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

vickh

Active Member
Dec 16, 2018
3,148
503
az
I want to avoid using 12v front bumper connectors but do it internally thru the car 12v port ( like my noco link above shows) but it says only some car 12v ports allow that
 

tga

Supporting Member
Supporting Member
Apr 8, 2014
3,997
2,865
New Hampshire
I want to avoid using 12v front bumper connectors but do it internally thru the car 12v port ( like my noco link above shows) but it says only some car 12v ports allow that
Read the thread pointed to by @dhanson865 - it shows how to replace the relay with a jumper to keep the cigarette lighter powered constantly. Which, IMNSHO, is how God intended them to be before manufacturers went all nanny-state and felt the need to protect you from your own stupidity from running down the starter battery.
 

iCharge

Member
Jul 30, 2018
248
69
New England
but will it work thru Canbus with 4 or more amps directly from the 12-volt port?
though I wont be able to answer your question, I do have a few comments
  1. In doing so, you might have to keep the charger INside the car, this to me is a fire hazard given the heat most chargers generate, [ which is why they are encouraged to be used in spaces with 'adequate ventilation' ] plus kinds of materials used in upholstery etc.
  2. Even if you keep the charger outside, you will be running a wire to the inside through some channel / conduit, which if not sealed properly, could be an issue [critters bugs going in, and reproducing ]
  3. Depending on where/how you run the wire, if a door is kept open, it may prevent the car from going into sleep mode, draining the TB even more especially if your 12V charger is not powerful enough.
  4. Given how the 12V male adapter connects to the 12V female, I would not be in support of pushing 1 Amp through it, esp over extending periods of time, let alone 4 or more commonly 5-7 amps [ which most of us do as it covers charging + vampire drain + powering electronics this preventing deep cycling]. Would I use CANBus mode In my beater - heck yes, not but not in my nice car. D rings exist for a reason, keep gaps near zero and reduce resistance and hence prevent heating up of contacts which reduces fire risk. They cheap and work safely.
  5. Also not sure of Insurance would cover should a fire result in damage to the interior.
This all being said, if you do it, I hope none of the above becomes an issue for you, pls do report your experience here.
 

iCharge

Member
Jul 30, 2018
248
69
New England
Thanks for the reply!

When you just plug it in, what LED lights are illuminated on the NOCO G7200 charger? I had to close the Frunk latch by pushing down on the latch with a screwdriver so I can keep the Frunk open (the car thinks it is closed). This allows me to monitor the battery voltage and current draw with a DC current probe. Before I closed the latch with a screwdriver, my Model 3 would not go to sleep.

Thanks Again, Ron
Not sure if you are still following but I am noting some strange behavior
Battery Tender / NOCO and Phantom Drain, ? by preventing the MS to go into Sleep Mode
 

iCharge

Member
Jul 30, 2018
248
69
New England
RRolsbe, I have the same set up as you do and I just plug it in and let the NOCO do it's thing. I don't change it to Cold/AGM, just plug it in. I see about 120watts being pulled a few times a day. Over a 3 day period it will use about 1.2kw. I believe the DC to DC doesn't kick on until the battery gets to 12.6 or 5. If my car is sleeping is doesn't loose any over 3 days, this is the longest my car was sleeping at one time. Once it's awake it will over power the NOGO I believe then does engage the DC to DC.
May be this shall interest you
Battery Tender / NOCO and Phantom Drain, ? by preventing the MS to go into Sleep Mode
 

chrissmith007

Member
Feb 28, 2019
12
18
Upper Marlboro, MD
I do not work for Tesla; I am not a mechanic; I am not promoting doing this; do at your own risk:

I have a 2016 Model S P90D. I installed the quick connect cable directly to the 12v battery (purchased on amazon called, "SPARKING 6FT Ring Terminal Cable - SAE to O Ring Terminal Harness Quick Connect/Disconnect Ring Terminal Assembly, 10A Fuse-6FT). This is the same way that I have done for years with my motorcycles and other cars. When you have the tub removed from the frunk its very simple (4-6 screws to remove; look on YouTube). With the tub removed I drilled a hole in the plastic tub and the material tub liner insert for the battery tender connection cable to go through and just sit inside the frunk when not used. I then purchased a 12 v 5 amp Battery tender charger (I initially had the 12 v 1.25 amp battery tender for my motorcycle but I read on the tread that you need more amps. It did work and charged the battery but the 5 amp works way faster). I can still close the frunk while it charges without pinching wires. Works great. 12v battery was on solid green within 10 min. Later I installed a more fancier style docking port for the battery tender to just plug into called a
"Kohree SAE Power Socket Sidewall Port Connector" purchased on Amazon.

The main thing to remember not to do is to not drive off while you still have the battery tender still plugged in. Its not like the main battery that wont allow you to drive or send you a warning message. There is no warning message.

