From: ReganRanch@aol.com
Message-ID: <ad6f7209.360bbdce@aol.com>
Date: Fri, 25 Sep 1998 11:59:10 EDT
To: lancair.list@olsusa.com
Subject: Batteries and Varistors
Mime-Version: 1.0

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>>
In yesterdays episode, Dr. Modulus (played by Scott) asks:

<< 1.) is it possible to use a 24 volt alternator to charge 2 12 volt
batteries
 in series and then just use 2 12 volt and 1 24 volt buses in the plane?
 seems like this could give you the best of both worlds. anyone done this?
 anyone willing to send in the wiring on how it is done?>>

Bad idea. All Pb-H2SO4 (lead acid) battery cells are nominally 2 volts. They
are combined in series to get higher voltages. Placing two 12 volt batteries
is OK to do if they are of the came capacity and type. Tapping off of two
would only work if the loads were matched and you could tolerate a "ground"
floating at 12 volts on one of the busses (potentially very dangerous. PTP).
The problem comes in charging battery cells that have not been evenly
discharged. Sooner than you would think, one of the cells will fail. I know
this because back in the mid 70s I built an electric car and tapped off 12
volts to run the radio, lights etc. I soon found out that one dead battery
meant that all were as good as dead. The tapped batteries soon failed and had
to be replaced.

Another way to look at is that although you are regulating the voltage to the
cell stack, the individual cells form a voltage divider and are unregulated
and therefore must be matched to charge evenly. Putting a load across of them
upsets the balance.

Of course you could put a charge regulator across each battery but by the time
you get done with that you bight as well have put a DC-DC converter to run
your 12 volt aux buss.
 
<<<
 2.)  I went up to see Dick Mitchell's 320 in park city. fantastic work(7000+
 hours!!) and he had put capacitors on all of his solenoids. he could not
 remember the size. has anyone else done this, and do you know the size of
 the capacitor? does it extend the life of the units?
  >>

I don't think they were capacitors. Likely they were varistors (they LOOK like
caps). Varistors turn into short circuits above a specified voltage and they
are needed because of the flyback voltage from the solenoid coil. YST, when a
solenoid is turned off it's magnetic field collapses causing a voltage pulse
at the solenoid terminals of several hundred volts. This pulse can acr across
the opening switch can cause mischief in the electrical system.

TLA Glossary:
Pb-H2SO4 = lead acid
PDP = pardon the pun
TLA = three letter acronym
YST = You see, Timmy

Regards
Brent