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

Dec 15, 2012
167
64
Beaverton, OR
I’ve read somewhere that the M3 12V charger is internal to the main pack and keeps the 12V battery charged t all times. If true, this should avoid the frequent battery failures of the MS.

But is it true?
 

miimura

Well-Known Member
Aug 21, 2013
6,493
6,255
Los Altos, CA
I’ve read somewhere that the M3 12V charger is internal to the main pack and keeps the 12V battery charged t all times. If true, this should avoid the frequent battery failures of the MS.

But is it true?
No. You can still hear the high voltage contactors frequently cycling on and off. That inherently means that the 12V charging is intermittent. However, they may have used contactors with more cycle life, so they may not drain the 12V as deeply as they do on S and X.
 

Brass Guy

Active Member
Jan 5, 2014
1,167
960
Holbrook, MA
I don't think these responses completely refute the possibility of Model 3 having a DC-DC converter built into the battery pack. In such a setup, I would expect contactors to disconnect the high voltage from the rest of the car; but that doesn't preclude the battery pack from supplying 12V to the car separately. It's an interesting idea, and I would not be surprised in such an advance.
 

miimura

Well-Known Member
Aug 21, 2013
6,493
6,255
Los Altos, CA
I don't think these responses completely refute the possibility of Model 3 having a DC-DC converter built into the battery pack. In such a setup, I would expect contactors to disconnect the high voltage from the rest of the car; but that doesn't preclude the battery pack from supplying 12V to the car separately. It's an interesting idea, and I would not be surprised in such an advance.
If this were true (12V is maintained from the traction battery even when the contactors are open), there would be no reason to close the contactors as soon as you open the door. It would only need to close the contactors when you step on the pedal and prepare to drive away or run the air conditioning or cabin heating.
 

Brass Guy

Active Member
Jan 5, 2014
1,167
960
Holbrook, MA
If this were true (12V is maintained from the traction battery even when the contactors are open), there would be no reason to close the contactors as soon as you open the door. It would only need to close the contactors when you step on the pedal and prepare to drive away or run the air conditioning or cabin heating.
This could be true, but maybe that behavior is just carried over from the Model S software; or maybe they have another reason for it to behave that way.
 

iCharge

Member
Jul 30, 2018
248
69
New England
To stalwarts, my MS will be sitting idle for much of winter and I am planning to put it on a Trickle Charger.
I need some options
Other than Noco 7200, what other chargers have folks used here successfully on their MS ?
 

miimura

Well-Known Member
Aug 21, 2013
6,493
6,255
Los Altos, CA
To stalwarts, my MS will be sitting idle for much of winter and I am planning to put it on a Trickle Charger.
I need some options
Other than Noco 7200, what other chargers have folks used here successfully on their MS ?
I recall from prior discussions about the Model S 12V cycling that you want a maintenance charger that can output 4 amps. So, the Noco 7200 would be good, but the 3500 may not be. I see several others on Amazon that meet that criteria that are much cheaper than the Noco units, but some superficially don't appear to be good quality. Since I don't have experience with them, I won't link to them.
 
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iCharge

Member
Jul 30, 2018
248
69
New England
I recall from prior discussions about the Model S 12V cycling that you want a maintenance charger that can output 4 amps. So, the Noco 7200 would be good, but the 3500 may not be. I see several others on Amazon that meet that criteria that are much cheaper than the Noco units, but some superficially don't appear to be good quality. Since I don't have experience with them, I won't link to them.
Thanks for replying !
I agree. I also saw a BT unit that does 5 AMP https://www.amazon.com/Battery-Tender-022-0209-DL-WH-Selectable-Switchable/dp/B00EDFPN1O?th=1 and then the NOCO which does 7.2AMP.
looking at the discharge cycle charts from our friend Russell, Syonyk's Project Blog: Tesla Model S 12V Battery Analysis ] it seems to me that the sooner this 12V battery is brought back to fuller state of charge, the better it is for its health.
But my old fart brain remembers "you dont want to charge a LA battery too fast" otherwise apparently there is a risk of frying it. Trying to balance safety / battery life / sanity :)
Thoughts ?
 

