REFLECTOR: POWER GUARD

Thu Aug 25 00:52:09 CDT 2005

If you want max redundancy, it's best to be able to separate the 
batteries.  Tying them together doubles your risk of failure for a 
single battery shorting its plates.  Of course this rarely happens, and 
it's all statistics.

Assume a battery shorts itself on average once every 100 hours.  Yes, 
this would be a terrible battery, but the math is easy.  That's .01 
chance per hour.

Tie the two batteries together, and that's .01 + .01 => .02, or an 
average failure rate of once every 50 hours.  This is the power failure 
rate your electronic ignition will see.

Keep them separate, or have the ability to separate them, then the odds 
are much better.  They would both have to fail simultaneously, which is 
.01 * .01 => .0001, or once every 10,000 hours.  Your electronic 
ignition system will probably be replaced with some newer gizmo within 
10,000 hours.

Of course, statistically, this could happen on the first flight.

Also remember that you still have the 50 hour failure rate on the 
batteries.  The difference is that they both have to fail simultaneously 
  for a total failure.  Adding more batteries, extra alternators all 
adds up to increased frequency of parts replacement.  They all have a 
mean time to failure.  If they are added to increase redundancy that's 
good, but the maintenance does not go down, it goes up.  Add a third 100 
hour battery, and you'll be replacing a battery every 33 hours.

David Scharfenberg wrote:
> Hi Terry,
> 
> 
> Glad to hear that the building is going well. I guess in the beginning I 
> was concerned about the electronic ignition, and wanted to keep a fully 
> charged battery just for it in case of an alternator failure.
> 
> Dave
> 
> 
> 
> On Aug 24, 2005, at 7:49 AM, Terry Miles wrote:
> 
>     Hi Dave,
>      
>     Good to hear from you.  Thanks for the explanation.  That is my
>     first inclination on this wiring setup too--separate relays. But the
>     literature on these Marine Switching units sound pretty slick.   But
>     let me ask--if I don't sound too dumb--why (in the case of
>     alternator inop) would you isolate your batteries?  Why not just
>     leave them in parallel ops when on batt power only, as opposed to
>     running them down one at a time?  Except to do a preflight separate
>     voltage check, I see am inclined to see them as one big batt.  A big
>     part of the why of dual batts for me is wgt and bal. 
>      
>     For wiring simplicity, I am moving away from dual/multiple bus
>     configs in event of alternator failure, and plan on a manual
>     download myself of on/off switches and deliberately grouped CBs for
>     fast load shed.  Only exception might be a hot battery bus on batt
>     #2 alone with a feed an EFIS that (by manufacturer design) will
>     accept three separate power sources and automatically grabs the
>     highest Voltage sense. 
>      
>     Seems like I would have to be asleep a long time for the Alternator
>     to go out, have the low volt warning illuminated, and then run the
>     primary batt down, and have things go dark on you before I woke up
>     and switched to a "standby" battery.  This paragraph is the only
>     reason I can come up with to isolate dual batts in flight.  ...but I
>     am a willing student.  Lightening strike? 
>      
>     Thanks for your thoughts!
>      
>     Big day down here today.  We are planning to put the top on.  We had
>     2 heat guns on the top flange of the firewall bulkhead due top
>     flange was angled up and seemed to be the high point.  While it was
>     still hot and malleable we dropped the top (still just dry fit) in
>     hopes of bending it back flatter.  It helped some.  Two noses lined
>     up great.  No big problems with sidewall line up.  The dust will fly
>     today!
>      
>     Terry
> 
>         -----Original Message-----
>         *From:* reflector-bounces at tvbf.org
>         [mailto:reflector-bounces at tvbf.org] *On Behalf Of *David
>         Scharfenberg
>         *Sent:* Tuesday, August 23, 2005 11:46 PM
>         *To:* Velocity Aircraft Owners and Builders list
>         *Subject:* Re: REFLECTOR: POWER GUARD
> 
> 
>         On Aug 22, 2005, at 8:52 PM, Terry Miles wrote:
> 
> 
>             Does anybody else out there have a dual batt, single alt
>             configuration?
>              
>             Regards,
>             Terry
> 
> 
>         Terry,
> 
>         I have two 17 amp/hr sealed batteries and one alternator. Each
>         battery has it's own relay, and switch on the panel. If I get a
>         low volt light (which comes on within seconds of the alternator
>         going off line), I can simply switch one battery off if I want
>         to isolate it. That way both batteries get full charging
>         voltage, as opposed to charging through a diode with it's
>         resultant voltage drop.
> 
>         Dave Scharfenberg
>         std/rg
> 
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