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 ?

This site may earn commission on affiliate links.
That Battery Tender is only 1.25 Amps. With the thirst of the Vampire, you really want a smart charger that can put out more than 4 Amps to keep up.

Here is my choice: Amazon.com: BatteryMINDer 12 Volt 2/4/8 Amp Wet/Gel/AGM Battery Charger. It's multi-stage, temperature compensated, and will put out 8 Amps. The only problem with this choice is that you have to select "AGM" and 8 Amps every time you plug it in.

And if the grid power fluctuates while it's in long term storage, then it defaults to the 2 amp setting and non-AGM?

I think any "plug it and forget it" option needs gross simplicity for starters.

"The Battery Tender Plus is a 1.25 amp battery charger designed to fully charge a battery and maintain it at proper storage voltage without the damaging effects caused by trickle chargers. Included is a quick connect harness for hard to reach areas. This unit is perfect for charging motorcycle, ATV's or any powersport vehicle and will do an excellent job maintaining car and farm equipment batteries. Temperature compensated to ensure optimum charge voltage according to ambient temperature. Automatically switches from full charge to float charging mode. Battery Tender at 1.25 amps will charge as fast or faster than any 3 amp charger available. Reverse Polarity Protection to ensure user safety. Red & Green Lights Alternately Flash in this condition. Complete 4-step charging program (Initialization, Bulk Charge, Absorption Mode, Float Mode). 10 year Warranty!"

What do you suppose they mean with the bold text?
 
And if the grid power fluctuates while it's in long term storage, then it defaults to the 2 amp setting and non-AGM?

I think any "plug it and forget it" option needs gross simplicity for starters.

"The Battery Tender Plus is a 1.25 amp battery charger designed to fully charge a battery and maintain it at proper storage voltage without the damaging effects caused by trickle chargers. Included is a quick connect harness for hard to reach areas. This unit is perfect for charging motorcycle, ATV's or any powersport vehicle and will do an excellent job maintaining car and farm equipment batteries. Temperature compensated to ensure optimum charge voltage according to ambient temperature. Automatically switches from full charge to float charging mode. Battery Tender at 1.25 amps will charge as fast or faster than any 3 amp charger available. Reverse Polarity Protection to ensure user safety. Red & Green Lights Alternately Flash in this condition. Complete 4-step charging program (Initialization, Bulk Charge, Absorption Mode, Float Mode). 10 year Warranty!"

What do you suppose they mean with the bold text?

I think it means battery tender likes snake oil.

Personally I have the CTEK 3300 US (a 3.3a unit) but If I didn't have that I'd go for the 4.3 amp version Amazon.com: CTEK (56-864) MUS 4.3 12 Volt Fully Automatic 8 Step Battery Charger: Automotive
 
And if the grid power fluctuates while it's in long term storage, then it defaults to the 2 amp setting and non-AGM?

I think any "plug it and forget it" option needs gross simplicity for starters.

The BatteryMinder uses the 12V side for continuity. If the grid goes down and come back it keeps its settings. If for some reason, it did go back to the default, 2 Amps and non-AGM are safe for the AGM, it just won't keep up with the thirsty Tesla Vampire.


Personally I have the CTEK 3300 US (a 3.3a unit) but If I didn't have that I'd go for the 4.3 amp version Amazon.com: CTEK (56-864) MUS 4.3 12 Volt Fully Automatic 8 Step Battery Charger: Automotive

That unit seems to meet the most important requirements of more than 4 Amps and temperature compensated; plus, it seems to be plug and play. On the other hand, I have no experience with it. I do have a lot of good BatteryMinder experience.


Why the 4 Amp requirement?

  • Depending on sleep mode and model, the Model S Vampire seems to drink about 1 kW-hr or 3 rated miles per day.
  • 1 kW-hr/24 hours = 42 Watts
  • 42 Watts/12 Volts = 3.5 Amps
  • To add a little margin, and for round numbers, get a 12 Volt Battery Maintainer that can put out more than 4 Amps continuously.
 
