use the car.

 

[smatthew]

I guess that’s what I’m asking. How good is the car really at keeping a 12v battery in peak health, not just topped up but topped up in the right way to extend 12v life ?

I find my CTEK invaluable for ATV, sit-on mower and tow-behind mower electric start systems, and they all have batteries of a similar capacity and type to the model 3.

Is it of benefit to have the car charged by an external device like a CTEK once a month, couple of times a year or just no need because the BMS is measuring 12v health and giving “pulse float” to keep all the electrons nice and happy ?

The batteries in your yard equipment are used, and then basically put away with no drain (except internal parasitic drain) until the next time you turn the key on. A battery tender keeps them topped up despite that minor drain.
A Tesla will always use a small amount of power to keep the cell modem online, and to watch for door handle activations. This slowly drains the 12V battery.
But, VCFront is constantly measuring voltage from the 12V battery. If the high voltage pack is disconnected, and the battery voltage drops below a certain level, the car engages the HV pack and charges the battery.

 

[Rocky_H]

A battery tender is a joke on a Tesla. The car has one built in.

It's not a joke.

I guess that’s what I’m asking. How good is the car really at keeping a 12v battery in peak health, not just topped up but topped up in the right way to extend 12v life ?

And this is why. The car handles this by letting it run down and then refilling it back up and then down/up/down/up/down/up... This is hard on the life of a lead acid battery. So no, the car is not keeping it at a good steady float charge, which would be beneficial for the 12V.

But, VCFront is constantly measuring voltage from the 12V battery. If the high voltage pack is disconnected, and the battery voltage drops below a certain level, the car engages the HV pack and charges the battery.

Yes, that's how it does it--running that cycle over and over, which is what is hard on these 12V batteries.

I would not think it's worth bothering with for a day or two, but if you're storing the car for a month or two at a time, this might be worth the trouble.

 

[smatthew]

The car via the battery specifically. I was summarizing the findings of the thread I mentioned - the roughly 200A the PCS can source is apparently not applied directly to the battery. The battery appears to be on a separate "charger", which comes off VCFront IIRC but that thread contains the info.

Ingineerix did a teardown of VCFront and found nothing like a "charger module" in VCFront. All they found in between the PCS and the 12V battery is an e-fuse.

 

[LongRanger]

@Rocky_H thank you, that’s really helpful. Going to go ahead and fit the comfort connector for CTEK - just need to know whether M8 or M10 ring eyelets are most suitable for the battery connectors

 

[Timbo2]

@Rocky_H thank you, that’s really helpful. Going to go ahead and fit the comfort connector for CTEK - just need to know whether M8 or M10 ring eyelets are most suitable for the battery connectors

I've heard that some of the newer German cars monitor the current the 12V battery provides and if you throw a battery tender directly on the 12V battery terminals they will throw a code. The way to get around that is to use a chassis ground. Now given that the negative terminal on the battery is also connected to the chassis I can't figure out what the issue is here and my quick Google failed me.

OTH, I can't see a downside with using some point on the chassis for your tender's negative instead of directly to battery. So you might want to consider that.

 

[FalconFour]

Euh. Okay, less conjecture, slightly more experience here, I hope... I have a clean set of findings here (I'm just sifting through threads looking for stuff I can contribute, so mind my intrusion here!). I haven't set-out to prove or disprove any theories or concepts in this thread, but just sharing my experience on the subject so far - maybe to provide new ideas to try or chew on.

The car only ever "sleeps" on the 12v battery. Nothing ever "runs from" it. When asleep, only the bare essential functions are awake. This draw is very, very minimal - and the car wakes so easily and stays awake for so long, it's not likely draining the 12v battery more than a few percent.

There is a current sensor on the 12v battery. One of the car's controllers tells the PCS (power conversion system, or DC-DC converter) to adjust its output voltage - which actually ends up changing the entire 12v bus voltage - to maintain a 10-amp charging rate. Yeah, this is totally practical and it actually works, even if a little rough, because the 12v battery only consumes as much current as its voltage (state-of-charge) dictates. So, simply modulating the PCS voltage is all it takes to limit the charging rate to 10 amps.

Once it reaches 14.4v and the amperage is around zero for a while, it'll linger there for a bit (presumably as an equalizing effort) and then back off to around 13.5v where it's considered "done" and will allow the car to sleep. If the battery hasn't gone through this complete charge cycle, it won't sleep 'til it's done.

During the charge cycle, it seems to also characterize (in new firmware around 2020.28.x) the 12v battery by considering its voltage vs. charge current compared to what a "good battery" should look like, and it'll give that "12v battery needs to be replaced" warning if it doesn't like what it sees. This is why even a brand-new lithium 12v replacement battery can set-off the 12v warning in new firmware.

