I just bought a bluetooth adapter for my car. It's input is 5V/0.3A and is powered via a 5V/1A USB power adaptor that is plugged into the 12V/10A/120W max. socket in the center console of my 2004 Honda Accord.

It turns on automatically when I turn my key halfway, but when I start my engine it shuts off, and I have to unplug it and plug it back in to get it to power back on.

I emailed the company, and this is what they said:

"We are sorry for all the inconvenience it caused to you. As you know, there is no internal battery for the SoundSync. The power is from the car or car charger. For some cars, when we turn the car ON, the SoundSync will power on at the same time. However, when we start the engine, the power shut off for a very short time. It shuts the SoundSync off and the SoundSync will not auto power on again. Therefore, we will change it to button control version in the short future."

I'm trying to figure out a way that I can fix this. Unfortunately, my knowledge of electronics is limited. If I had a capacitor between the device and my car, would it solve the problem? I.e. the capacitor takes 1-2 seconds to charge, which interrupts the on-off-on of the car starting, and provides continuous power to the bluetooth module?

I have a soldering iron and am pretty tech-savvy, I just don't know where to begin troubleshooting the issue. Any input would be greatly appreciated.


Edit: How would I go about adding a capacitor? Solder together a circuit board, capacitor, and male/female USB pieces? Cut an existing USB cable and use it instead of the USB components?

  • \$\begingroup\$ I have the same issue. It is way easier to just buy one with a built in battery and just keep it continuously charging via USB car adapter. Here is one of many out there: amazon.com/Mpow-Bluetooth-Receiver-Streambot-Hands-Free/dp/… \$\endgroup\$ – Vince Patron Jan 10 '17 at 21:29
  • \$\begingroup\$ Thanks for the tip. I looked at what you linked, but it still kind of defeats the purpose. It looks like I would still have to manually turn it on; I want it to turn on automatically when I start my car. \$\endgroup\$ – Minty_Tiger Jan 10 '17 at 21:36
  • \$\begingroup\$ It looks like the "AnkerDirect" is the company to avoid. Their "solution" to brown-out car power is dilettantish. Instead of providing "Manual Reset Button" for future product, they should implement a simple voltage monitor IC, and drive their hardware reset from it. \$\endgroup\$ – Ale..chenski Jan 10 '17 at 23:09
  • \$\begingroup\$ Related: "How do I create a USB cable with a capacitor in-line?". \$\endgroup\$ – SamGibson Jul 30 '18 at 15:04

It looks like you bought a badly designed adapter, which is not tolerant to "brown-out" conditions. In cars, when starting an engine, the 12V outlet voltage drops or even toggles several times. All car-intended equipment are designed to withstand this power shortage.

However, adding a simple capacitor to USB cable might not be able to solve this problem. Since your power requirement is 0.3A at 5V, it means that the load is about 5/0.3=17 Ohms. To hold the VBUS power for, say, 1 second, on 17 Ohm load, you will need more than 60mF (milliFarad!!!) of capacitance, based on crude estimation from time-constant RC = 1s. This should be a super-capacitor to do the job.

ADDITION: You can try this 100mF super-cap/5.5V, it is just about $2.80. To avoid back discharge into the 12V - 5V converter, I would use a medium Schottky diode before the capacitor. enter image description here

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Instead of trying to keep the device powered under brown-out conditions, you could make sure it is properly reset when the input voltage becomes low. Several designs are possible, depending on how low the voltage becomes. I would start with this simple circuit:


simulate this circuit – Schematic created using CircuitLab

When the voltage on the 12V line becomes low enough, the relay opens, completely cutting off the power to the device, resulting in a proper reset. When the 12V comes back on, the device is powered again.

Note that I would expect your car to have a proper "Engine Starting" signal which you could use, but you'll need a maintenance & repair manual for your car to locate it.

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  • \$\begingroup\$ Awesome, this looks like a much smarter fix than using a capacitor. Would something like this work? amazon.com/SMAKN%C2%AE-LCUS-1-module-intelligent-control/dp/… \$\endgroup\$ – Minty_Tiger Jan 17 '17 at 18:17
  • \$\begingroup\$ @Minty_Tiger My suggestion was to modify the 12V part of the circuit, so you would need a 12V-driven relay for that. I believe this approach is more robust, since the voltage on 12V side has to drop below around 6V for the 5V side to notice, and a 12V relay would certainly switch off at 6V. On a 5V side, anything below 4V could cause problems, and a 5V relay might remain latched at 4V. \$\endgroup\$ – Dmitry Grigoryev Jan 17 '17 at 20:26

If the shut off at the engine's start is not long (1-1.5 secs), then you could put a large buffer capacitor (ie. 1500uF) on the 5V USB output of your cigar lighter converter. That could provide enough energy until the power comes back on.

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  • \$\begingroup\$ Seems easy enough. How would I go about doing so? \$\endgroup\$ – Minty_Tiger Jan 10 '17 at 21:01
  • \$\begingroup\$ @Minty_Tiger You'll need to measure the current draw before any calculations or design can be made. You can measure it with a specialized USB power meter or just hack something together using a DMM. \$\endgroup\$ – Samuel Jan 10 '17 at 21:11
  • \$\begingroup\$ @Samuel Thanks. Why do I need to measure it? \$\endgroup\$ – Minty_Tiger Jan 10 '17 at 21:16
  • \$\begingroup\$ @Minty_Tiger So you can determine the size of the capacitor required. \$\endgroup\$ – Samuel Jan 10 '17 at 21:41
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    \$\begingroup\$ @Samuel is the 5V/0.3A specification that's listed not adequate? I'd rather not have to but a DMM or specialized power meter. \$\endgroup\$ – Minty_Tiger Jan 10 '17 at 21:43

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