Welcome to Tesla Motors Club
Discuss Tesla's Model S, Model 3, Model X, Model Y, Cybertruck, Roadster and More.
Register

Any reason not to hook up a battery tender to the 12 volt battery ?

iCharge

Member
Jul 30, 2018
248
69
New England
questions for @whitex or other stalwarts
have the newer Model Ss also experienced battery issues ? like 2017 onwards
someone tells me that Tesla replaced the 'old battery type' with a deep cycle battery - [ I know newer MSs have a DCB] but also that Tesla fixed this constant cycling of 12V battery so 12V battery in newer Tesla does not fail as much
any word folks ?
 

whitex

Well-Known Member
Sep 30, 2015
6,705
8,047
Seattle area, WA
I have not been tacking the 12V battery upgrades over time, so can't answer whether and what they upgraded it to. Knock on wood, never had any 12V issues with my Teslas. They are almost always plugged-in overnight though.

To the original question about a battery tender (sorry if already answered by someone before, thread TLDR), no reason at all to connect one - just keep the whole car plugged in instead. If you don't have a charger where the car is, just plug it into the same 120V socket where you were thinking to plug the battery tender into. The car does the job of topping off and managing the 12V battery.
 

SteveS0353

Member
Aug 23, 2014
365
54
San Diego, CA
I have not been tacking the 12V battery upgrades over time, so can't answer whether and what they upgraded it to. Knock on wood, never had any 12V issues with my Teslas. They are almost always plugged-in overnight though.

To the original question about a battery tender (sorry if already answered by someone before, thread TLDR), no reason at all to connect one - just keep the whole car plugged in instead. If you don't have a charger where the car is, just plug it into the same 120V socket where you were thinking to plug the battery tender into. The car does the job of topping off and managing the 12V battery.

Sorry, but this advise is not correct. Keeping the car plugged in will certainly keep the 12 battery charged, but it won't prevent the cycling that wears out the 12V. The car when plugged in will NOT "float" charge the 12V, it will wait until the 12V is discharged to some level by the car systems, then close the breakers in the traction battery, turn on the DC-DC converter, recharging the 12V. This cycling of the 12V battery is what kills it. A far better solution is to use an external 12V tender to "float" charge the 12V, thus providing the energy to run the car systems via the tender and eliminating / reducing the cycling of the 12V battery.
 

whitex

Well-Known Member
Sep 30, 2015
6,705
8,047
Seattle area, WA
Sorry, but this advise is not correct. Keeping the car plugged in will certainly keep the 12 battery charged, but it won't prevent the cycling that wears out the 12V. The car when plugged in will NOT "float" charge the 12V, it will wait until the 12V is discharged to some level by the car systems, then close the breakers in the traction battery, turn on the DC-DC converter, recharging the 12V. This cycling of the 12V battery is what kills it. A far better solution is to use an external 12V tender to "float" charge the 12V, thus providing the energy to run the car systems via the tender and eliminating / reducing the cycling of the 12V battery.
While you absolutely correct about the tender avoiding battery discharge/charge cycle, the point is the batteries are actually designed for such cycling. So, first, I am not certain whether keeping the battery fully charged for prolonged periods of time is good for the battery. Remember that Tesla doesn't do full/deep discharge then charge cycles, they keep topping it off instead. Second, even if the additional wear does shorten the life of the battery a little, I suspect the battery will dies of old age before the wear hits is. So far, I haven't had any problems with 12V batteries on any of out Teslas and the oldest one is 3.5 years old, and it sits in the garage most of its life (I don't even drive it most days as I work from home) while the 12V battery is being drained by regular vampire drain, plus a dual dashcam running 24/7, and hasn't had any indications of a 12V problem. When driving, I drain it even faster with a phone charger and a dual radar detector + laser jamming system.

So, to clarify, I wasn't saying that plugging in the car will be 100% equivalent of being plugged into a battery tender, rather that it wouldn't make much practical difference, if any to eliminate this wear since that is exactly what car batteries are designed for (charge while driving, discharge while parked).
 

Brass Guy

Active Member
Jan 5, 2014
1,167
960
Holbrook, MA
I am not certain whether keeping the battery fully charged for prolonged periods of time is good for the battery.
The 12V battery supplied by Tesla is not of the lithium type, and is best stored full.
When driving, I drain it even faster
When driving, the DC-DC converter is always powering the 12V systems and keeping the 12V battery topped off.
 

iCharge

Member
Jul 30, 2018
248
69
New England
Sorry, but this advise is not correct. Keeping the car plugged in will certainly keep the 12 battery charged, but it won't prevent the cycling that wears out the 12V. The car when plugged in will NOT "float" charge the 12V, it will wait until the 12V is discharged to some level by the car systems, then close the breakers in the traction battery, turn on the DC-DC converter, recharging the 12V. This cycling of the 12V battery is what kills it. A far better solution is to use an external 12V tender to "float" charge the 12V, thus providing the energy to run the car systems via the tender and eliminating / reducing the cycling of the 12V battery.
Hey
Do you have a float charger hooked to your car, and if yes, can you pls share what kind / amperage ? and how you went about to get access to the 12V battery ?
thanks much !
iCharge
 

iCharge

Member
Jul 30, 2018
248
69
New England

whitex

Well-Known Member
Sep 30, 2015
6,705
8,047
Seattle area, WA
When my 12 volt died at ~56,000 miles, I got this as a replacement.

