Received-SPF: none
 receiver=logan.com; client-ip=140.172.240.2; envelope-from=bdube@al.noaa.gov
Message-Id: <5.2.1.1.0.20050216072939.02e080f8@mailsrvr.al.noaa.gov>
Date: Wed, 16 Feb 2005 07:54:48 -0700
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
From: Bill Dube <bdube@al.noaa.gov>
Subject: Re: [FlyRotary] Re: Battery voltage (was: Racetech RV6A Forced
  Landing)
In-Reply-To: <list-741114@logan.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 05:11 PM 2/16/2005 +1000, you wrote:
>Bill,
>What would you expect the battery to show under starting load.
>George ( down under)

         The Hawker Odyssey PC 680 that most folks use has an internal 
resistance of 0.007 Ohms. A typical starter draws perhaps 200 amps in cold 
weather. This means that the battery voltage will drop 200 x 0.007 = 1.4 
volt initially. If the battery is fully charged,the voltage will be 
about  12.8 - 1.4 =  11.4 volts for the first few of blades. As you 
continue to crank, the voltage will continue to decline. If you continue to 
draw 200 amps for 60 seconds (at 77 degrees F) the battery voltage will sag 
down to 7.2 volts.

         If you do crank for this long, it might take as long as ten 
minutes for the battery to charge back up to 13 volts. (Of course this 
depends on the size of your alternator and the draw from other loads on the 
system.)

         There are nifty charts and graphs at:

http://www.batterymart.com/pdf_files/odyssey_guide.pdf