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

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

In article . com,
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

Wouldn't it be easier to either re-wire the cigar lighter to the ignition
circuit or provide a second outlet so wired?

--
*Speak softly and carry a cellular phone *

Dave Plowman London SW
To e-mail, change noise into sound.

wrote in message
ups.com...
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
....

That reminds me of the power supply I made for my first high level language
computer - a UK101 - a kit that involved soldering hundreds of connections
by hand. I decided I wanted to make the supply really smooth, so I put a
10,000 microfarad capacitor across the outlet. It was big, but it would run
the TTL based computer for several seconds after I disconnected the power.

Colin Bignell

Dave Plowman (News) wrote:
In article . com,
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

Wouldn't it be easier to either re-wire the cigar lighter to the ignition
circuit or provide a second outlet so wired?

--
*Speak softly and carry a cellular phone *

Dave Plowman London SW
To e-mail, change noise into sound.

Thanks for your replies, I hadn't realised it would have to be that big
so I think I will take Dave's advice and hook it into the ignition
circuit instead.
Regards
Trevor Smith

The message . com
from contains these words:

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 he stuck a diode in the feed so it wasn't trying to backfeed the rest
of the car and provided the drain on the socket wasn't too high he'd get
away with it.

--
Skipweasel
Pay no attention to that man behind the curtain.

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.

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


If you dig-out your old school physics books you'll find that CV=IT so
for particular values of load current (I), hold-up time (t) and
acceptable voltage droop (V) you can work out the size of C ... but much
more practical to feed the phone kit from somewhere else.

Dave

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

Wire it direct to a permanent live with its own fuse, much easier than
having to hide a "coke can" somewhere!!

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.

wrote in message
ups.com...

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.

Does that actually work? My rather simple understanding of capacitors was
that if you stick them in parallel, they just behave like one larger one. Or
is the resistance of the wiring somewhere providing a useful effect?

cheers,
clive

No ammount of rewiring the sockets will get round the fact that voltage
on the main bus drops to a few volts during cranking

This is the scheme I used for the radio and
GPS in my last but one car. The NICAD supplies the voltage when the
supply is less than 9volts and even when the main battery is removed.
The Nicad trickle charges through the 330R resistor



SUPPLY LOAD
----------|-------||--------------
/ |
\ 330Resistor |
/ |
\ |
|-------||----+
|
-------
--- 9 VOLT
------- NICAD
---
------
---
|
|
GROUND

In article . com,
dcbwhaley wrote:
No ammount of rewiring the sockets will get round the fact that voltage
on the main bus drops to a few volts during cranking

So what? It's unlikely the phone needs more than 10 volts.

--
*All generalizations are false.

Dave Plowman London SW
To e-mail, change noise into sound.

On 27 Sep 2006 15:57:13 -0700, "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.

Well it would do that.

snip

Any advice would be appreciated

Forget the capacitor idea it's ridiculous. Rewire the socket or add
another one suitably fused directly back to the battery. Or you could
just not use the phone while driving or alternatively chuck the whole
lot away and use a phone box.


--

Clive George wrote:
wrote in message
ups.com...

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.

Does that actually work? My rather simple understanding of capacitors
was that if you stick them in parallel, they just behave like one larger
one. Or is the resistance of the wiring somewhere providing a useful
effect?


More that the coiled up-ness of the big capacitor puts substantial
inductance in series with it. Also electrolytic capacitors have a
measurable internal resistance.


cheers,
clive

Staffbull 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

Wire it direct to a permanent live with its own fuse, much easier than
having to hide a "coke can" somewhere!!


Trouble having to remember to switch it off when I leave the car,
memory not as good as it used to be. No I will wire it into a ignition
feed as I only really need it when driving not parked up with the
engine off. Thanks again for the feedback.
Trevor Smith

Staffbull 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

Wire it direct to a permanent live with its own fuse, much easier than
having to hide a "coke can" somewhere!!


Trouble having to remember to switch it off when I leave the car,
memory not as good as it used to be. No I will wire it into a ignition
feed as I only really need it when driving not parked up with the
engine off. Thanks again for the feedback.
Trevor Smith

On Thu, 28 Sep 2006 12:49:04 UTC, "dcbwhaley" wrote:

This is the scheme I used for the radio and
GPS in my last but one car. The NICAD supplies the voltage when the
supply is less than 9volts and even when the main battery is removed.
The Nicad trickle charges through the 330R resistor

I have a Pama handsfree kit that does exactly that....
--
The information contained in this post is copyright the
poster, and specifically may not be published in, or used by
Avenue Supplies, http://avenuesupplies.co.uk

The Natural Philosopher wrote:

More that the coiled up-ness of the big capacitor puts substantial
inductance in series with it.