https://www.element3batteries.com/product-page/tesla-lithium-12v-battery
If the folks suggesting that cycling the battery while parked on shore power is what kills the batteries, mileage isn't important, rather how long has the car spent parked since new. Heck, the more you drive the less 12V is used (since car runs on on dc-dc while driving). Tesloop cars (Tesla taxis, driven many hours per day) must still be on their original 12V batteries even though they have replaced the main battery under the unlimited mileage warranty.
 
  • Like
Reactions: Rocky_H

SteveS0353

Member
Aug 23, 2014
365
54
San Diego, CA
When driving, I drain it even faster with a phone charger and a dual radar detector + laser jamming system.

So, to clarify, I wasn't saying that plugging in the car will be 100% equivalent of being plugged into a battery tender, rather that it wouldn't make much practical difference, if any to eliminate this wear since that is exactly what car batteries are designed for (charge while driving, discharge while parked).

When driving, the traction battery provides all of the energy, including charging the 12V; you are not draining the 12V while driving despite the extra "widgets".

Also, 12V batteries were not designed for very thirsty vampire drain when parked. Calculations indicate that the vampire drain on the 12V is roughly equivalent to a 40 watt incandescent lamp running continuously. No ICE car has that sort of drain on the 12V when parked. There is s wide variance in MTTF on Model S vehicles, but the statistics indicate that early Model S vehicles had 12V battery replacements about every 12-18 months. The battery type has changed, and the vampire has been made less thirsty, and more recent vehicles have been reporting 3+ years MTTF. But that does not avoid the fact that the cycle based wear out of even the more recent better deep cycle AGM batteries will eventually bring their demise.
 

tga

Supporting Member
Supporting Member
Apr 8, 2014
3,997
2,865
New Hampshire
Thank you for the link ! that looks like a good product. Obviously the demand shows [ its OOS as of now ]
I ll probably replace it if my current 12V fails, I am trying to get more knowledge here, to prevent it from failing esp in cold NE winters.
Hence my question on battery tender-ing the 12V in my tesla
I would not run a 12V lithium in NE (or anywhere with extreme winters). The battery will fail quickly when charged below freezing.
 
  • Helpful
Reactions: Rocky_H and iCharge

SteveS0353

Member
Aug 23, 2014
365
54
San Diego, CA
Hey
Do you have a float charger hooked to your car, and if yes, can you pls share what kind / amperage ? and how you went about to get access to the 12V battery ?
thanks much !
iCharge
To avoid 12V cycling, you need a tender that can provide about 50 watts -- at 12V, that's a tad over 4A. There are many recommendations on this thread for suitable 12V tenders of that specification. I popped the nose cone, connected some terminals to the battery posts, routed them inside the "frunk" of my classic P85+. I drive a lot too (~70k miles in ~4 years), and I'm still on the original 12V, 4.5 years later. I've probably jinxed myself for posting this ;-)
 

whitex

Well-Known Member
Sep 30, 2015
6,705
8,047
Seattle area, WA
When driving, the traction battery provides all of the energy, including charging the 12V; you are not draining the 12V while driving despite the extra "widgets".
You are correct. DC-to-DC basically works like an alternator on an ICE car and it's on whenever you drive the car (and sometimes when 12V needs a top off). I incorrectly statined that the battery was draining during the drive, the 12V load simply subtracts from the DC-to-DC max capacity to charge the battery.

Also, 12V batteries were not designed for very thirsty vampire drain when parked. Calculations indicate that the vampire drain on the 12V is roughly equivalent to a 40 watt incandescent lamp running continuously. No ICE car has that sort of drain on the 12V when parked.
Not sure why you mentioned incandescent lap here, wouldn't any 40 Watt lamp be just as equivalent?

There is s wide variance in MTTF on Model S vehicles,
What does that mean? MTTF is mean time to failure, which is the an average time to failure over many vehicles - so all averaged vehicles share the same MTTF, there isn't a different MTTF per vehicle.

but the statistics indicate that early Model S vehicles had 12V battery replacements about every 12-18 months.
Do you have any statistics on that, or is it mostly anecdotal evidence? If anecdotal, I had a 2013 Model S60 which I kept 2 years before I sold it with the original battery without any problems. It spent most of its life with me connected to a Mobile Connector.

more recent vehicles have been reporting 3+ years MTTF.
Which more recent vehicles are you talking about here, which average failure time to 3+ years? If you took 2015 cars for example, the early ones could report failures almost 4 years (less 2 months), and the latest 2015's less than 3 years (since they haven't been alive for 3 years yet if in December 2015). Now, if you average a set of data with maximum ~3 years, you're not getting 3+ years MTTF. To get a reported MTTF to be 3+ years you'd probably have to be looking at 6+ year history, unless the newer cars you are looking at all failed at 3 years time.

