[Roadsters] battery not charging

[Tom Hendricksen]

Tim,

Engine hot or cold does not matter when testing your alternator.  Once the
engine reaches 1000-1500 RPM (depending upon how the excitation field is
wired - or not wired) alternator goes into the charge mode. You should see
about around 13.6-14 volts or so, across the battery terminals.  The voltage
at the alternator may be slightly higher depending upon how much voltage
loss is in the wiring between the alternator and battery.

If you are using the original wiring arrangement, which goes from the
alternator, to the ammeter on the dash, and then back to the battery the
alternator voltage will be slightly higher than the battery because of
voltage loss in the wiring between the alternator and battery.  Since you
say the ammeter goes slightly negative when you turn the headlights on I
would not start with a burnt out ammeter.  If it were burnt out I would not
expect to see the needle move at all.....  BUT when the KISS theory begins
to fall apart keep the ammeter or wiring between the alternator and battery
in mind.  A little resistance from a bad connection can cause headaches in
the charging circuit.  If that is the problem you might find a warm (HOT)
connection somewhere in this circuit because power loss equals heat.

In my case the GM alternator is connected directly to the battery, bypassing
the ammeter.  There is still some ammeter movement, but it is very small and
usually only visible when the headlights are on.

One thing I did not see in your notes is what the battery voltage does after
disconnecting the external charger and letting the battery sit for a while.
If there is a bad cell in the battery the voltage at the terminals may drop
a volt or so from the 11.5 - 12 volts you might see on a good battery to
around 10 volts after not being charged for a while.  A bad cell in the
battery may also cause the engine to turn over slow, or not at all, after
only a short time off of a charging source.  Bill Cole once came to my
rescue when I fuelled a car near his home.  In the time it took to put gas
in the tank the battery would not turn the starter over.  I drove the car 75
miles home at night without any problems and a new battery was installed the
next day.  Thank you Bill.

Having the battery load tested is a good move.  If your battery has a bad
cell or other problems they will show up on the tester.  Many auto parts
stores are able to test the alternator for you if the battery is in good
condition.

Tom
69 2000 - Mr. Hyde
Las Vegas
http://www.datsun2000.com
http://www.nowroc.org


On 6/21/2011 6:06:34 PM, Tim (tputland at charter.net) wrote:
> First off: I am an electrickery novice. Big time.
>
> This is a new condition this spring: 70 1600 w/ gm alternator upgrade
> done three or four years ago. Alt is putting out over 12 volts at 600
> rpm idle; more at higher rpm. Battery is over 12 volts once charged with
> battery charger. But car won't start if battery does not have most of a
> fresh charge from the charger. Amp meter will go into the negative if I
> turn on the head lights but never goes into the positive any more.
>
> Is this my regulator going (gone) bad? How do I test it?
>
> Thanks
>
> Tim
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