On Wed, 25 Apr 2018 09:54:28 +0100, Roger Hayter wrote:

Rod Speed wrote:

"Roger Hayter" wrote in message
...
The Natural Philosopher wrote:

On 24/04/18 09:42, Chris Green wrote:

OK, yes, I agree. 12 volts across 82 ohms is around 160mA.
No, its around 146mA.

What's the voltage of a '12V' lead acid battery?

Usually taken as 13.8 but it obviously varys a bit.

That was my point: 13.8/82=0.168

SLAs tend to have an off charge resting voltage around the 12.75 to
12.85 mark which can last a whole 12 months before reaching a low of
12.70 whilst SLI batteries typically show 12.7 to 12.6 for months
afterwards on older vehicles, maybe less than a month for some newer
models with a relatively high parked up drain from their electronics.
It's never as low as 12 volts 'on-the-nose' unless they're close to
discharged.

In any case, I was assuming the typical 2.2W 12v incandescent panel
lamp's rated current on 12v which worked out to 180mA and hardly varies
with voltage anyway due to the pronounced positive temperature
coefficient of resistance of incandescent lamp filaments even at the much
lower operating temperatures that suffice for the functional requirement
of a panel lamp (no need to be *so* fekin' bright - a long reliable life
is what's needed here - petrol pump filament displays even had life
ratings from 50 to 100 thousand hours!).

By the time the battery voltage hits the 13.8v mark (and rising), it
becomes somewhat academic since the warning light has long since
extinguished with the regulator now fully in control of the excitation
current, confirming the normal engine running condition that the
alternator has kicked in and is charging the battery and supplying power
to the electrical loads.

As for when a red LED indicator lamp is used for this function, I
couldn't say whether the 10 or 20 mA LED lamp current would suffice on
its own to bootstrap the alternator's field current without the addition
of another 50 to 100mA's worth of current[1] with a 5W[2] 120 to 240 ohm
resistor in parallel with the LED and its own current limiting resistor
(circa 1000 to 2200 ohms).

It might well be that any current greater than 10mA would have sufficed
to bootstrap the regulator and a 2.2W panel lamp current was simply a
convenient way to supply this initial current.

[1] I'm just guessing that 60 to 120mA would suffice to cover the most
pessimistic worst case boot strap requirement here.

[2] I'm also guessing that reliability would be the key requirement in
specifying a resistor wattage rating with plenty of margin to eliminate
any risk of 'burn out'.

--
Johnny B Good