Overview - I would like to install an Apple AirTag in a vehicle and have it gets its primary power from the vehicle's 12V battery. In the event that the 12V Battery is disconnected, I would like for a backup 3V Battery to continue to provide the power required to run the AirTag.

Approach - Here is what I think I need for this circuit solution to achieve

1. The Apple AirTag runs on 3V and about 230mA.
   1. I would like to hide this unit where it is hard to get to, the coin battery typically lasts 1 year. I would like to power off the 12V
   2. I will be removing the AirTag speaker to help make it's location obscure.
2. I would like the Vehicle Battery to be the primary power source of the Apple AirTag.
3. Use of a Buck Converter to bring the Auto Battery 12V down to 3V (actually 3.7V for Diode voltage loss)
4. I would like ton include a 3V Battery for backup in case the Vehicle Battery is disconnected.
5. Place a 3V LIR2032 Coin Battery in parallel to the Buck Converter power supply
   1. Wanting the 3V Battery to sit idle and not provide any power until needed.
   2. If it is not used, there should be no need to trickle charge it.
   3. Trying to avoid placing another Diode in the 3V Battery circuit due to slight voltage drop.
   4. The LIR2032 is a Re-chargeable version of the CR2032, as I believe the current diagram may attempt to charge this battery.
6. Place a Diode between the 3V Battery and Buck Converter for converter protection.
7. The AirTag does require 3V to 2 separate terminals and Ground to 1 terminal.

Current Diagram - Here is my current diagram for your review. Looking for your suggestions.


Thank you in advance for your review.

Mike

 

Great Stuff !

 

Accolades to Adam Cantley - for details/specs on Apple AirTag
Apple AirTag Reverse Engineering - Adam Catley

 

Accolades to Adam Cantley - for details/specs on Apple AirTag
Apple AirTag Reverse Engineering - Adam Catley

Thanks... Yes, Adam's page is an incredible resource on the AirTags.

 

That circuit will cause the battery to supply all the power until it is half discharged. Is that what you want?

 

The battery is a CR2032 with a capacity of 210mAh, which is claimed to last about a year. So the average current is 210mAh/8670 hours = 23uA
A switched mode will be less efficient than a linear regulator at that sort of current, so use something like a LP2951 to supply 3V to the airtag. Then supply it from its standard CR2032 via a schottky diode (or a P-channel FET ideal diode circuit). That would give you a year's use when disconnected from the vehicle battery - won't that be enough?

 

That circuit will cause the battery to supply all the power until it is half discharged. Is that what you want?

When you say "will cause the battery"... are you talking about the 12V car battery? My thoughts are that it should draw all of it's current from the 12V battery. That battery is recharged every time I use the vehicle, so let's assume it will stay at 12V+.

 

Additional Details - The original post will not allow me to edit, so here are some additional details.

I was researching Car alarms and they all have GPS "add-on" features at a cost and with a monthly subscription. Therefore, why no use the Apple AirTag. As I previously mentioned, I would like to hide the AirTag deep under the dash and with an expected 1 year battery life, why not have it powered by the car's battery.

My original design only included the Buck Converter and was straight forward. I wanted to add the backup as an added feature and for me to figure out how it could be done. My research did not uncover a definitive answer. Without wanting to give up, I am turning this into a learning exercise on my part.

If someone stole the vehicle AND they have an iPhone... they would get a message that an AirTag is following them and they would have the opportunity to have the AirTag identify itself by making a chirp (this is why I removed the speaker).
But now that they know an AirTag is present and the Car Alarm, they would highly consider disconnecting the battery. I would like the AirTag to continue to operate with the 3V coin battery.

My concerns for the overall circuit are:

1. If the 3V coin battery is truly in idle and not being used, it should be good for 10-15 years.
2. I was not looking for this solution to provide any "re-charging" functions to the 3V Battery.
3. I did not want to place a Diode to block the 12V from charging the 3V as that would drop the Voltage of the 3V when it was in use.
4. Without this extra Diode, will the 3V attempt to charge, over-charge, Explode, CATCH FIRE?
5. I was hoping that as long as the 12V Battery in connected, that it would provide the energy for the AirTag and the 3V would be idle.

Is there a better method to what I am wanting to do? Can you sketch (napkin is perfectly fine) and provide a better circuit?

 

When you say "will cause the battery"... are you talking about the 12V car battery? My thoughts are that it should draw all of it's current from the 12V battery. That battery is recharged every time I use the vehicle, so let's assume it will stay at 12V+.

No, I am talking about the rechargeable lithium battery. When fully charged it is 4.2V, so the diode is reverse biased and will not conduct anything from your buck converter. The power will come from the coin cell until its voltage drops to 3.7V. At that point it is half discharged.

And, if you are worried about the the voltage drop, what do you think the battery voltage is at near the end it’s 1 year life? Hint: it will be down by more than the 0.3V drop you can expect from a Schottky diode.

So my recommendation is to use a normal, non-rechargeable battery, use two Schottky diodes and lower the buck output to just above the voltage of a fresh battery.

 

So my recommendation is to use a normal, non-rechargeable battery, use two Schottky diodes and lower the buck output to just above the voltage of a fresh battery.

Thank for the explanation and suggestion!

I have revised my diagram to what I believe you are suggesting. Changes are in Red.


1. Using 2 Schottky Diodes. I placed them in the orientation that made sense to me. Is the Diode number OK?
2. Swapped out the LIR2032 3V to a CR2032 (non-rechargeable)
3. Plan to adjust the Buck Converter output to 4.3V. I assume that will make this the primary power source.

Many thanks for your help. Does this appear to be correct?

 

4.3 is too high, that would’ve give about 4V to your AirTag, which might be okay, but might damage it or shorten its life. I would measure the new fresh battery and add about 0.2 to that.

 

Thank you Bob. I was going off your note of that battery having 4.2V fully charged. I will measure the actual and add 0.2 as your mention. That is a great approach.

Did I get the Diodes illustrated in the correct position and orientation? Many thanks! Mike

 

You probably already know that you can get a linear regulator that draws way less then the idle current of any buck converter I have read about recently (75 uA for lnear).

Check out LP2950L-33-T92-R

 

You probably already know that you can get a linear regulator that draws way less then the idle current of any buck converter I have read about recently (75 uA for lnear).

Check out LP2950L-33-T92-R

Thanks Dick for the reference. I will keep that for a future project. I have already bought a handful of Buck converters, so I plan to go that route.

 

I have already bought a handful of Buck converters

You might check the current for those converters from the 12V battery to make sure it's not excessive.

 

Thank you Bob. I was going off your note of that battery having 4.2V fully charged.

That is the LIR one one your schematic. A normal non-rechargeable will be something like 3.2V.