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NoSpam

wrote:
Trevor Smith wrote:

give the phone kit enough juice to keep it powered up during
the time it takes to start the engine

I'd use 3 capacitors in parallel. 22uF (which is probably adequate),
then a 1uF and a 100nF all in parallel, because I'd be worried about
filtering high frequency electrical noise from the starter. Feed them
from a diode.

If 22uF isnt enough, then either fix the engine to start more quickly,
or parallel up another one. As we don't know just how much the
bluetooth gadget draws for current, it's not practical to calculate
this beforehand.

But it's possible to make some reasonable estimates. Assume that 2V
droop is the max acceptable and that the load current is 10mA ... CV=IT
will tell you that 66uF will give about 13mS hold-up ... it would be a
rare car that starts in 11mS ;-)

As somebody else has suggested the best way is to wire it from somewhere
else or take a diode to a small rechargeable battery and power it from that.

Dave