But that does not avoid the fact that the cycle based wear out of even the more recent better deep cycle AGM batteries will eventually bring their demise.
Do you have data backing this? Any studies published, any specifications from battery manufacturers, or any statistically meaningful data proving this?

While I can't state with certainty what the impact of frequently topping off a car battery is, your explanation doesn't really hold up to even basic scrutiny. Tesla bumper-to-bumper warranty is 4 years / 50K miles. Even if the battery dies within that time, you get a new battery for free, instead of spending time and money setting up a battery tender. Even for older cars, what do you think is the return on investment for using a battery tender vs. just keeping the car plugged in (which you probably want to do anyways to keep the large battery happy)?
 
Last edited:

FlatSix911

Porsche 918 Hybrid
Jun 15, 2015
6,889
6,504
Silicon Valley
You are correct. DC-to-DC basically works like an alternator on an ICE car and it's on whenever you drive the car (and sometimes when 12V needs a top off). I incorrectly statined that the battery was draining during the drive, the 12V load simply subtracts from the DC-to-DC max capacity to charge the battery...

While I can't state with certainty what the impact of frequently topping off a car battery is, your explanation doesn't really hold up to even basic scrutiny. Tesla bumper-to-bumper warranty is 4 years / 50K miles. Even if the battery dies within that time, you get a new battery for free, instead of spending time and money setting up a battery tender. Even for older cars, what do you think is the return on investment for using a battery tender vs. just keeping the car plugged in (which you probably want to do anyways to keep the large battery happy)?

Much better to spend $50 on a battery tender than have your 12V battery fail in the middle of nowhere and have the inconvenience of towing and replacement... time and money :cool:
 
  • Like
Reactions: iCharge

FlatSix911

Porsche 918 Hybrid
Jun 15, 2015
6,889
6,504
Silicon Valley
To avoid 12V cycling, you need a tender that can provide about 50 watts -- at 12V, that's a tad over 4A. There are many recommendations on this thread for suitable 12V tenders of that specification. I popped the nose cone, connected some terminals to the battery posts, routed them inside the "frunk" of my classic P85+. I drive a lot too (~70k miles in ~4 years), and I'm still on the original 12V, 4.5 years later. I've probably jinxed myself for posting this ;-)

Good idea on the remote terminals in the trunk... do you have any photos of the installation? :cool:
 

whitex

Well-Known Member
Sep 30, 2015
6,705
8,047
Seattle area, WA
Much better to spend $50 on a battery tender than have your 12V battery fail in the middle of nowhere and have the inconvenience of towing and replacement... time and money :cool:
Except that that $50 does not guarantee that your battery won't fail in the middle of nowhere. We have no solid data whether it helps at all.

If battery failing suddenly is your main concern and it's important to you, a more reliable way (though still not guaranteed) might be to regularly change your 12V battery. Also, bring one of those small Lithium Ion car battery boosters with you, which you can use to start your car in case of 12V failure.
 
Last edited:

tga

Supporting Member
Supporting Member
Apr 8, 2014
3,997
2,865
New Hampshire
We have no solid data whether it helps at all.
You can argue that we don't have a good sample set, but it just common knowledge that a lead acid battery will wear out faster with cycling than sitting on a float charger. The deeper the cycling, the more the wear/shorter the life. AGM's are better than flooded lead acid for this, but are still not immune to damage/wear from cycling.

It is completely reasonable to assume a 4 year old lead acid battery that has spent most of it's life on a float charger will be in much better shape than one that is deep cycled 3 times a day.

I've used AGM's in all my ICE vehicles, tractor, and boats for years, and my experience backs that up.

But yes, float charger or not, the battery can still fail in the middle of nowhere. Keeping a self contained jump starter in the frunk (which can be opened with a dead 12V) is a good idea if your 12V is iffy.
 

FlatSix911

Porsche 918 Hybrid
Jun 15, 2015
6,889
6,504
Silicon Valley
Except that that $50 does not guarantee that your battery won't fail in the middle of nowhere. We have no solid data whether it helps at all.

If battery failing suddenly is your main concern and it's important to you, a more reliable way (though still not guaranteed) might be to regularly change your 12V battery. Also, bring one of those small Lithium-Ion car battery boosters with you, which you can use to start your car in case of 12V failure.

I have a number of sports cars that are occasionally driven and are on CTEK 12V battery maintainers... all with 10+ year lifespan.
The small Lithium chargers are also a good idea to carry on board. :cool:
 

Products we're discussing on TMC...

About Us

Formed in 2006, Tesla Motors Club (TMC) was the first independent online Tesla community. Today it remains the largest and most dynamic community of Tesla enthusiasts. Learn more.

Do you value your experience at TMC? Consider becoming a Supporting Member of Tesla Motors Club. As a thank you for your contribution, you'll get nearly no ads in the Community and Groups sections. Additional perks are available depending on the level of contribution. Please visit the Account Upgrades page for more details.


SUPPORT TMC
Top