DC Volts + Inductance ?

Not really an issue is it.

Also electrolytic capacitors have a
measurable internal resistance.

Yes, to a DC voltage a Capacitor has infinite resistance because it is
open circuit!

I presume the OP was querying strapping 3 capacitors of the same value
in parallel i.e. 3 x 22 uF instead of a single 66 uF cap.

--
http://gymratz.co.uk - Best Gym Equipment & Bodybuilding Supplements UK.
http://trade-price-supplements.co.uk - TRADE PRICED SUPPLEMENTS for ALL!
http://fitness-equipment-uk.com - UK's No.1 Fitness Equipment Suppliers.
http://Water-Rower.co.uk - Worlds best prices on the Worlds best Rower.

On Thu, 28 Sep 2006 16:21:10 UTC, "Pet @ www.gymratz.co.uk ;¬)"

wrote:

The Natural Philosopher wrote:

More that the coiled up-ness of the big capacitor puts substantial
inductance in series with it.

DC Volts + Inductance ?

Not really an issue is it.

Also electrolytic capacitors have a
measurable internal resistance.

Yes, to a DC voltage a Capacitor has infinite resistance because it is
open circuit!

His point, I think, is that an electrolytic capacitor does not have
infinite resistance. (actually, no capacitor has infinite
resistance....)

--
The information contained in this post is copyright the
poster, and specifically may not be published in, or used by
Avenue Supplies, http://avenuesupplies.co.uk

In article ,
"Pet @ www.gymratz.co.uk ;¬)" wrote:
Yes, to a DC voltage a Capacitor has infinite resistance because it is
open circuit!

Eh?

--
*If love is blind, why is lingerie so popular? *

Dave Plowman London SW
To e-mail, change noise into sound.

Bob Eager wrote:
On Thu, 28 Sep 2006 16:21:10 UTC, "Pet @ www.gymratz.co.uk ;¬)"

wrote:

The Natural Philosopher wrote:

More that the coiled up-ness of the big capacitor puts substantial
inductance in series with it.
DC Volts + Inductance ?

Not really an issue is it.

Also electrolytic capacitors have a
measurable internal resistance.
Yes, to a DC voltage a Capacitor has infinite resistance because it is
open circuit!

His point, I think, is that an electrolytic capacitor does not have
infinite resistance. (actually, no capacitor has infinite
resistance....)

The issue is RF suppression in the original comment about paralleling
capacitors. To preserve a low impedance across a broad range of
frequencies it is usual to have up to three capacitors - a large
electolytic, which is reaosanbly low impedance in the audio and
kilohertz sort of arena then something like a mylar layered capacitor,
which will do well up to a Mhz or tow and then something like a ceramic
disc, which can absorb frequencies in the 10-100MHz range, or higher.

Beyond that you are into black magic...every mm of wire will do SOMETHING.

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

The Natural Philosopher wrote:

The issue is RF suppression in the original comment about paralleling
capacitors. To preserve a low impedance across a broad range of
frequencies it is usual to have up to three capacitors - a large
electolytic, which is reaosanbly low impedance in the audio and
kilohertz sort of arena then something like a mylar layered capacitor,
which will do well up to a Mhz or tow and then something like a ceramic
disc, which can absorb frequencies in the 10-100MHz range, or higher.

Beyond that you are into black magic...every mm of wire will do SOMETHING.

Apologies.
My mistake. Your explanation above clears up the error of my confusion.
:¬)

--
http://gymratz.co.uk - Best Gym Equipment & Bodybuilding Supplements UK.
http://trade-price-supplements.co.uk - TRADE PRICED SUPPLEMENTS for ALL!
http://fitness-equipment-uk.com - UK's No.1 Fitness Equipment Suppliers.
http://Water-Rower.co.uk - Worlds best prices on the Worlds best Rower.

Dave Plowman (News) wrote:

In article ,
"Pet @ www.gymratz.co.uk ;¬)" wrote:

Yes, to a DC voltage a Capacitor has infinite resistance because it is
open circuit!

