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 ?

scottm

Legacy account
Jun 13, 2014
3,070
2,236
Canada
I don't know about cycle counting... I'd think "voltage speaks" and volt drop over time is the measure of a sick battery, no matter how many times it has cycled.

And for sure we know Tesla is watching health, even newer, lightly cycled battery will be flagged by the car if it's not holding charge over time.

Frankly, I don't like the notion of planned obsolescence devices such as cycle counts. Manufacturers gimmick for ensuring quality but really just forces to buy more. Especially if they put hard limits on a count. And the device when tested is still in spec, more life in it.

But seeing as the 12v battery is part of the critical systems list of devices on the car, maybe it warrants proactive maintenance checks?
 
Last edited:

KArnold

Member
May 21, 2017
626
576
Columbus OH
Spot on. The 12V battery only cycles when the car is parked and "off". Cycles kill the 12V battery
Apparently regardless of whether or not the car is plugged in?

I recently had to leave the car idle for 3 weeks but plugged into my Nema 14-50. When I returned my 7-month old 12-volt needed to be replaced. I assume those 2 events were not coincidence.
 
  • Informative
Reactions: .jg. and navguy12

N..8

Member
Dec 20, 2017
204
151
Sanford, NC
Apparently regardless of whether or not the car is plugged in?

I recently had to leave the car idle for 3 weeks but plugged into my Nema 14-50. When I returned my 7-month old 12-volt needed to be replaced. I assume those 2 events were not coincidence.
The car can't pull DC power from the plug, it would need another separate 12v batter charger put in place.
 
  • Like
Reactions: KArnold

rrolsbe

Member
Feb 18, 2017
239
132
Albuquerque
My neighbor who does not work on cars just had his 12 volt battery replaced. He does not look forward to doing this every 12-18 months. He asked me if I could hook up a Battery Tender to his 12 volt battery. It looks pretty simple to me to do this. He would like to top off his 12 volt battery for 8 hours once a month. He intends to do this while not charging the main battery. Any one have any concerns on why he should not do this ?

After reading many posts in this thread (but by no means all), I see mostly positives to using a battery tender. I only have a few questions that may have been addressed in some of the message I did not read. Charging ICE vehicles I have never had the charger connected to the battery while the vehicle was running ie.. with the alternator AND battery charger BOTH charging the vehicle. With that i mind, I have two concerns: 1) If the tender charging wires always remain attached to the battery AND the tender is plugged in to wall power, would anything go wrong if the high voltage contactor enabled (either because a door was opened or traction battery needed a charge). In this scenario the battery tender and the DC-to-DC converter would BOTH attempt to charge the 12V battery. 2) Same question as 1 but with the tender NOT plugged into wall power but still having the charging wires attached to the 12V battery. The car could even be driven in this scenario. I could make a quick disconnect for the charging wires which would eliminate scenario 2 but would not help with scenario 1 (ie.. I still want the tender to maintain the 12v charge level). Bottom line, will anything bad happen if BOTH the tender and the DC-to-DC converter are attempting to charge the 12V battery in parallel?

Thanks in Advance for any Enlightenment!
Regards, Ron
 

Rocky_H

Well-Known Member
Feb 19, 2015
6,660
7,956
Boise, ID
I guess the real question is "why bother?" If the car is plugged in, as it should be, it will automatically be taken care of.
I guess you haven't read any of this thread, huh? The car only recharges from that wall connection at its next scheduled charging time, which will be the next day. Meanwhile, the very small 12V battery gets drained and refilled, drained and refilled, drained and refilled, repeatedly, about 4 to 5 times a day. That is a lot of stress and cycling on that 12V battery, which is what leads to these frequent replacements. If you have a tender just providing that energy directly, the 12V battery never goes low, thus significantly extending its life.

So...yeah...that's why to bother.
 
  • Like
Reactions: .jg. and navguy12

jerry33

(S85-3/2/13 traded in) X LR: F2611##-3/27/20
Supporting Member
Mar 8, 2012
19,963
23,906
Texas
I guess you haven't read any of this thread, huh? The car only recharges from that wall connection at its next scheduled charging time, which will be the next day. Meanwhile, the very small 12V battery gets drained and refilled, drained and refilled, drained and refilled, repeatedly, about 4 to 5 times a day. That is a lot of stress and cycling on that 12V battery, which is what leads to these frequent replacements. If you have a tender just providing that energy directly, the 12V battery never goes low, thus significantly extending its life.

