PC925, mounted on its side in a -10. I've replaced it every 2 years, same symptom: it loses about 30% (?) of its capacity. Still starts just fine. Symptom: battery is at 12.7 or 12.8 volts. Turn on avionics, and the voltage drops within 1 minute to about 12.1-12.2 volts. After that it declines at a much slower rate. And, put it back on the charger, and it shows fully charged after about 2 minutes on the charger. I believe I have lost the top 30% capacity. Not sure why. Because of mounting it on its side? Buss voltage too low (14.2 volts from Plane Power internally regulated alternator)?
PS Anyone know for sure: Can I bump buss voltage up to 14.8 volts or so by putting a diode in series with the field line? Is this likely to be too high?