I am sure some other old-timer will correct me if I am wrong on this, but I think we may be painting an overly simplified picture. I believe a battery must be really REALLY flat to be unable to excite an alternator. Flatter than can usually be imagined.
Even a virtually flat battery should still have enough energy to excite an alternator. And once the alternator is excited, it extracts energy from the turning engine and recharges the battery, so long as the battery is connected. BUT, (and here, to coin a phrase, "is the rub") If the battery is too flat, it will not have the necessary energy to close the master solenoid switch, i.e. the battery will not get connected to the alternator. That WILL keep the alternator from getting ANY excitation current. No excitation current = no electricity from the alternator. So, the problem is not that the battery is too weak to excite the alternator. The problem, as a rule, is actually that the battery is too weak to close the master solenoid switch, so the alternator never gets connected to the battery at all, and accordingly gets no excitation current, and therefore does not get a chance to even TRY to recharge the battery. The alternator and the battery never even see each other. OK, all, I've hung it out there. Take your fair pot-shots and shoot me down if I am wrong. Grins, Dave Winters N2797H PS My mechanic promises that I will have a flyable bird by Autumn..But, now that I think back on it, he did not say what year. -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Percy Sent: Wednesday, July 30, 2008 6:15 PM To: [email protected] Subject: [ercoupe-tech] RE: Radio/battery failure Frank wrote: "I recently learned that an alternator needs a current in order to generate electricity. In other words, you can't put a dead battery in the system and expect the alternator to recharge the battery." I learned that just spring term in A&P class myself, Frank. And when you have an alternator installed, you should also have a volt meter on the panel. Some installations allow a "dual" gage = volts and amps. Neither voltmeters nor ammeters are called out by FAR 91.205, so don't expect them there. There is a volt meter as well as an ammeter in that `coupe I'm selling. This can be tricky - one can prop the plane and launch with a low/dead battery, and the alternator will not charge the system. This leaves one in flight with no electricity for radios, etc. Percy