So...yeah...that's why to bother.
The first battery did go quickly, I believe they had a bad batch back then, but the current 12V has lasted four years and 100K miles. If it goes tomorrow, it won't owe me anything.
 

dhanson865

Active Member
Feb 16, 2013
4,685
7,816
Knoxville, Tennessee
After reading many posts in this thread (but by no means all), I see mostly positives to using a battery tender. I only have a few questions that may have been addressed in some of the message I did not read. Charging ICE vehicles I have never had the charger connected to the battery while the vehicle was running ie.. with the alternator AND battery charger BOTH charging the vehicle. With that i mind, I have two concerns: 1) If the tender charging wires always remain attached to the battery AND the tender is plugged in to wall power, would anything go wrong if the high voltage contactor enabled (either because a door was opened or traction battery needed a charge). In this scenario the battery tender and the DC-to-DC converter would BOTH attempt to charge the 12V battery. 2) Same question as 1 but with the tender NOT plugged into wall power but still having the charging wires attached to the 12V battery. The car could even be driven in this scenario. I could make a quick disconnect for the charging wires which would eliminate scenario 2 but would not help with scenario 1 (ie.. I still want the tender to maintain the 12v charge level). Bottom line, will anything bad happen if BOTH the tender and the DC-to-DC converter are attempting to charge the 12V battery in parallel?

Thanks in Advance for any Enlightenment!
Regards, Ron

If you are using a modern smart charger like the NOCO Genius it senses that the voltage of the 12v system it is charging is higher and stops charging based on that. So if the car decides to kick in and do the charging the external charger backs off. No problem. It will end/interrupt the desulfation/charging cycle and force the external charger to start over later but if you are leaving it plugged in for a long period there will be enough time for both systems to do their thing independently (taking turns).

If you used a dumb charger like they made in the 70s that just has a set charge rate or maybe a couple of switches then that charger would fool the car into thinking the battery is healthy and the car wouldn't do it's thing (the car wouldn't try to charge the 12v battery so long as a dumb charger is feeding it). That isn't bad on the spot, but it's not helping like having a smart charger would. If that dumb charger is undersized it could be worse than letting the car do the charging.

Long and the short get a proper smart charger that knows the difference between LA/SLA, AGM, and Li-Ion and don't worry about it, the two systems will play nice.
 
Last edited:
  • Helpful
  • Disagree
Reactions: Troy and Rocky_H

rrolsbe

Member
Feb 18, 2017
239
132
Albuquerque
If you are using a modern smart charger like the NOCO Genius it senses that the voltage of the 12v system it is charging is higher and stops charging based on that. So if the car decides to kick in and do the charging the external charger backs off. No problem. It will end/interrupt the desulfation/charging cycle and force the external charger to start over later but if you are leaving it plugged in for a long period there will be enough time for both systems to do their thing independently (taking turns).

If you used a dumb charger like they made in the 70s that just has a set charge rate or maybe a couple of switches then that charger would fool the car into thinking the battery is healthy and the car wouldn't do it's thing (the car wouldn't try to charge the 12v battery so long as a dumb charger is feeding it). That isn't bad on the spot, but it's not helping like having a smart charger would. If that dumb charger is undersized it could be worse than letting the car do the charging.

Long and the short get a proper smart charger that knows the difference between LA/SLA, AGM, and Li-Ion and don't worry about it, the two systems will play nice.

Thanks for the reply!

Pluses as I see it.

Keeps 12v battery topped off extending its life and the possibly of being stranded or without a car waiting for a new battery. (A good 12V lead acid battery has lasted me at least 3 years many last 5 years or more) Note: in very hot climates longevity will be reduced. The main problem with so called maintenance free lead acid batteries is there NEEDS to be a way to check/add distilled water to the cells (they will dry out). I have found a way to check/add water to every 12V battery I have ever owned and almost always got more than 3 years of service. The battery in my Model 3 has what appears to be a lid glued over the front side of the battery hiding the fill ports. Can anyone verify this? If there is no access to add water, the battery will probably fail earlier due to drying out. That may be the main reason why many new car batteries (not just Tesla) fail at around the two year point.

Stops frequent cycling of High voltage contactor extending its life?

Extends the life of the traction battery because it SHOULD not need to go through many discharge/charge cycles to maintain the 12V battery.

Negatives:
Requires the purchase of a good charger.
Would need to unplug the battery charger before driving.

Again, Thanks Very Much for your reply!!!
Regards, Ron
 

tga

Supporting Member
Supporting Member
Apr 8, 2014
3,997
2,865
New Hampshire
A good 12V lead acid battery has lasted me at least 3 years many last 5 years or more)
You can't look at your past experience with a starting battery in an ICE. That delivers a tiny fraction of its power to start the car and is recharged immediately. The battery in a Tesla is a totally different animal - sealed AGM (though still lead acid), deep cycle vs starting, and is cycled several times a day.

