DIYbanter - View Single Post

The Natural Philosopher

wrote:
Trevor Smith wrote:
In my car I have a bluetooth phone kit that plugs into an auxiliary
cigar lighter socket that is controlled by the ignition switch, which
means it turns off when you turn the key to the start position. I would
like to put an electrolytic capacitor across the terminals of the
socket to give the phone kit enough juice to keep it powered up during
the time it takes to start the engine but I'm not sure how they are
rated, ie: a high number uF a lower number. I know the current that the
kit draws will be needed to get the right capacitor for the right time
but I haven't checked this yet, and I wouldn't the formula to work it
out any way so I was going to go for the biggest.
Any advice would be appreciated
Regards
Trevor smith

This is not a good idea, as you'd need a capacitor the size of atlantis
plus the V_out would ramp down not stay steady. If youre deterimned to
do this you could just wire the lighter to an always on fuse,
disconnecting the old feed. But this wont really work either, since
system v drops heavily during cranking.

NT

Indeed. One reason that aircraft used to use (and may still do) rotary
converters so that huge electrical drains like lowering undercarriages
or flaps do not result in instrument blackout. A big heavy rotor
spinning at many thousand RPM stores a lot more energy than the same
sized capacitor.

Which leads to a potential third suggestion. And auxiliary 12v (or less)
battery with a low Vf diode in series with it (shottky) that powers the
phone..and is recharged at a slightly lower voltage from the main battery..

In fact thats a very nice case for a 3 cell lithium battery. With the
diode it will never get overcharged.