Eh?

But if the 'DC' voltage changes a current will flow, so you can no
longer describe it as presenting infinite resistance - although of
course energy is being stored or released, rather than dissipated as
heat as it would be in a resistor.

For a perfect capacitor i = C * dv/dt, for a practical capacitor there's
always some additional loss present which does result in heat being
dissipated. This is usually thought of in terms of an equivalent series
resistance, ESR.

There's no such thing as DC really, unless you've been hanging around
since before the big bang.

--
Andy

In message . com,
Trevor Smith writes
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

Buy Drivel's rusty old Prius

it's got a big battery in it

--
geoff

So what? It's unlikely the phone needs more than 10 volts.

--
But the main bus voltage can fall below 6 volts when cranking the
engine, especially if it is very cold.

In article . com,
dcbwhaley wrote:
But the main bus voltage can fall below 6 volts when cranking the
engine, especially if it is very cold.

No it can't.

--
*I wonder how much deeper the ocean would be without sponges*

Dave Plowman London SW
To e-mail, change noise into sound.

In article ,
Nick2 wrote:
But the main bus voltage can fall below 6 volts when cranking the
engine, especially if it is very cold.

No it can't.

Why not?

6v gets you max power delivered from the battery...

I've got an accurate voltmeter sitting across my car battery at all times
the ignition is on, and I can assure you it never drops under nine point
something volts even on the coldest day when starting. Usually the high
10s. If you think about it, old cold start ignition systems are designed
for 9 volts minimum.

--
*Verbs HAS to agree with their subjects *

Dave Plowman London SW
To e-mail, change noise into sound.

dcbwhaley wrote:
So what? It's unlikely the phone needs more than 10 volts.

--
But the main bus voltage can fall below 6 volts when cranking the
engine, especially if it is very cold.

you won't even start a modern car on 6v.

I had two batteries die on me this year on different cars. Around 9v was
the startable limit. I stuck a meter on them to see.

Nick2 wrote:
On Fri, 29 Sep 2006 00:20:42 +0100, Dave Plowman (News) wrote:

In article . com,
dcbwhaley wrote:
But the main bus voltage can fall below 6 volts when cranking the
engine, especially if it is very cold.
No it can't.

Why not?

6v gets you max power delivered from the battery...

And enough power absorbed inside it to explode it.

The Natural Philosopher wrote:

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.

This is completely wrong. Although aircraft certainly did (and some
still do) use rotary converters, they didn't use them to supply the
high power loads. The high power system was fed by engine generators
(DC, then later AC) at 100V and above and the rotary inverters were
used to supply the older 28V standards for existing instrumentation
systems. Nor do these rotaries have particularly massive rotors with
appreciable stored energy in them.

I posted a summary of aircraft electrical power to this ng a while ago.
http://groups.google.co.uk/group/alt...a7213ccfd91f85

Andy Wade wrote:

Yes, to a DC voltage a Capacitor has infinite resistance because it is
open circuit!

But if the 'DC' voltage changes a current will flow,

If it changes, then it isn't DC (for the purposes of reactive theory),
you've imposed some AC component upon it. It may be of very low
frequency, but it's still an AC component and thus the impedance of the
capacitor is no longer infinite.

dcbwhaley wrote:

But the main bus voltage can fall below 6 volts when cranking the
engine, especially if it is very cold.

If the terminal voltage of a lead acid car battery falls below about
9V, the effective internal resistance (for high currents) will also
have become so high as well that they're no longer capable of supplying
starter-levels of power. Try it and you'll find that voltage at the
starter might be 6V when off, but drops to approximately zilch when you
pull starting current.

There's also the problem that trying to start at low voltages can burn
the starter windings out. The engine is often cold and reluctant to
start anyway, so the cranking time goes up. As the efficiency of a DC
motor (mechanical power out / heat wasted) drops dramatically at lower
voltages, then the temperature rise in the starter is significantly
worse when trying to start from depleted batteries - even if they're
not even delivering the same power. This is one reason why the Swedes
were so keen to switch to permanent magnet starters. In their cold
winters it was one less source of failure - it's not that they work
significantly better, it's that they don't kill themselves when trying
to.

The message
from The Natural Philosopher contains these words:

you won't even start a modern car on 6v.