Personally I have the CTEK 3300 US (a 3.3a unit) but If I didn't have that I'd go for the 4.3 amp version Amazon.com: CTEK (56-864) MUS 4.3 12 Volt Fully Automatic 8 Step Battery Charger: Automotive

The BatteryMinder uses the 12V side for continuity. If the grid goes down and come back it keeps its settings.
If for some reason, it did go back to the default, 2 Amps and non-AGM are safe for the AGM, it just won't keep up with the thirsty Tesla Vampire.

That unit seems to meet the most important requirements of more than 4 Amps and temperature compensated; plus, it seems to be plug and play.
On the other hand, I have no experience with it. I do have a lot of good BatteryMinder experience.

Why the 4 Amp requirement?

  • Depending on sleep mode and model, the Model S Vampire seems to drink about 1 kW-hr or 3 rated miles per day.
  • 1 kW-hr/24 hours = 42 Watts
  • 42 Watts/12 Volts = 3.5 Amps
  • To add a little margin, and for round numbers, get a 12 Volt Battery Maintainer that can put out more than 4 Amps continuously.

I agree on the 4 Amp requirement for the Model S and have used the CTEK 4300 with no problems.
 
So, you "disconnect" it by leaving a 15amp fuse connected? Maybe that makes sense to somebody, but 15 amps will kill a 40 amp/hr battery in just a few hours. By the way, for all the cheap folks who think this is a smart way to "save" your 12v battery, just know that the BMS works 24/7 and needs the 12 volt battery to operate.

Get a simple battery trickle charger / tender.
That pictured disconnect is marketed as a security device for ICE vehicles, when "disconnected" there's still 15A of power available to keep all your accessories powered (mainly it's just to keep the radio presets and the clock working) but if someone tries to start the engine the fuse will blow and they won't be able to go anywhere, without knowing what happened they'll likely give up and not bother.

I still don't understand the desire to trickle charge the battery, for short term (eg. overnight or a weekend) it doesn't seem to make any sense as the onboard charger is perfectly capable of this task, and for long term, you use a whole bunch of electricity, to avoid using electricity from the charger, seems disconnecting makes more sense for that use case.

a 4A charger to avoid firing up the Tesla chargers? are you actually saving anything? you're certainly adding complexity!
 
I still don't understand the desire to trickle charge the battery, for short term (eg. overnight or a weekend) it doesn't seem to make any sense as the onboard charger is perfectly capable of this task, and for long term, you use a whole bunch of electricity, to avoid using electricity from the charger, seems disconnecting makes more sense for that use case.

a 4A charger to avoid firing up the Tesla chargers? are you actually saving anything? you're certainly adding complexity!

It's not about saving energy; it's about saving the 12V battery. The 12V battery gets somewhere around 6 discharge / charge cycles per day, each about 30% depth, to supply the vampire load. The 12V battery has a specified cycle limit to wear out. Doing the calculations, it would seem that the mean time to wear out of the 12V battery is 18 months, with a large variance due to all sorts of factors. Lots of owners have reported 12V battery replacement in the 12-18 month time frame.

Assuming you leave the 12V battery connected, one way or the other, you will need to supply the vampire load, either from the HV Li-Ion battery recharging the 12V (and literally cycling it to death at some point) or by supplying the vampire drain directly from the trickle charger and avoiding any cycling of the 12V battery. Losses and efficiency aside, trickle charging the 12V battery from shore power will use the same energy as charging the 12V battery from the Li-Ion battery pack via the DC-DC converter.

Certainly, disconnecting the 12V battery will eliminate any discharge / charge cycling and reduce energy consumption (if that's your goal), but the vehicle will be unpowered and totally isolated (no 3G/LTE/WiFi, no phone app, no key fob radio). Eventually the 12V battery will self-discharge and need to be charged before the car can be brought back to life.

Personally, I would prefer to keep the car powered, secure and connected, but eliminate the 12V cycling to increase that battery's life expectancy.
 
