stereo turns off during engine cranking

the way the vehicle is set up, when the engine cranks, the switched power wire to the stereo is not energized. 

It is not a low voltage situation, because if I override the switched power wire, and just put battery power to the back of the stereo, and crank the engine, the stereo stays on while the engine is cranking.

I was wondering if I could add a small capacitor to the switched power wire, so that when the engine cranks, the capacitor maintains power for a short period of time to the switched power wire.

I don't know how many micro farads I would need to achieve this. 

has anyone done this before.  I searched and couldn't find anything other than stiffening capacitor talk.

What's the vehicle?  Sounds like this is the way it was designed to work...Support the12volt.com

it was designed that way to conserve all power during engine cranking, but I upgraded to a much larger battery that has plenty of power to run the stereo while cranking the engine.  it is annoying that the radio shuts off when the engine cranks.  

Say I take the wife to the store, I wait in the car with the engine off, but the radio still on.  when she gets back to the car, when the engine is started, the radio (aftermarket headunit) shuts off.  Then I have to wait for it all to power back up, which takes 15-20 seconds, plus allow it to run thru the USB stick of mp3's, which takes another 15-20 seconds. 

So ultimately 1/2 a minute goes by before the stereo is actually fired up and playing music. 

If I can add a capacitor to keep feeding power to the switched power wire, the headunit will keep running while the engine is cranking, thus it never shuts off requiring the 1/2 minute boot up process. 

The accessory circuit in all vehicles does this. If you wish to retain power on the radio during cranking, you need a relay, a diode, and a capacitor. The diode is required online on the accessory wire from the car to terminal #86. The band of the diode must face towards the relay. This will prevent the charge in the capacitor from discharging to the vehicle. Depending on how well the vehicle starts, the size of capacitor will vary. I would suggest at least 10,000 microfarads. If it isn't enough, simply connect another capacitor in parallel. The capacitors must be connected across the relay coils (terminals 85 and 86). Make sure you use polarized capacitors and insure their polarity is correct, otherwise they will explode if their polarity is backwards when you apply power.sparky

What kind of car? You should be able to cut the switched accessory wire going to the radio and replace it with a switched ignition wire. Ignition wires stay powered through the crank cycle.Kevin Pierson

POST EDIT: Thanks KP!! Geez guys, I just transfer to the IGN circuit. Actually I power direct from the battery through a fuse on its own dedicated cabling (both battery + & -) for the usual noise reasons, and for audio anytime I want. If I wanted it key switched, to the above I'd add a relay which is energised by the IGN position - ie, during cranking. If I also wanted audio on ACC, I'd connect both IGN and ACC each thru a diode to the relay (86). But if the OEM wiring is good and simple enough, I'd simply transfer the audio's power feed from the ACC circuit to IGN. But a cap-held relay? Just move to IGN. And surely 2 diodes is simpler, smaller, cheaper and more reliable if the ACC & IGN solution is required - and you can crank to your heart's content. But as I wrote, in my case, no relay/s - just it's own dedicated cabling. And my sense to turn it off. Incidentally, IMO the cranking disconnection is more to protect the audio from spikes or noisy output since their loading is minimal. (If loading were such a problem, more cars would have the headlight cut-out). Other things like wipers may be cutoff to preserve power (or motor damage). But maybe the above are also cost saving - they are required on the ACC position, to add IGN requires a relay etc. (A typical 12V DIN audio rarely consumes more power than 4 brake bulbs except near full volume.)   

But that adds a (diode) voltage drop to the EQ supply, and requires a BIG cap for larger loads like radio-etc-amplifiers. It may be ok if it's used to hold on the unit's relay, but what timing should be used - ie, what cap size for long cranking? But diodes - if required - have no timing issues, they don't dry out, and incorrect connection doesn't cause much damage (unlike reverse-polarised electrolytic caps), and there is no off delay.