The neighbour's Kia got upset with me jumpstarting it. Had to charge the
battery even though it was cranking over easily fast enough for a petrol
engine. I didn't actually measure the voltage, but it was damned
annoying.

--
Skipweasel
Pay no attention to that man behind the curtain.

wrote:
The Natural Philosopher wrote:

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.

This is completely wrong. Although aircraft certainly did (and some
still do) use rotary converters, they didn't use them to supply the
high power loads.

I never said they did.

If you READ what I said, the rotary converters are there to keep the
INSTRUMENTATION running when the BIG power stuff ****s the batteries.

The high power system was fed by engine generators
(DC, then later AC) at 100V and above and the rotary inverters were
used to supply the older 28V standards for existing instrumentation
systems. Nor do these rotaries have particularly massive rotors with
appreciable stored energy in them.


Thats is almost exactly what I SAID ****wit. However the rotaries have a
lot more energy in them than a bank of similar weight capacitors.

I posted a summary of aircraft electrical power to this ng a while ago.
http://groups.google.co.uk/group/alt...a7213ccfd91f85

Yawn.

wrote:
dcbwhaley wrote:

But the main bus voltage can fall below 6 volts when cranking the
engine, especially if it is very cold.

If the terminal voltage of a lead acid car battery falls below about
9V, the effective internal resistance (for high currents) will also
have become so high as well that they're no longer capable of supplying
starter-levels of power. Try it and you'll find that voltage at the
starter might be 6V when off, but drops to approximately zilch when you
pull starting current.

Not quite true, as those of us who built go karts powered by starter
motors can attest. Its a bit of a race as to whether the battery
explodes, the start current in the motor burns the windings out, or the
kart actually gets up to speed, but for a split second, the voltage is
pretty low..

We estimated that from te Kart duration on a stock battery, peak
currents were up to a Kiloamp, and normal running was a couple of
hundred plus.. About 2-3KW

...of coourse these days you can buy model aircraft motors of less weight
and similar power. I think I juts read that someone has a 15KW one in
pre production. Bang in a thousand quids worth of lithium batteries and
you have something that will take a 20lb plane vertically upwards. Takes
all the fun out of it.

In message .com,
writes

The Natural Philosopher wrote:

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.

This is completely wrong. Although aircraft certainly did (and some
still do) use rotary converters, they didn't use them to supply the
high power loads. The high power system was fed by engine generators
(DC, then later AC) at 100V and above and the rotary inverters were
used to supply the older 28V standards for existing instrumentation
systems. Nor do these rotaries have particularly massive rotors with
appreciable stored energy in them.

So, an Austin Allegro with an APS tied to the roof rack then ...

--
geoff

Nick2 wrote:
On Fri, 29 Sep 2006 09:47:58 +0100, The Natural Philosopher wrote:

6v gets you max power delivered from the battery...
And enough power absorbed inside it to explode it.

A few hundred watts for several seconds? I doubt that would result in much
of a bang.


About 5Kw...and guess what, enough hydrogen coming off the plates..

Nick2 wrote:
On Fri, 29 Sep 2006 09:41:03 +0100, Dave Plowman (News) wrote:

I've got an accurate voltmeter sitting across my car battery at all times
the ignition is on, and I can assure you it never drops under nine point
something volts even on the coldest day when starting. Usually the high
10s. If you think about it, old cold start ignition systems are designed
for 9 volts minimum.

I wonder how responsive it is to transitions? I think you are right
though, most modern cars never seem to really load their batteries on
starting.

Doesn't always apply to diesels though... many a time I've heard a diesel
van start up after barely turning over.


We bought a second hand diesel, and tha battery was duff..I had to jump
it. Even at 10v it would not 'catch' - gloplugs too cold probably. I had
to fire up the 'donor' vehicle to get the volts up enough for it to
fire, and the bloody thing ran a bit rough for half a minute till the
volts came up on the battery. I had a meter on and that was about 10.5
volts that DIDN'T start it.

Strangely enough I had traded it for a jaguar with also a dead battery.

That fared even worse. Half the electronics didn't work properly even
with the engine running (finally) and I drove it to the garage with the
ABS and power brakes warning lights on, and the bloody windows wouldn't
stay closed till I got there and rebooted them..

Its not a question of the starter motor these days as much as the
likelihood that half the computers in the thing are designed for some
voltage above 10v at least.

Old cars are a lot more tolerant.