I am making a youtube video not to encourage this or to promote doing this. This is at your own risk.
YouTube Video that I made:
 
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Kenz

Member
Oct 10, 2017
326
257
Hebron, Indiana
When you have your car plugged into a 120 or 240 volt outlet with a mobile connector the BMS system will take care of charging your 12 volt battery for you. You should not need to worry about the 12 volt battery as long as the car is plugged in. The 12 volt battery is charged by the BMS as needed. I must be missing something here but I see no reason to use a separate charger for the 12 volt battery.
 
Last edited:
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Watts_Up

Active Member
Mar 4, 2019
3,648
2,462
In a galaxy far, far away
When you have your car plugged into a 120 or 240 volt outlet with a mobile connector the BMS system will take care of charging your 12 volt battery for you. You should not need to worry about the 12 volt battery as long as the car is plugged in. The 12 volt battery is charged by the BMS as needed. I must be missing something here but I see no reason to use a separate charger for the 12 volt battery.
May be you want to keep Sentry mode on when parked for a long duration and you don't want to have the internal DC charger running too often?

Beside, when the car is asleep, and you don't drive your car for a long time, does a low 12 V battery would wake up the car to get charged?
 

RandyS

Fan of Elon
Jul 8, 2012
726
942
San Diego
When you have your car plugged into a 120 or 240 volt outlet with a mobile connector the BMS system will take care of charging your 12 volt battery for you. You should not need to worry about the 12 volt battery as long as the car is plugged in. The 12 volt battery is charged by the BMS as needed. I must be missing something here but I see no reason to use a separate charger for the 12 volt battery.

Kenz, here is the issue for me...I have a 2013 Model S. These cars are pretty well known for going through 12 volt batteries. I had a couple of replacements under warranty. Without a battery tender, the 12v load is somewhere around 50 watts from what I have read. This causes the 12v battery to drain and be recharged a few times a day, which means closing the contactors on the main traction battery and firing up the DC to DC circuitry to charge it back up. This draining and recharging puts "cycles" on the 12v battery that eventually wear the battery out.

By attaching a battery tender to the car, the 12v battery is kept in a charged state and the ~50 watt load is now carried by the battery tender. The car doesn't have to wake up and charge the 12v battery a few times per day. My current 12v battery (off the top of my head) is probably about 4-5 years old, so I've been able to work around the premature wear issue successfully.

Is it the end of the world if you have to replace the 12v battery every year or two? No, it's not. I just think the 12v battery tender solution is a more elegant solution rather than having the car's power system supply the ~50 watt load and gyrate all day long.

Newer Teslas (Model 3 in particular) don't have this issue because the design was improved...

Anyways, that's my two cents worth....
 

Tim-in-CA

Member
Apr 23, 2018
316
141
SoCal
Reviving and old thread. Instead of using a battery tender, could I plug into a wall charger and set the charge level below my current SOC? In this case will the car run off the wall to keep electronics running vs using the 12V battery? There doesn’t appear to be a convenient way to hook up a battery tender in a facelift Model S. With Covid, I may go 2 weeks without driving. I have battery tenders on my ice vehicles and they work great.
 

chrissmith007

Member
Feb 28, 2019
12
18
Upper Marlboro, MD
I do not work for Tesla; I am not a mechanic; I am not promoting doing this; do at your own risk:

I have a 2016 Model S P90D. I installed the quick connect cable directly to the 12v battery (purchased on amazon called, "SPARKING 6FT Ring Terminal Cable - SAE to O Ring Terminal Harness Quick Connect/Disconnect Ring Terminal Assembly, 10A Fuse-6FT). This is the same way that I have done for years with my motorcycles and other cars. When you have the tub removed from the frunk its very simple (4-6 screws to remove; look on YouTube). With the tub removed I drilled a hole in the plastic tub and the material tub liner insert for the battery tender connection cable to go through and just sit inside the frunk when not used. I then purchased a 12 v 5 amp Battery tender charger (I initially had the 12 v 1.25 amp battery tender for my motorcycle but I read on the tread that you need more amps. It did work and charged the battery but the 5 amp works way faster). I can still close the frunk while it charges without pinching wires. Works great. 12v battery was on solid green within 10 min. Later I installed a more fancier style docking port for the battery tender to just plug into called a
"Kohree SAE Power Socket Sidewall Port Connector" purchased on Amazon.

The main thing to remember not to do is to not drive off while you still have the battery tender still plugged in. Its not like the main battery that wont allow you to drive or send you a warning message. There is no warning message.

I am making a youtube video not to encourage this or to promote doing this. This is at your own risk.
YouTube Video that I made:


UPDATE August 29, 2020: Since the posting of this video this is still working out great for me. I park for long periods of time. I installed in other car as well and the two cars stored in the garage are all set when I drive the other two commuter cars. It’s stored and on both chargers right now.
 

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