  • Like
Reactions: Lodo1000
I keep one car on a CTEK charger to maintain the battery using the lighter plug that is always on.
It works well and never lets the battery discharge and deep cycle to prevent premature failure.

th?&id=OIP.M5b06b418137d9a86559ac2268891c57bo0&w=300&h=300&c=0&pid=1.9&rs=0&p=0.jpg


Regarding the Tesla, the lighter plug is a switched power supply that will not work to charge the battery.
So we have to use the direct wiring of the charger directly to the front trunk 12V battery terminals.


th?&id=OIP.M6d42e47b35d37ee05f5b05a85af64858o0&w=300&h=300&c=0&pid=1.9&rs=0&p=0.jpg
 
Last edited:
Regarding the Tesla, the lighter plug is a switched power supply that will not work to charge the battery.
So we have to use the direct wiring of the charger directly to the front trunk 12V battery terminals.
See this post. There is a relay dedicated to the 12v port. Just remove it and plug a jumper wire in its place.

Near annual replacement of 12V battery is typical according to Tesla Service Tech - Page 15

This shows the relay location:
Telsa 12V adapater fuse 1.1040.jpg


and a closeup showing where to plug in a short jumper with spade lugs:
Telsa 12V adapater fuse 2.1040.jpg
 

Attachments

  • Telsa 12V adapater fuse 1.640.jpg
    Telsa 12V adapater fuse 1.640.jpg
    64 KB · Views: 106
  • Informative
Reactions: scottm
Good post. I am going add the jumper to keep the 12V accessory plug always on.
Then I will use my CTEK charger to keep the 12V battery topped off in the garage.

See this post. There is a relay dedicated to the 12v port. Just remove it and plug a jumper wire in its place.
Near annual replacement of 12V battery is typical according to Tesla Service Tech - Page 15
This shows the relay locationand a closeup showing where to plug in a short jumper with spade lugs:

BTW, it's a simple matter to keep the 12v outlet in the console live all the time if desired. It's just a simple relay in fuse box #2 (the one on the passenger side under the removable cowling cover - see owners manual). Simply make up a jumper consisting of short piece of 14AWG or better wire with a couple of male 1/4" spade terminals. Pull the front most passenger side relay out and insert the jumper into the now exposed 1/4" female receptacles, and you're done, the 12v outlet is now on all the time. You can quickly change it back anytime by removing the jumper and reinstalling the relay.

Here is a photo of the 12V battery location on a P85D above the front electric motor.
Note that the article states it is not an easy task to replace the deep cycle battery...
http://insideevs.com/wp-content/upl...it-really-looks-like-under-the-hood-Imgur.jpg

Want-to-know-what-it-really-looks-like-under-the-hood-Imgur-750x562.jpg
 
Last edited:
Why the 4 Amp requirement?
  • Depending on sleep mode and model, the Model S Vampire seems to drink about 1 kW-hr or 3 rated miles per day.
  • 1 kW-hr/24 hours = 42 Watts
  • 42 Watts/12 Volts = 3.5 Amps
  • To add a little margin, and for round numbers, get a 12 Volt Battery Maintainer that can put out more than 4 Amps continuously.
Just one question! Why are you calculate with 42 watts? As I remember this was measured 'from the wall', so it includes the
  • HPWC relay's
  • AC-DC converter's
  • DC-DC converter's
  • HVDC relay's consumption
and also affected with the efficiency of these components (battery chemical efficiency etc)!I think the big part of the vampire load is used by the 'recharging process' itself not by the standby devices!siai47 measured about half-amp load on the 12v circuit when feed directly with 12v!
 
Just one question! Why are you calculate with 42 watts? As I remember this was measured 'from the wall', so it includes the
  • HPWC relay's
  • AC-DC converter's
  • DC-DC converter's
  • HVDC relay's consumption
and also affected with the efficiency of these components (battery chemical efficiency etc)!I think the big part of the vampire load is used by the 'recharging process' itself not by the standby devices!siai47 measured about half-amp load on the 12v circuit when feed directly with 12v!

