> Matt wrote:
       > On a brand new, fully charged battery, 15Ah at 5A is 3 hours of run
       > time.  Add a difficult start, some idle time on the ground, and some
       > other flight loads like radios and lights, and you may be lucky
      to get
       > 1.5hours of run time from the battery.  That's not enough in my
book,
       > and certainly won't allow you to use up all of your fuel.
       > ---------
       > Actually if you mean a 15A-Hr Battery, the above statement is
      incorrect.
       > Battery amp-hours are usually rated at the 20 hour discharge rate.
At
       > higher draws, the capacity goes down significantly.
       >
       > Bill Schertz
       > KIS Cruiser # 4045
       >
       > --------------------------------------------------------------
       >
       > Let me do the math here Bill and correct me if I am wrong.
       > 15A-Hr / 20 hours is .75 amp draw.
       >
       > Here are a few curves. It also appears there are significant
      temperature
       > effects on the discharge rate.
       >
       > http://www.batteryuniversity.com/partone-16a.htm
       >
       > Paul Lamar ...No rotor no motor.

      What you want is Peukert's Equation.
      http://www.smartgauge.co.uk/peukert.html
      At the bottom of this page is a nifty calculator:
      http://www.geocities.com/CapeCanaveral/Lab/8679/battery.html

      If you have the A-hr capacity and the reserve minutes, it will
calculate
      the Peukert's exponent. Then you can pick a discharge and see how long
      the battery will do it.

      For example an Odyssey 680 has a Peukert's Exponent of 1.17. It is
rated
      at 16 A-hr at the 10 hr rate. This translates to 17.7 A-hr at the 20 hr
      rate. A three hour run could draw almost 4.5 amps while a 2 hour run
      could draw as much as 6.3 amps.

      Temperature is, indeed, a BIG factor with lead-acid batteries. A
      lead-acid battery will lose about 1% capacity per degree Celsius below
      20 C. It will gain the same amount above 20 C.

      Bill Dube'



Thanks Bill,

My SWAG numbers weren't too different from yours.

I believe that the Peukert's Exponent changes as a battery ages?


Matt-




A number of things recently discussed on the subject of batteries and
electrical systems raised my interest but also bothered me a bit.  Among
some other checks I made, I just went to Concorde's aircraft batteries web
site and have this little grain of salt to add to the discussion:

Concorde Batteries rate their aviation batteries at 1-HR DISCHARGE RATE and
not at the 20-hr rate automotive battery manufacturers seem to do (for
mainly marketing reasons I suspect).  It makes bloody good sense to me.  I
don't care much about the energy that may still be available in 20 hours if
I lose my only alternator in flight, but I sure care about what's likely to
happen in the next 60 minutes! ... and my E6B calculator doesn't have a
Peukert equation function!

Well, it's good news:  On my 35 Amp.hr battery, I now know I CAN draw 35A
for a full hour and 20A for 2 hours, provided the battery cells are at the
rated temperature (I believe it to be 74°F) and provided the battery is in
good shape.

On the subject of single or dual battery(ies) in the "all electric
airplanes" that are acoming, I have a little story to tell which happened to
me in night IFR with wife and kids aboard.  The battery brutally failed and
nearly set the aircraft on fire... Reason: the airplane was just coming out
of annual.  The mechanic who put the battery back in place tightened the
connections finger-tight... and forgot to tighten them further.  It passed
the IA's as well as my inspection unnoticed.  The connection was fine with
the engine's not running and could start it 100% normally.  But then, engine
vibrations loosened one connector and it started arcing.  The connecting pod
of the battery finally broke off, melted!  There was high-temperature damage
all around it.  So, silly mistakes happen and batteries can fail!

On the subject of single or dual alternator(s), please be a little patient
and save your saliva!  Mistral is hopefully going to make it a non-issue
soon.

Have a great new flying year!

Francois Badoux


Bad things happening coming out of annual is an all too frequent
story. It should be changed from annual to 100 hours or maybe
150 hours.

Paul Lamar ...No rotor no motor.

After my last annual (last month)l, I took the cowl off, found a large rag left there to catch the oil that the filter might drip when it comes off-a real fire hazard in that location. Also, the cowl landing light wires were not re-attached......just for starters! And I had made the mistake of flying it once since the annual. I never seem to learn about annuals and certified a/p's (of which I am one).
I have found tools (on top of the engine and under the cowl), parts, etc after annuals many times in the past! Buyer beware.
Marc Wiese


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