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Date: Thu, 17 Feb 2005 10:54:10 -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)
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         Most folks don't understand batteries well. Even the tech support 
folks at the battery manufacturer often are poorly trained.

         Here is the real scoop:

 >>> Deep Cycle <<<
         Deep cycle batteries are the best selection if you tend to drain 
down the battery on a regular basis. Sailboats, forklifts, and electric 
cars do this. They run from the batteries for hours and then charge them 
back up when they are half discharged or more. Generally, when the vehicle 
is in use, the battery is NOT being charged.

 >>>Starting <<<

         Starting batteries are best to use if you hardly ever deeply drain 
your battery. Cars, and airplanes tend to only lightly discharge the 
battery between charges. Generally, when the vehicle is in use, the battery 
is being charged.

         The deep cycle batteries are optimized to allow repeated deep 
cycles, at the expense of peak power output and cold weather performance.

         The starting batteries are optimized for peak power output, 
especially in cold weather, at the expense of deep cycle lifespan.

         If you live in a warm climate, a deep cycle battery might be a 
good choice for you airplane. If you live in a cold climate, a deep cycle 
battery might be a poor choice for your airplane, because you might have 
trouble starting it in cold weather.

         There are 4 main factors that cause a battery to wear out:

1) Sulfation. This comes from not fully charging the battery for long 
periods of time. As the lead sulfate builds up, it reduces the capacity and 
power of the battery.

2) Paste degradation. When you discharge and charge the battery, the paste 
contracts and expands. This causes it to mechanically disintegrate. 
(Vibration also causes this in flooded batteries, but not in AGM sealed 
batteries.) The battery loses electrical contact with the disintegrated 
paste and loses capacity.

3) Grid corrosion. When you charge a battery, oxygen is produced. This 
oxygen corrodes the support structure of the battery (grids) that hold the 
paste and conduct the electricity. The battery loses power and eventually, 
capacity.

4) Electrolyte loss. In a seal battery, there is no way to add electrolyte. 
When you fully charge a battery (as you must to prevent sulfation) some of 
the electrolyte is lost. This leads to a reduction in capacity, and 
eventually a reduction in power.

         The manufacturer selects the charging specifications based on the 
best compromise between these wear mechanisms. Everything you do activates 
one or more of these wear mechanisms. If you charge a battery "properly" it 
is a balance between them all that results in the longest battery life and 
best performance over that life.

At 05:38 PM 2/16/2005 -0500, you wrote:
>Alex Madsen wrote:
>
>>It seems to me people should be using deep cycle marine batteries for 
>>there aircraft. A deep cycle marine battery will give much better and 
>>longer performance with a dead alternator.
>>
>>
>I don't think that's true, Alex, unless by 'dead alternator' you're 
>referring to the guy with getthereitis who quick charges a low battery to 
>get juice to get to a destination when he has a known alternator problem.
>
>When a battery isn't charged, the acid eats the lead and the resultants 
>gunk drops to the bottom of the battery where it can never be reconverted 
>back to a usable state through the charging process.  The deep cycle 
>batteries avoid this problem by not exposing all of the lead to the 
>acid.  Some still gets eaten away when the battery is completely 
>discharged, but the lead plate is thick enough not to care.  It doesn't 
>hold any more electrons is store, and since there is less contact area 
>between the plates and acid, it offers more resistance when the terminals 
>are shorted (which is what happens when you throw the starter switch).
>
>They are more robust (as far as charging regime is concerned), but carry a 
>weight penalty.  A much better solution is a well maintained electrical 
>system, and a trickle charger at the hangar.
>
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