The measurements that I have seen, and that I have done, have been done with the car unplugged; the rough number is 3 rated miles per day lost from the traction battery. From there to the 12V battery, the only thing left is the DC-DC converter and the traction battery relay. The DC-DC converter is probably 90% or better efficiency and even if the traction battery relay draws 40 Watts, its duty cycle is only a few percent. That leaves an overall efficiency of close to 90%; that means my calculations for 12V current could be reduced by 10%. I would still recommend a battery maintainer of with 4 Amps or more capability if you want to feed the vampire.
 
The measurements that I have seen, and that I have done, have been done with the car unplugged; the rough number is 3 rated miles per day lost from the traction battery. From there to the 12V battery, the only thing left is the DC-DC converter and the traction battery relay. The DC-DC converter is probably 90% or better efficiency and even if the traction battery relay draws 40 Watts, its duty cycle is only a few percent. That leaves an overall efficiency of close to 90%; that means my calculations for 12V current could be reduced by 10%. I would still recommend a battery maintainer of with 4 Amps or more capability if you want to feed the vampire.
I think it's not about the charger size rather than about the entire fleet's vampire drain, so how could TM re-design this part of the car. Are the coolant pumps starts when maintaining 12V from the main pack etc? I think the current 12V standby consumption can be only estimated by measuring current from external 12V feed and not calculated from rated miles. If just few watts (0.5A-->6W) as I think than the question is not how to reduce vampire but how to increase the efficiency of the 12V charging circuit. For example a second AC-DC charger is a good idea as brkaus wrote here.
 
See this post. There is a relay dedicated to the 12v port. Just remove it and plug a jumper wire in its place.

Near annual replacement of 12V battery is typical according to Tesla Service Tech - Page 15

This shows the relay location:
View attachment 100813

and a closeup showing where to plug in a short jumper with spade lugs:
View attachment 100814
Tree, I have 3 questions:
  1. Where did you get the parts to make the jumper, specifically the silicon boots that go around the spade terminals?
  2. Do you leave the window open or leave the door ajar to plug the male plug of the charger into the 12V socket on the console?
  3. Where do you put the charger while the car is charging? On the garage floor outside of the car, or sitting on the seat inside the car.
I just bought the CTEK 4.3A charger and adapter plug to plug into the console 12V plug. My S85D is 15.5 months old and the I have not had to replace the battery yet. I think I may wait until the battery is replaced before I start using the charger. Getting the battery replaced under warranty and then start using the charger sounds better than using it now that replacing the battery when it is out of warranty.
 
Tree, I have 3 questions:
  1. Where did you get the parts to make the jumper, specifically the silicon boots that go around the spade terminals?
  2. Do you leave the window open or leave the door ajar to plug the male plug of the charger into the 12V socket on the console?
  3. Where do you put the charger while the car is charging? On the garage floor outside of the car, or sitting on the seat inside the car.
I just bought the CTEK 4.3A charger and adapter plug to plug into the console 12V plug. My S85D is 15.5 months old and the I have not had to replace the battery yet. I think I may wait until the battery is replaced before I start using the charger. Getting the battery replaced under warranty and then start using the charger sounds better than using it now that replacing the battery when it is out of warranty.

I can't speak for Tree, however I can share my experience...hope this helps.:cool:

1. I used crimp on terminals that have the silicon boots attached.
2. I just leave the window open about a 1/4 inch for the charging cord.
3. My CTEK charger is plugged into my overhead garage door opener above the car.
 
What I did: replaced the weak 12V bat with a 85Ah deep cycle battery, put one of those in the trunk:

View attachment 101039


Works like a charm :cool:

Does "what" like a charm? And how is the 85Ah battery wired?

Did you even do this in Tesla, or trunk of some other car (there's nothing in your sig to know)?

Did you truly replace (omit) the factory 12v battery and stick a 85Ah battery to do its duty?
And do you disconnect this replacement battery each time you park?
(a bit of a pain and now nothing works on the car, and I'm assuming you have a manual hatch ??)

An idea of your wiring plan would go a long way toward explaining what is clear in your mind, but a mystery to me.

Just not getting the usage pattern and purpose of the switch in a Tesla.