iCharge

Member
Jul 30, 2018
248
69
New England
With a Tesla on a tender, you're not really charging the battery, you're satisfying the car's power draw so that the 12V battery doesn't discharge.
agreed, my hope is that the 'charger' will supply the computers when need be, so they dont drain the battery [ as much when they are active], hence reducing the number of charging cycles the 12V battery has to go through.
 

miimura

Well-Known Member
Aug 21, 2013
6,493
6,255
Los Altos, CA
agreed, my hope is that the 'charger' will supply the computers when need be, so they dont drain the battery [ as much when they are active], hence reducing the number of charging cycles the 12V battery has to go through.
Exactly right. My point was that a 5A or 7.2A charger isn't going to "Fast charge" the lead acid battery because it's already nearly full. On a Model S, the car will never wake up to charge the 12V from the traction battery because it's voltage will stay above that threshold as long as your tender can delivery the wattage necessary to satisfy the 12V loads.
 
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Reactions: iCharge

iCharge

Member
Jul 30, 2018
248
69
New England
Exactly right. My point was that a 5A or 7.2A charger isn't going to "Fast charge" the lead acid battery because it's already nearly full. On a Model S, the car will never wake up to charge the 12V from the traction battery because it's voltage will stay above that threshold as long as your tender can delivery the wattage necessary to satisfy the 12V loads.
what you are saying makes total sense. Now I am wondering if I really need the 7.2 Amp NOCO or can I get away with a lower 4 amp or even 3 Amp unit ? ;-)
 

miimura

Well-Known Member
Aug 21, 2013
6,493
6,255
Los Altos, CA
Like I said up-thread, 4 amps or higher. The 5 amp one you linked above looks great. It comes with a ring terminal cable with quick disconnect, just like the Noco units do.
 
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Reactions: iCharge

iCharge

Member
Jul 30, 2018
248
69
New England
Like I said up-thread, 4 amps or higher. The 5 amp one you linked above looks great. It comes with a ring terminal cable with quick disconnect, just like the Noco units do.
yes, that is my preferred one as well. Have 2 of the BTs already, one on my bike and another on a beater car, no issues in 5+ yrs of ownership. NOw I have to start researching how to get to the 12V battery & hopefully attaching this does not void the [very fragile] Tesla warranty
 

fbitz777

Member
Apr 6, 2016
432
567
Wexford, PA
Has anyone compared the vampire drain with and without the tender.. I am talking about the MILES lost per day when parked ...I see easily 8 miles per day on my '17 X which is atrocious (and yes I have all the setting optimized not do loose so many miles)
 

fbitz777

Member
Apr 6, 2016
432
567
Wexford, PA
Another idea I had yesterday:
Why not use a solar 50-100W PV instead of a tender...much easier to install permanently...some of my military trucks have very tiny 24V PV cells glued on the hood to keep thinks alive.
Fisker and some Toyotas used to have Solar on the roof..
 

iCharge

Member
Jul 30, 2018
248
69
New England
Has anyone compared the vampire drain with and without the tender.. I am talking about the MILES lost per day when parked ...I see easily 8 miles per day on my '17 X which is atrocious (and yes I have all the setting optimized not do loose so many miles)
I Can report back when I have the BT hooked up, am wondering how hooking the BT will affect warranty, any ideas ?
 

iCharge

Member
Jul 30, 2018
248
69
New England
Another idea I had yesterday:
Why not use a solar 50-100W PV instead of a tender...much easier to install permanently...some of my military trucks have very tiny 24V PV cells glued on the hood to keep thinks alive.
Fisker and some Toyotas used to have Solar on the roof..
my MS sits in garage when parked over a long time
 

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