The Cessna 120-140 Association Forum

Over the summer our Delco-Remy 20 amp generator failed.
Our mechanic removed it. The brushes were worn and replaced. The insides looked great.
Additionally a new, modern regulator was installed.
Recently a 4 1/2 flight was made using the pull starter twice, radio, rotating beacon and during the last 20 minutes the nav lights as well.
Upon landing no battery power to operate the radio or landing light was left.
It is being suggested that a generator may not be able to keep up with these electrical requirements.
I prefer to avoid installing an alternator since our regular flying is local.
Can the membership advice on this issue?
Thank you

Question, previously when you flew the aircraft before this past summer was the generator able keep the battery charged, and have you added any new items that put an additional load on the generator?

To be truthful, a 20 amp generator should be able to keep the battery charged after using the starter, radio, and typically a single bulb rotating beacon that draws about 4.5 to 5 amps (more if it’s a double bulb beacon) when in operation. Where you see the current really go down hill is operating the landing light unless you have an LED type. Regular incandescent bulbs like the GE unit typically pull a very high amp load, and it’s difficult for the generator to keep up if you utilize the light for longer periods of time.

Your problem may be your battery, have you had it tested for load lately. Additionally, your mechanic should be able to check the generator’s charging capability with a meter or you could take the unit off the engine and take it to an alternator/generator repair shop in your area, and ask them to test it and check the voltage regulator for proper output.

GE4522 250 Watt incandescent landing light for Grimes retractable about 17 amps

Aerolite LED FUSION PAR46 landing light about 8.9 amps

7512-12 incandescent position lights about 2 amps each

LED positon lights about .2 amps each

7079-12 rotating beacon incandescent lights about 2.85 amps each

7079 equivilent LEDs about 1.5 amps each

LED's can cut your amperage use by more than half

What did your ammeter show? If it was charging for most of the flight (everything plus the landing light may be more than it can handle long term) then you should be OK.

Add up all your continuous loads (radio in receive mode, nav lights, beacon, etc.) Don't include intermittent loads like the starter or landing light. The total shouldn't exceed 80% of your generator capacity (16 amps.)

Test your generator to make sure it can really put out 20 amps. (Is your new regulator a 20 amp regulator? The original will limit the output to 12 amps.)

How old is your battery? Have it load tested.

I prefer to avoid installing an alternator since our regular flying is local.

An alternator will work just fine (better, in fact) on local flying. It can put out full power at much lower RPMs than a generator.

thanks everyone for all your suggestions.