At 11:05 AM 8/10/98 -0400, Edwin McCarroll wrote:
>.... I have a small meter available and was hoping for advice on 1) how to
check the charging system
Put the volt meter on the battery terminals, directly on the battery
terminals, not on the cable ends. With the ignition switch off, the idle
battery in good condition should read 12.6 volts. If it reads 12.0 volts
or less it needs to be charged. Check the water level first just to be
sure. If it has been on the charger for hours and still reads 12.0 volts
or less, the battery is defective and needs to be replaced.
Next, put the volt meter on the cable ends at the battery. You should
still get 12.6 volts. If the reading is less here than directly on the
battery terminals, then the cable terminals are dirty or corroded and need
to be cleaned before anything else.
Start the engine and check the voltage at the battery again. You should
see an increase in voltage with the generator charging, at least 1 volt
higher, nominally 14 volts. You may have to rev the engine up a little to
get this higher voltage, but not more than a couple thousand RPM. This
higher voltage is required to charge the battery, to cause the smoke to
flow into the battery against the resistance of the base 12.6 volts of
smoke pressure inside. If you do not get this increase in voltage at the
battery with the engine running, then the generator is not functioning
properly. Note that this may not be the fault of the generator, it could
be a bad voltage regulator (control box) or a broken wire.
If you do get a increase of voltage at the battery with the engine running,
then you need to check that it is not overcharging. Under no circumstances
should the voltage at the battery exceed 15.5 volts. If it does, it will
eventually overcook the battery by boiling the water out of it, possibly
warping the plates or otherwise degrading the internal materials of the
battery.
Put the voltmeter on the generator with the engine running, one lead
directly on the big terminal on the back of the generator and the other
lead on a good ground connection. You should see a reading a bit higher
than base battery voltage at idle, maybe 13-14 volts. When you rev the
engine a bit you should see an increase in generator output. At about 2500
rpm engine speed you should see about 16 volts at the generator output
terminal. Less than 14.5 volts here will not charge the battery
adequately, and more than 16.5 volts here will overcharge the battery. You
should see no additional voltage increase at higher engine speeds.
If your voltage test reveals no charging, undercharging, or overcharging,
consult the workshop manual for test and repair proceedures for the
generator and/or control box.
>and 2) how to test for any electrical leakage that might be causing any
problem, assuming that the old battery didn't die of old age.
Turn off the ignition switch. Remove one battery cable fron the battery.
Touch the cable terminal against the battery terminal and look for a small
spark. There should be no spark here, indicating little or no current
flow. Some modern electronic accessories such as a digital radio
continuously take a very small amount of power to maintain their internal
memory, but this should not be enough current to cause any visible spark.
Connect an amp meter between the battery terminal and the cable terminal.
Start on a scale of 1 amp and work you way down to smaller scales in the
milliamp range. A stock MGA should be open circuit with the ignition
switch off, but you may get a small amount of power loss due to some
components leaking power across dirty outer surfaces to ground. This
should not be more than a few milliamps.
For practicle purposes, you would like to be able to let the car sit for 90
days and still have enough power in the battey to crank the engine. A
typical small car battery in good condition may have about 40 amper-hour of
storage capacity, and you can drain at least 75% of that power out before
you see any significant voltage drop. So you do not want to lose more than
30 amper-hour of power in 90 days (2160 hours). That discharge rate works
out to be 0.014 amps, so any leakage greater than about 10 milliamps would
be a problem, and you need to trace down where it's going and put a stop to
it.
A common form of electrical leakage is through the dirt on the top of the
battery to the earthing connection of the battery hold down clamp, or
between the battery terminals. If a battery is quite dirty, and especially
if it is wet on top or shows visible signs of corrosion around the battery
ternimals, it is a candidate for serious surface leakage. Set the
voltmeter on the millivolt scale, make a good connection with one lead on
the non-earth (hot) battery terminal, and poke the other lead around to
various spots on top of the tar or plastic surface of the battery. When
doing this, be careful not to touch anything metal with that second lead to
avoid blowing the meter from over voltage. You can also repeat the process
with one lead connected to the earth terminal of the battery and probing
the other lead around the top of the battery. You should get no voltage
reading when doing this, nada, none, zip. A reading of even 1 millivolt on
the top of the battery indicates significant leakage, and the battery needs
to be thouroughly cleaned.
To clean a battery, flush the outside with water and have at it with a
stiff bristle brush and some baking soda (lots of baking soda). As long as
the mixture is still bubbling anywhere on the surface there is battery acid
present. Continue cleaning in this manner until there is absolutely no
more bubbling, and then rinse the battery clean and let it dry. The same
goes for the area on the body or chassis around the battery. To clean a
particularly stubborn battery cable end terminal, immerse it in a small tub
of water and baking soda solution and let it soak for a few hours, then
clean it with a tooth brush and flush with fresh water.
Barney Gaylord
1958 MGA with an attitude
|