# How not to lose power to cigarette lighter when starting the car's engine?

#### averagejoe1

Joined Jul 18, 2018
3
Hello,

I'm new to this forum and I have a basic knowledge of electronics, so keep that in mind.
I was wondering if someone could help me with my car's audio.
So I have this Bluetooth device that has an AUX output which I've connected to the car's AUX input and that's how I can play songs from my phone to my car's speakers via Bluetooth. Now, the problem is that when I turn off the engine (the music is still playing) and then go to start it again, the starter motor for the engine draws a lot of power and the Bluetooth module loses power and therefore the connection with my phone is being terminated. And when the engine starts I have to reconnect the phone and BT module and it's quite annoying. I've connected a simple USB power bank to get around this, but it's just a temporary fix and not very practical (i have to recharge it at home because it won't charge if you're drawing power from it).

So, my question is this: How can I make the BT module not lose power for those few seconds while the engine is being cranked?
Do I need some kind of a capacitor, or a relay or something? I have no idea. Can you please suggest a solution to this?
The BT module doesn't draw more than 2.5W of power.

Joined Mar 10, 2018
4,057
Bulk capacitor with a diode, preferably a schottky between it and battery.
Schottky to prevent cap discharge when starting.

You decide what V drop you can tolerate on BT module. Then using its
current you can compute size of cap.

Q = C x V
I = C x dV/dT
C = (I x dT) / dV

This approach, when you purposely turn off car, leaves the BT
running when you may not want it to. If that's the case add a Cap
discharge circuit, like a comparator to detect "off" and drive a mosfet
to discharge the cap.

Regards, Dana.

#### averagejoe1

Joined Jul 18, 2018
3
Bulk capacitor with a diode, preferably a schottky between it and battery.
Schottky to prevent cap discharge when starting.

You decide what V drop you can tolerate on BT module. Then using its
current you can compute size of cap.

Q = C x V
I = C x dV/dT
C = (I x dT) / dV

This approach, when you purposely turn off car, leaves the BT
running when you may not want it to. If that's the case add a Cap
discharge circuit, like a comparator to detect "off" and drive a mosfet
to discharge the cap.

Regards, Dana.
Wow, thanks for the quick and thorough reply

#### shortbus

Joined Sep 30, 2009
9,635
Not knowing what make of car you have, I can only talk to GM, but think they all work like this. There are two different forms of vehicle power, one that is constant and one that is wired with the key. The constant is used to power parts of the computer(s) in the car, and other things with a memory, like the radio. Those things need constant power to keep the logic alive when the car isn't running, stuff like the radio presets and computer memory, because the ECM computer and more, learn how you drive and store that. So after all of that, which power are you using for your device? Constant or keyed? It may be as simple as changing where you are getting the power for the device.

#### averagejoe1

Joined Jul 18, 2018
3
Not knowing what make of car you have, I can only talk to GM, but think they all work like this. There are two different forms of vehicle power, one that is constant and one that is wired with the key. The constant is used to power parts of the computer(s) in the car, and other things with a memory, like the radio. Those things need constant power to keep the logic alive when the car isn't running, stuff like the radio presets and computer memory, because the ECM computer and more, learn how you drive and store that. So after all of that, which power are you using for your device? Constant or keyed? It may be as simple as changing where you are getting the power for the device.
It's a Mazda 3, the cigarette lighter uses "keyed" power. On position 1 and 2 it has power, but if you turn it to "off" or however is labeled, the cig. lighter doesn't have any power, nor does the radio.

If I connect it directly, the BT module will work all the time and it will be draining the car's battery. I like how it's currently connected.

I'll probably have to get some help from an electronics guy.

#### shortbus

Joined Sep 30, 2009
9,635
It's a Mazda 3, the cigarette lighter uses "keyed" power. On position 1 and 2 it has power, but if you turn it to "off" or however is labeled, the cig. lighter doesn't have any power, nor does the radio.
Off is the same as start to keyed wired things. When in start mode things shut down to allow all power to be used by the starter.

#### olphart

Joined Sep 22, 2012
113
I agree with the "right-sized" capacitor, but I'd just let it discharge through the load on shutdown.
Five to ten seconds runtime should do.

#### brissieboy

Joined Jul 21, 2018
1
You might find that the cigarette lighter power is connected to the 'accessories' power distribution which is ON with the key in the ON position, but OFF when the key is turned to the 'start' (or OFF) position. If so, it can be re-routed quite easily.

#### cork_ie

Joined Oct 8, 2011
428
"You might find that the cigarette lighter power is connected to the 'accessories' power distribution which is ON with the key in the ON position, but OFF when the key is turned to the 'start' (or OFF) position. If so, it can be re-routed quite easily."

Brissieboy is correct.
In the Mazda3 the cigarette lighter is powered through the accessories position of the ignition switch and power is cut to accessories when the starter motor is engaged. Just wire your cigarette lighter socket through ignition or battery positive. Make sure the circuit is appropriately fused.