It's also been observed that the car will "play yo-yo" with the 12v battery a bit, lowering the PCS voltage below the 12v battery's resting voltage and letting the car be powered by the 12v for a while, back and forth, discharging and recharging it, maybe to test and characterize the battery, maybe to exercise it.

My experience with lead 12v batteries has been that "if it's just sitting there doing nothing, it'll last forever". I've got a big blue Optima spiral AGM battery that was made in 9/2010, and it's been sitting on the ground most of its 10-year life, picked up and started using recently, and it behaves 100% like brand new. It was resting at about 11.6 volts when I saw it - almost completely discharged. However, I've also seen UPS batteries that were never used - and were kept fully charged their whole life - and they were basically inert bricks because the UPS was constantly maintaining a charging voltage on them. It seems like it's possible to abuse them in both ways - to leave them fully charged is just as bad as with a lithium battery, but to constantly cycle it is also bad as it'll wear them out. I think Tesla is walking a fine line here and doing their best to manage the 12v battery as well as they can.

FWIW, my mid-2018 Model 3's 12v battery performs today just as well as I'd expect it to when it was new. Quite impressed.

 

[LongRanger]

Excellent thanks @FalconFour - alternative connection option is under the rear bench. However this isn’t ideal for slow charge as door needs to be open.

 

[FalconFour]

Excellent thanks @FalconFour - alternative connection option is under the rear bench. However this isn’t ideal for slow charge as door needs to be open.

Actually, absolutely not... that's the PCS output, and it's just another device on the 12v bus (that supplies power when enabled - a big part of the bus, but still merely a switched part of the bus, not a hard tap into it). As I detailed here, you can lay a wrench across the PCS terminals and the car just responds with a "hmm, whoopsie?" and shuts the PCS down, falling back to 12v battery power and dropping those terminals to 0v near instantly. The PCS shuts down and those terminals go dark when the car is asleep - which is the only time you'd want to put a charger on the battery anyway. If there remains voltage on the PCS after it tries going to sleep, it throws a fault and temporarily "crashes" (I won't say bricks - it's easily reset but the car is non-driveable) the car.

In my opinion, there's no correct place to put a 12v charger, as the car does a perfect job of doing this on its own. All you'd be doing is confusing the car, and possibly powering its idle power consumption (150-200W) using the charger instead of the PCS (but that'd confuse the hell out of the car and the PCS would probably fight back and win anyway, leaving the charger running at 0A and "done" all the time). My advice: don't do this. lol.

 

[LongRanger]

The only thing I can see is a 10A charge rate being set on a very cold battery in winter conditions for a relatively small Ah battery size.

Do you know if the car systems steadily ramp up to 10A from “trickle” based on temperature or does the 10A just get thrown in regardless of starting voltage and ambient ?

The CTEK is much more intelligent as it has modes for winter and “rescuing” unhappy cells. The Tesla battery is basically the same as my JD Z series mower, and the CTEK keeps that in perfect health.

 

[FalconFour]

It knows the 12v battery's temperature, I know that much (via Scan My Tesla - 12v battery temp is a measurement it reports)... and I'd be very surprised if, considering Tesla's cold weather testing, they didn't take it into consideration. If the battery isn't absorbing current while voltage is applied, it probably takes the temperature into consideration and maybe boosts the voltage a little higher.

TBH this is the point where I enter the realm of conjecture myself, so I can't really speak to cold weather behavior of lead batteries or what Tesla does.

 

[electrongeek]

FalconFour is right that a 12V lead acid battery that does not see elevated temperatures will self-discharge very very slowly. The problem is that Tesla will frequently charge it up to about 14.4V. This is great for ensuring that the battery is fully charged, but it is actually hard on the battery long term - when done frequently. It is mentioned that UPS lead acid batteries regularly fail after a few years despite being kept fully charged, but I would fault a poor charging circuit for this myself. If the charging source is not well filtered, a continuous ripple voltage will be detrimental, even if the ideal average float voltage is maintained. Temperature may also have an effect on UPS battery life.

I do believe that if the vehicle computer can be prevented from its frequent 14.4V top ups with a good battery maintainer, the 12V battery will last longer. Whether it is really worth the effort is another issue.

 

[FalconFour]

I do believe that if the vehicle computer can be prevented from its frequent 14.4V top ups with a good battery maintainer, the 12V battery will last longer. Whether it is really worth the effort is another issue.

I don't think that's avoidable. It'll give it a 14.4v top-off every time it wakes from sleep, as best I can tell. Since it doesn't really take "how long have I been out?" into consideration, it figures that 14.4v is the completion point of charging, so it'll do that whenever it gets the chance.

Unless you've got the car left unused for quite some time... but I've never given my car a dull moment in all its life. Pretty much every day, it gets some action. With that constant cycling of 14.4v, it's held up pretty well, so I'm legitimately clueless as to how Tesla is having so many 12v problems. (but I just wonder if what we're seeing is the 1% of failures among tens of thousands of sold cars... while 99% are working just fine)