> The "idiot" light tells you (when it is on) that the alternator output
> voltage is lower than the bus/battery voltage. And nothing else.
> When the light is out, the battery is not necessarily charging ( as output
> voltage may be below the c.13.4 volts required for charging).
> My preference is for a voltmeter; which will tell the state of charge of
> the battery before start-up and whether charging is taking place during
> engine run. The amount by which charging is taking place (as indicated by
> an ammeter), I find a little academic.
I don't think a volt meter can tell you anything about the battery.
1) A cell goes open circuit after starting the engine (it does happen) or
one of the leads falls off the battery. Your volt meter will show a >13V
reading, but the battery is getting zero charge (an ammeter would show this
fault).
2) Battery not holding a charge. You arrive at the airfield and find you
can't turn your engine, so you jump it from your car. When running, it may
appear to be charging and the voltage may be >13V, but it won't start the
engine again (even an ammeter won't be much use here).
3) A duff battery may show a reasonable voltage but may have too high an
internal resistance to let the current out (or in). It may even let the
avionics work, but not quite let the starter turn the engine. You will see
the avionics die when you try to crank the engine though.
I think I'd rather have an ammeter than a volt meter.
Cheers,
Mark.
________________________________
Mark Jackson - +44 (0)7050 645590
europa-builder@ntlworld.com
http://harley.pcl.ox.ac.uk/~mark/Europa
|