The main problem with so called maintenance free lead acid batteries is there NEEDS to be a way to check/add distilled water to the cells (they will dry out). I have found a way to check/add water to every 12V battery I have ever owned and almost always got more than 3 years of service. The battery in my Model 3 has what appears to be a lid glued over the front side of the battery hiding the fill ports. Can anyone verify this? If there is no access to add water, the battery will probably fail earlier due to drying out.
Tesla uses a sealed AGM battery. AGM batteries must be (and remain) sealed. There is very little free acid (by design). If they are opened they will dry out and fail in short order. I've switched all my non-Tesla vehicles (boat, truck, tractor, track car) to AGM's and have had multiple years off them. My boat battery is 7 years old and shows no sign of getting tired.
 
  • Helpful
Reactions: Rocky_H

scottm

Legacy account
Jun 13, 2014
3,070
2,236
Canada
.. tteries is there NEEDS to be a way to check/add distilled water to the cells (they will dry out). I have found a way to check/add water to every 12V battery I have ever owned and almost always got more than 3 years of service. The battery in my Model 3 has what appears to be a lid glued over the front side of the battery hiding the fill ports. Can anyone verify this? If there is no access to add water, the battery will probably fail earlier due to drying out. That may be the main reason why many new car batteries (not just Tesla) fail at around the two year point.

Caution, you're in a model S forum talking about a model 3 battery charging system.

The model 3 is probably like the newer generation model S, in that there is a small DC-DC charger always connected to main pack that is supplying idle loads when the car is off (contactor open). This means your 12v battery doesn't drain appreciably and probably not enough to cause a top-up recharge (contactors close) all the while the car is parked if that's all the load your using.

If however, you are using more than the basic 12v loads, computer on, internet connected... say, you've got something plugged into the lighter socket, have a dashcam, whatever.. then maybe the onboard DC-DC is not enough to keep the 12v floated. In this case, a stronger external battery minder is probably a good idea to keep the 12v recharge cycle from happening.
 
  • Informative
Reactions: navguy12

N..8

Member
Dec 20, 2017
204
151
Sanford, NC
I use the NOCO after doing a bunch of reading here. I can tell you that if I don't plug my car into the NOCO I hear it cycle randomly through out the day and night. Once I plug the NOCO in I don't think I've ever heard it cycle, this isn't to say that it doesn't but not enough for me to catch it. I use the G7200 and have seen at times that the battery level will drop to 75 percent without the car cycling then go back to 100%. I would say that with that being seen the car pulls at least 7.2 amps from the battery since the NOCO is barely able to keep up with the draw.
 

Davidoconn

Member
Mar 23, 2018
7
3
Virginia
I’m sorry in advance to not quite understand how the 12v charging system operates. If I leave my car plugged in for long periods how would the 12v be charged any differently than if driven every day? I was under the impression the traction battery charges the 12v whether driving or sitting parked as there is no alternator that I know of.

Am I missing something? I have a CTEK MUS 4.3 that I use on other vehicles but just don’t understand the charging system on the Model S and why it is needed. Thanks.
 
  • Helpful
Reactions: Snerruc

Rocky_H

Well-Known Member
Feb 19, 2015
6,660
7,956
Boise, ID
I’m sorry in advance to not quite understand how the 12v charging system operates. If I leave my car plugged in for long periods how would the 12v be charged any differently than if driven every day? I was under the impression the traction battery charges the 12v whether driving or sitting parked as there is no alternator that I know of.
The difference is in what the car does when it is "on" versus when it is "off". First off, where you say there is no alternator, that's true that there is not literally a device that is an alternator, but there is a different device that serves the same function. It is just a DC to DC voltage conversion circuit.

But here's the operational difference. When the car is on and being used and driven, the main battery pack is connected to the rest of the system, and the DC to DC circuit is drawing from the main battery and keeping the 12V battery topped up and full all the time. When the car is off, though, for safety, they have these big switches, called contactors, that open up and isolate the main battery from the rest of the car. Then, the 12V battery is all on its own. It is getting drawn down lower and lower and lower, having to supply all of those minor computer loads that run in the background. There is monitoring for it, and when the 12V gets too low, the car will close the contactor switches to re-engage the main battery, turn on the DC to DC circuit again, and refill the 12V. Then, it will disconnect the main battery again.

So the difference is like this. If you're driving for a couple hours and then only away from the car for any hour or two, and then driving some more and then only away from the car for another hour or two, the "on" periods are keeping the 12V full, and the "off" periods are short, where it's staying mostly full and topped up most of the time and not getting drawn down very low. That's healthy for a lead acid battery. But if the car sits for days at a time, the 12V is being left on its own for really long periods of time. It gets really low, and then refilled, really low and then refilled, over and over, and that is some harder cycling on a lead acid battery.

So really frequent "on" use of the car keeps the charging circuit on more and keeps the 12V battery topped up. Sitting "off" for long periods creates more of those drain/refill cycles.
 

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