Narrative:

I was on a routine flight from ZZZ to ZZZ1 to conduct aerial imaging. When I leveled off at 9;500 ft. I noticed an indication on some of the imaging equipment indicating low voltage. Thinking I was dealing with a software problem; I started troubleshooting. The equipment is only linked to the airplane by the alternator; which charges it. After my attempts to fix what I thought were a problem with the imaging equipment failed; I turned my attention back to the panel. My scan showed a discharge indication on the alternator. My next reaction was to verify that I was below gear extension speed; and lower the gear lever. The landing gear only extended about halfway down. I ran through the checklist; and pumped the gear down manually. Although I had followed the procedure; I was not getting a green light due as there was not enough power to illuminate it. I headed toward ZZZ2; and called the tower on my cell phone. We arranged a low pass; and everyone in the tower cab agreed they were down and locked. I then climbed out and held over the ZZZ2's VOR. I again reviewed the checklist and everything in the poh; as well as a few 210 training manuals I keep in the airplane. My knowledge of these airplanes told me that if I had followed the procedure; the gear should be down. As a precaution; I called an a&P familiar with 210s; and he agreed that this was the case after conferring with several other people. At that point; I called ZZZ2 tower again; and was cleared for an uneventful; no-flap landing. After some ground troubleshooting; it was confirmed that the alternator was the likely cause. A replacement alternator seems to have fixed the issue.contributing factors;the failure of the low voltage warning lightmy unfamiliarity with the aerial survey equipment; which could have been used as a backup voltmeter. The low voltage indication I was looking at was indicating that the survey equipment was not receiving power from the airplane.my fixation on troubleshooting individual problems; rather than recognizing an alternator failure immediately.this is a valuable lesson; as a similar chain of events could have led to a dangerous situation in IMC. For example; a pilot with a vacuum failure might fixate on troubleshooting a vacuum driven autopilot; and be in an unusual attitude before recognizing the true nature of the situation. It is better to assume the cause of a problem is the worst possible answer; and work backward from there. Useful lessons;cell phones are a valuable tool. While a handheld radio is great; unless you have one that can be wired to a headset it is loud; and the range usually isn't great. A cell phone works just as clearly as a radio; and once paired with a bluetooth headset requires no additional input. Controllers are more than willing to help work a problem. I was very impressed with how everyone in the palm springs tower handled the situation.keep a backup battery to power cell phones with you. I did; but many people are in the habit of charging off of a receptacle in the airplane. Keeping a good scan going is important.

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Original NASA ASRS Text

Title: Pilot reported that an alternator failure resulted in a diversion.

Narrative: I was on a routine flight from ZZZ to ZZZ1 to conduct aerial imaging. When I leveled off at 9;500 ft. I noticed an indication on some of the imaging equipment indicating low voltage. Thinking I was dealing with a software problem; I started troubleshooting. The equipment is only linked to the airplane by the alternator; which charges it. After my attempts to fix what I thought were a problem with the imaging equipment failed; I turned my attention back to the panel. My scan showed a discharge indication on the alternator. My next reaction was to verify that I was below gear extension speed; and lower the gear lever. The landing gear only extended about halfway down. I ran through the checklist; and pumped the gear down manually. Although I had followed the procedure; I was not getting a green light due as there was not enough power to illuminate it. I headed toward ZZZ2; and called the tower on my cell phone. We arranged a low pass; and everyone in the tower cab agreed they were down and locked. I then climbed out and held over the ZZZ2's VOR. I again reviewed the checklist and everything in the POH; as well as a few 210 training manuals I keep in the airplane. My knowledge of these airplanes told me that if I had followed the procedure; the gear should be down. As a precaution; I called an A&P familiar with 210s; and he agreed that this was the case after conferring with several other people. At that point; I called ZZZ2 Tower again; and was cleared for an uneventful; no-flap landing. After some ground troubleshooting; it was confirmed that the alternator was the likely cause. A replacement alternator seems to have fixed the issue.Contributing Factors;The failure of the low voltage warning lightMy unfamiliarity with the aerial survey equipment; which could have been used as a backup voltmeter. The low voltage indication I was looking at was indicating that the survey equipment was not receiving power from the airplane.My fixation on troubleshooting individual problems; rather than recognizing an alternator failure immediately.This is a valuable lesson; as a similar chain of events could have led to a dangerous situation in IMC. For example; a pilot with a vacuum failure might fixate on troubleshooting a vacuum driven autopilot; and be in an unusual attitude before recognizing the true nature of the situation. It is better to assume the cause of a problem is the worst possible answer; and work backward from there. Useful Lessons;Cell phones are a valuable tool. While a handheld radio is great; unless you have one that can be wired to a headset it is loud; and the range usually isn't great. A cell phone works just as clearly as a radio; and once paired with a bluetooth headset requires no additional input. Controllers are more than willing to help work a problem. I was very impressed with how everyone in the Palm Springs Tower handled the situation.Keep a backup battery to power cell phones with you. I did; but many people are in the habit of charging off of a receptacle in the airplane. Keeping a good scan going is important.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.