Aviation From My Rocking Chair: Is the Battery Bad, Again?

Wow, you look outside and see a beautiful, Chapter 430-type Saturday. What a great time to put a couple hours on the old flying machine. Though arrival at the airport is slowed somewhat by Sequim traffic ( a two minute wait for the light at River Road and 101), you’re still in a nice mood as the aircraft is untied and preflighted. That is until you snap on the Master switch, the mags, and hit the starter. Instead of that healthy roar as the engine comes to life, all that is heard is a weak solenoid click. Through superior temper control, you manage to slow the red color rising in your cheeks long enough to do a battery check. Master switch On, Landing Light switch On. Battery voltage drops like a stone! Now you are really ticked. That’s the second battery in the last two months. They sure don’t make them like they used to. The preceding scenario may not have happened to you, but if it should, don’t automatically blame the battery. Here is a simple troubleshooting procedure for the aircraft battery charging system. (You will need a multimeter, either digital or analog.)

(1) Charge the battery to full capacity.

(2) Turn on the Master switch and then the Alternator switch.

(3) Set the multimeter to DCV and the “Scale” above normal system voltage.

(4) Place meter Red lead on the Alternator ‘Hot’ stud (or BAT terminal) and the Black lead on the Alternator case (ground). Voltage should read 12.5v (12 volt system) or 24.5v (24 volt system). If there is no bus voltage, suspect a burned fuse, popped circuit breaker, or a bad connection on the battery lead to the Alternator. (5) If the above is o.k., move the multimeter Red lead to the Alternator Field stud and reset the DCV “Scale” to a more sensitive range (such as 5v). This will check the loop from Alternator output to Field input, and includes Alternator feeder, Circuit Breaker and Switch, Voltage Regulator, Connectors, Contacts, Splices, Terminals, etc. If the result is more than 1.5v, there is abnormal drop in the current path from Alternator through the Voltage Regulator and back to the Alternator. Note: 1.25v is normal for the Voltage Regulator. (6) If the drop above is more than 1.5v, check the Field circuit breaker, Alternator switch, Over voltage relay, voltage Regulator, and all the connections for failure. Check each of the components by working through the system backwards, starting with the Alternator.

(7) If the voltage is a lot less than 1.5v (e.g. less than 1.25v), it normally indicates a failed voltage regulator which is usually seen as an Over voltage when the engine is operating.

That’s it folks. I realize the beautiful day is shot, but maybe the next one won’t be. Rick Vaux TC 4130