An electrical question regarding capacitors

[Eric M.]

I'm wondering if anyone would have an idea of the size and rating of a capacitor I might need. OR, know of a good electronics site I might post this

I have a 69 Dodge Charger. I upgraded the radio to a modern CD player. My problem was that every time I came to a stop, the CD player would shut down until I hit the accelerator again. It turned out the old style alternator couldn't deliver a constant 13.4 volts at idle (low RPMs) which is what the CD player required. My solution was to install a capacitor in parallel to the power supply to the CD player which increased the voltage enough at idle to keep it from shutting down.

Now I have a 1991 Suburban with a rear power window that operates very slowly (sometimes stalls) when the engine is off, running of the 12 volt battery.. It works well when the vehicle is on and the charging system is putting out 13.4 volts. I wanted to install a capacitor at the rear widow motor to increase the voltage but am not sure what size or rating I'll need. I'm thinking the motor probably draws 10 to 15 amps (I can test if necessary).

Anyone have any ideas?

Thanks,

Eric M.

 

[73k5blazer]

I don't think that would really work so well. A capacitor is designed to supply short term (very short term) voltage during times of low voltage. Also, for them to work properly, they need a constant source of power so when load is low, they charge to capacity. (So, read, engine on, alternator running). When you shut off the engine, the capacitor will slowly bleed down after the next few seconds/minutes (depending how big it is). I'm actually surpised to find it worked in your PLymouth, the scenario you describe doesn't seem like a capacitor would help much more than a couple of seconds, but I guess if it's just your CD player, and it was in the trunk, and needed 13.4 all the time, there's not much load there, so a capacitor would help I guess.
But for your Sub's rear window, engine off, I doubt the capacitor will do anything. because after you shut the engine off, the capacitor will bleed down to battery voltage pretty quickly.

 

[Eric M.]

I didn't think about that. Actually, I was thinking of putting the cap. back at the motor, so, there would be no power to it at all to charge it until I operated the switch, at which point, it's too late.

Some sort of coil transformer maybe?

Any ideas?

 

[ntsqd]

Get in the habit of only running the window up & down with the engine running. It's what I did.

 

[btolenti]

A cap won't do anything for you if the motor is off. It would just charge up to the battery's level and stay there. It may help a bit with initial voltage drop, but to provide enough voltage for the few seconds you need to roll the window up and down would require a LARGE cap....and even then the voltage would only be as high as the battery. Your best bet is to run larger power wires back to the motor to decrease the voltage drop between the battery and the motor....