"Dave Plowman (News)" wrote:
In article ,
Chris Green wrote:
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

That's the 'on charge' voltage. A car battery which is not being
charged won't provide 13.8 volts, we're just used to car batteries
with alternators which mean that whenever the engine is running it's
being charged.

Take a look at:-

http://scubaengineer.com/documents/l...ing_graphs.pdf

Even a fully charged battery will only show just over 12.5 volts when not
being charged.

The point is when calculating near anything car electrics wise is you have
to make allowances for the voltage when running being rather higher than
the nominal 12v. As very little is only ever used with the engine
stopped. And when starting the engine, you also need to allow for the
voltage dropping considerably due to the starter load.

But the current we were debating is the one that flows through the
'lamp' often when the engine hasn't yet been started so it will
probably be when the battery voltage is around 12.5 volts.

It's not a critical current (as in its value isn't critical) anyway so
its exact value is unimportant anyway.

--
Chris Green
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