Al:
I recently replaced batteries and voltage regulator both that were failing. I have good voltage to the battery at idle but when I turn on my landing light I get an actual 24.7 volts to the battery and of course goes up to 28.8 at higher RPM. The problem is my ammeter at the 24.7 shows a discharge condition. Is there anyway to rezero the ammeter?
You don’t want to try to re-zero the ammeter, Al. It is not voltage-based, it is current-based. Any time the current used exceeds alternator output current, the ammeter should show a discharge.
It is quite probable that what you are describing is normal, but I can’t confirm that without more info. No alternator system provides enough wattage to support all typical “night loads” at idle and low RPM. I can’t tell with certainty from my materials at hand, but my guess is that you have a 60 Amp alternator. It probably can’t produce more than about 35 Amps at low idle, climbing to full output at about 1,500 RPM. Your landing light is supposed to be a #4596, which is a 28V, 250 Watt bulb. This bulb will draw about ten amps all by itself; nearly one-third of the alternator’s output at idle.
As a brief aside, one of the several reasons alternators replaced generators is because alternators could produce the generator’s typical full rated output of 35A, at idle or near-idle (and higher output at higher RPM). The generator required full cruise RPM just to get to its rated 35A.
If you increase RPM to 1,500, turn on your normal loads and confirm roughly 28 Volts, so far so good. If you then turn on the landing light and the ammeter stays at least neutral or slightly positive, all is probably well. It is also normal if voltage drops to 25-26 volts while this heavy load is applied, but the ammeter should not show a discharge.
Note that the 250W landing light should normally be used only in flight. Ground cooling is inadequate for the 250W bulb. During in-flight use, engine RPM is always plenty high enough to get full alternator output.
If it still looks like alternator output is low, after digesting what I have said and doing more testing, you may have an open diode or other fault in the alternator. Check with one of the major auto parts places such as Pep Boys, to see whether they can test a 28V alternator. You’ll have to use the term 24V rather than 28V, since aviation uses charging voltage as the rating nomenclature, while auto applications refer to the approximate rated battery voltage. Please keep me posted.