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18 amp alternator voltage output

in 912 / 914 Technical Questions 13 years ago
Hello all,

I have been reading the various posts on voltage regulators/rectifiers. I am trying to find out what voltage should the 18 amp alternator system on our 914 f put out. I am unable to find the nominal value to be expected from the unit. It does not show (or could not find) in both line maintenance and heavy maintenance manuals. A shy guess would have me thinking something smaller than the usual 14,2 Volts (40 amps alternators).

Our aircraft battery went bad after only 2 years yet was of apparent high quality. We now suspect the voltage not being sufficient to charge completely the battery. This of course with minimal electrical components activated. The ammeter is showing a charge indeed.

During runup today we were showing at 1500 prop RPM (3365 eng RPM): 12,5 Volts and at 1900 Prop RPM (4262 eng RPM): 13,2 Volts.

Any tought would be appreciated

Cheers

Marc

  • Re: 18 amp alternator voltage output

    by » 13 years ago

    Marc,

    I also had my battery go bad after less than 2 years. For the last 3 years I have had a "smart charger" hooked up to my battery constantly while the airplane is in the hanger. Several times during that 3 years the airplane has sat outside for 2 or 3 days, once for 3 weeks, without the charger, and was plenty charged up to quickly start the engine after those interludes. This tells me the battery is in good health. It might not be that your alternator is not fully charging the battery but that the battery is being discharged during down time, reducing the battery's lifespan. Lead acid batteries, including AGM batteries, work far better and last far longer if kept fully charged. You must use a quality "maintainer" type of charger that says 3 or 4 stage (the 4th stage is a desulferator stage) on the package and also allows you to keep it hooked up constantly while the vehicle is in storage.

    Alan
      
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  • Re: 18 amp alternator voltage output

    by » 13 years ago

    Hello Alan,

    Thank you for the reply. I agree keeping a battery fully charged is a must. I have had no problems before with other aircraft with "bigger"alternators, ie 40 amps. Nonetheless prior the runup yesterday, I also verified that there was no draining of the battery while the master switch was off. I used an accurate ampmeter connected between the battery negative pole and its lead (cable). The lowest scale (milli Amps) was showing nil current.

    My concern is whether the alternator is pushing sufficient voltage to charge the battery. My reasonning being that an alternator should provide sufficient current to charge the battery fully while in flight much like my former aircraft running traditional alternators.

    Indeed a charger while hangered would supplement and guarantee a charge to maximum capacity however it was in my case not a required procedure on previous aircraft.

    Best regards

    Marc
      
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  • Re: 18 amp alternator voltage output

    by » 13 years ago

    Your Rotax reg/rec and or external generator should have enough output to keep the battery charged. Batteries should last 3-5 years. That said a number of things can adversely affect a battery and shorten its life.

    Examples:
    1. Flying only once a month or less and not putting your battery on a "Smart Charger" that has "float" capabilities. Your do not want a trickle charger. If you fly once a week, that's good. Batteries like to stay charged and standard batteries will have a short life if you discharge it too far too often.
    2. Poor quality battery to start with. I use the Odyssey battery in the US, it is called Hawker in Europe.
    3. Really cold hard starts will shorten its life span. Use an engine pre-heater like Tanis if you live in cold country.
    4. Over loaded electrical system. Pulling to many amps with all you equipment on. You may need the external generator or reduce some of your electrical drain.
    5. Poor wiring that has too much resistance and doesn't let the battery charge normally. Even a poor ground for the battery will strain its charging capabilities.
    6. Extreme temperatures. Excessive heat of excessive cold. If you live in really cold country try keeping the engine compartment a little warmer during the winter and if you aren't flying during the winter take the battery out and put it on a "Smart charger"

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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  • Re: 18 amp alternator voltage output

    by » 13 years ago

    Hi Roger,

    The battery was indeed the Odyssey (PC 925) excellent technology here. The aircraft (Diamond HK 36) is almost brand new but you do ring a bell here. We use the aircraft to tow gliders. The temperature range of operation is -2ËšC to 34ËšC. I would not qualify this too extreme. But we do operate a cable retractor system which reels in the tow cable once the glider is relaesed. The motor however only functions for about 20 seconds. the remaining of the flight using cruise settings lasts about 5 minutes. The on board ammeter usually indicated "0" by the time we reduced power to land. This indicates that the battery is fully charged.

    What got us thinking of insufficient volatge was the low voltage reading on the battery and a continuous charge on the ammeter (positive sector of the ammeter).

    Marc
      
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  • Re: 18 amp alternator voltage output

    by » 13 years ago

    Hello , I check the voltage output at least once during every flight. I find at cruise normally 13.7 to 13.9 and it does fluctuate . Please understand that the flux are normal and I do keep my battery on a smart charger between flights. Hope this info helps.
    Thank you said by: Marc Arsenault
      
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