Forgot to Turn On Alternator After Start - Did I Damage My Battery?

edited November 2022 in General Discussion

After many years away from GA - I'm a senior captain at a major U.S. air carrier - last year, I purchased a 1977 172 Hawk XP II.

I hadn't been able to get up in my airplane for a couple of months, so last week, I went out to my hangar and cranked the engine and ran it for about 10 minutes, in an effort to charge my battery, and get the oil moving around the crank-case.

Yesterday morning, after flying about 40 minutes away from my home airport, upon landing, I realized I had forgotten to turn the alternator switch on after starting the engine. Instead of getting breakfast, as originally planned, I kept the engine running, with the alternator switch now on, and after about twenty minutes, flew back home.

(When I purchased the aircraft last year, one of the pre-purchase inspection "punch list" items I had done was the installation of a new battery.)

Did I damage my battery by allowing it to get to a deep state of discharge?

(As I'm sure I'm the only one on this forum who's ever pulled this kind of DS, flame on...)


  • planewrenchplanewrench IA 46yr A&P DAL A/C Inspector

    At this point i would just consider you did a deep cycle, it never went dead. Looks like it had a lot of reserve. 24 volters are not cheep and none of the batteries we have today seem to tolerate going dead. In old days we even used to put new acid in a weak battery and it was like a enengizer bunny. As long as you are getting good starts and you see the charge rate back to zero, i think you will be ok.

    Might suggest getting a F/E to set up cocpit before you come out and fly! LOL. I always had to work the heater for Paul, my dc10 driver buddy!

  • Thanks for the input.

    Would it matter if it were a 12-volt battery, vice a 24-volt?

  • planewrenchplanewrench IA 46yr A&P DAL A/C Inspector

    No necessarily, the last decade or so, both have suffered the same. I was lucky to find my 172n , early run that had the last of the 12v systems. REally did not want a 24. Every swinging fbo has a 12v charger for most part. I guess not as bad as used to be in that respect. I just changed my bat out last spring. Noted that after two week sit time, was coasting at 11.8. slower crank too. New bat.. 12.3 after month sit time. 12's lot cheaper if that what you have. If any bat goes dead dead, be very suspicous of life left.


  • Start and run an engine for 10 minutes that had not had been run for two months? Would this be good practice for engine care? Thank you.

  • If you start the engine you should fly for about 1 hour to get the oil above 180 degF. That vaporizes any water in the oil .

    The most wear on an engine is in the first few minutes, before the oil pressure comes up. A cold start puts fuel and water vapor into the crankcase, which if left there will start corrosion on the cam and lifers.

    If you are concerned about the battery, it's better t put it on a charger for a few hours to get the voltage up to 14.5 v [ or 29 v ]

  • I have owned a 1977 Hawk XP since 1989. The 12v battery is in the tailcone (similar to Cardinals) which means a long cable to the starter but also easy access. Avionics shop installed a battery tender (1.5 amp) and it is on continously in the hangar so the XP is ready to crank. Also has a Tanis heater and timer for colder weather.

    When extended forecast is temps in single digits and I have no immediate trips, I remove the battery and keep it in the basement. Concorde RG35AXC batteries last about 6 years.

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