calling all electro heads

[Tahrey1043]

OK, so i'm going on my MPG challenge, and it involves a fair bit of having the engine off at lights, cruising to an engine-off halt over a quarter mile distance, coasting in neutral with zero ignitions downhill etc...... and what's getting on my nerves is that this keeps making my radio cut out, when I inevitably have to run the starter motor. For the briefest of periods after I put the new ignition cables in, this stopped happening, but whatever temporary mojo they worked has long worn off.

I think the solution to my problem is a capacitor on the main power line for the radio, it being DC and all. Don't mind the slight delay this will cause when first powering up, as it will buffer the "no current" periods when all available power including life support and shields is being directed to starter-motor thrusters.

The question I guess I'm driving at is......... my electro-physics knowledge is highly rusty, and I have no textbooks. I know I'll have to hunt out a particular farad value of capacitor for this task, but what, and/or how do I work it out?

For a 12-volt (DC) supply,
carrying current to a consumer that can probably hit 200w (4x45w output plus display, tuner, wastage) at peak (though, during the periods it's likely to be needed, no more than 50w - but the capacitor must be able to handle the peaks to not melt?),
and providing the simulated power-spike smoothing for a duration of (what i stopwatch-measured to be) 1.4 seconds.... call it 2.0 seconds for safety.

So, 12V, upto 200w, for upto 2 seconds. There's something involving Coloumbs in this, i'm sure of it...

....and the result could likely be quite large. Something that i'll have to specially insulate and have trouble fitting/mounting behind the dash, will cost a full fiver from maplins and cause them to note my details in the big book of potential terrorists As a miseducated guess, 33.3F? and I know a 1F capacitor is chunky - normally they start in the pF range?? (picofarad, like, one-billionth or something)

just the first of what might be many electrical adventures, i think. this, then getting the fogs hooked to the sidelight feed, then onward to simulated hybrid drive

[david burton]

33.3?!

hahah good luck!



you'd be better off getting a second battery for the radio!

apparently one coulomb is one amp for one second, and coulombs = capacitance x volts. C = Q/V

200W is 16.7 amps. So for every second at 200W you need 1.4 Farads. This is HUGE. I would definitely source a second battery.

[Tahrey1043]

Christ! um ok then!
mebbe i'll wait til i get the balls up to start playing with electric motors, as they'll need extra batteries, and won't be under so much load when i'm turning the engine over (in fact, they might do away with the need for that "cover" altogether). as it is i think getting an extra battery just for the (£40) stereo, in a 13 year old 1-litre, just to cover it going "dead" for 5 or 10 seconds when i use the starter is a bit extravagant...

(how would i hook it up to stay charged, for one thing?)

not that i couldn't do with carrying a spare battery, the number of times i've run the main one flat.

still tempted by the idea of being on Maplins Most Wanted list though