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From: "Rumburg, William" <william.rumburg@cdicorp.com>
To: "'lancair.list@olsusa.com'" <lancair.list@olsusa.com>
Subject: RE: Diode Voltage Drop
Date: Fri, 21 Jul 2000 13:50:45 -0400
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>>
After reading all this stuff about diodes connected in series with the
charging circuit of a battery, I can not restrain myself any longer...
Bob Jude  

> Like Bob Jude, I'm having trouble 'restraining' myself after reading
> about the two batteries in parallel and the diodes being used to isolate
> them....
      Earl
>   
	Wow, am I getting a lot of flak regarding my "Keep It Simple" design
for electrical backup power! 
	My backup power system consists of a backup battery, a single diode
(to isolate and charge the backup battery from the main bus) and a DPDT
switch to manually switch backup power to critical devices (ignition and
gyros, etc.) if ever necessary. 
	A VMS 1000 continuously monitors charging amps and bus voltage. It
audibly and visually warns of any abnormal condition on the main bus. I'd
manually switch to "Backup Power", shutoff the alternator and disconnect the
main battery using the Master switch, if warranted.
	As I wrote yesterday, I'm using a Concorde RG-25 sealed lead-acid
battery for the main bus. The electrical loads and required duration of
backup system power are not great and therefore, I'm using a Sears Die-Hard
sealed lead-acid motorcycle battery for backup power (If you're flying with
a traditional split vacuum/electric system and either fails, you complete
your flight, but you don't fly the plane again until both vacuum and
electric power are operable. Likewise, if you have an all electric system
and there is a fault in the main battery or charging system and you have to
go on backup power, you complete your flight and make repairs. You need not
have an excessive number of hours of backup power available).
	Now, about the diode that I'm using to isolate and charge the
battery from the main bus...Concorde states that the required charging
voltage for both their RG and Sears Die-Hard lead-acid batteries is 13.8 to
14.2 volts, although testing has shown that a voltage as low as 13.5 V DC
will maintain full charge. My VMS 1000 indicates a steady-state charging
system voltage of 14.0 V DC. I didn't bother to look up a curve for the
voltage drop across a silicon diode vs. current, just assumed a worse case
of 0.6 V DC. So, at worst, the charging voltage [14.0 - 0.6] = 13.4 V DC
(minimum) to the backup battery is borderline for maintaning full charge.
	It wouldn't be difficult to periodically check the backup battery's
charge state.
	Bill Rumburg
	N403WR  (Sonic bOOm) 

	  
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