Keeping the stereo from rebooting or resetting when starting

[baxsie]

This mod is for Kick Azz--our 1975 Super Beetle project, but it can apply to any VW or really any car at all.

The older radios powered up instantly, so if they lost power during cranking, it was no big deal. As soon as the starter is done, they would snap back to life.

Newer stereos can take many seconds to wake up, and additional time to load the CD, or reconnect the bluetooth and what not.

The Kenwood stereo (KIVBT901) we have seems to be particularly sensitive to resetting itself when the starter cranks, even though it is connected directly to the battery. Well the battery is 12v, typically having 12.5v or more at rest, and my meter reads 11.2v when cranking. The Kenwood KIV901BT specifies 10.5v as its minimum voltage, so why is it resetting?

Let's take a look at the battery voltage with the scope during cranking:

So apparently the stereo cannot handle a gitch as short as less than 1/10 second below its 10.5v minimum before it resets.

How can we power the radio during this short time? Maybe a big capacitor? I chose a small SLA (Sealed Lead Acid) battery. It has the benefit of being the same chemistry as the main battery, so charging should be easy, it is small enough to tuck up under the dash, and it should be able to power the stereo for a few seconds easily. As a side benefit, the stereo should not lose its memory or settings even if the main vehicle battery is removed for a short time.

We chose a Werker WKA12-0.8WL (data sheet), mainly because it was the smallest SLA available at our local BatteriesPlus. Even at the full discharge rate of 2.4 amps it should give about 4 minutes before the voltage drops low enough to reset the radio. In real life, the stereo uses about 0.5 amps (the power amplifiers are on a separate, not-backed-up circuit) which should give around 30 minutes.

Here is the circuit we used:

Here is what it looks like wired up. The bump under the heat shrink is the four diodes:

We tucked the battery neatly into a small area behinf the fresh air ducts, just above the stereo. It sits on the stereo clamp, and is secured with a generous helping of SEM 1K seam sealer:

Essentially it is a very simple, small UPS for car stereos.

Now we can listen to the stereo, with the car off, restart the car, and never miss a beat -- or drop a bluetooth call.
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1975 LaGrande Super Beetle Build Log / Farm boy hinge pin puller / Farm Boy Chassis Rotisserie
"I'm not getting older, I'm getting bitter."

[glutamodo]

Hell, all I did when I had a touchy radio in my Baja many years ago was to use a Bosch WR1 starter relay kit to power the radio - and run a dedicated power supply wire straight from the battery to the radio.

[kamesama980]

[baxsie]

The battery is already oversized, and alll the connections are brand new. Battery cables are 2 AWG:

Using skinnier cables to the starter might have made the problem less noticeable

The starter draws ~130 amps . . . but I do not think that is out of line. The engine is still brand new and still a bit stiff.

I think the main problem is that the Kenwood's power supply is rather sensitive to glitching.

Many people may never even classify that the stereo rebooting is an issue, and if the Kenwood booted in 5 seconds, I wouldn't care.

If your stereo does not reboot when starting, or if it reboots quickly enough for you, then you are already good.
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1975 LaGrande Super Beetle Build Log / Farm boy hinge pin puller / Farm Boy Chassis Rotisserie
"I'm not getting older, I'm getting bitter."

[TheAmazingDave]

Thanks for the infoand the diagram! My stereo resets about 50% of the time when cranking. Very annoying, indeed.

I'm gonna have to put this together.
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Just call me Big D. (formerly xBigDx408x)
TheAmazingDave.net
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