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<DIV><FONT face=Arial size=2>Ron:</FONT></DIV>
<DIV><FONT face=Arial size=2>It's been a few years, but I've done tests with
NiCad packs, taking a 4 cell pack and measuring the current dump into it from a
5 cell pack, in order to simulate what happens when you have a 1 cell short in
one 5 cell pack in parallel with another 5 cell pack (no regs, no diodes).
As you say, the situation is very tolerable. For the typical pack sizes
we'll use in a pattern bird, and depending on the state of charge in both packs
at the time that the short happens, you can expect to see something on the order
of 100 ma flow at the beginning. Could be as high as 120, might not even
hit 80, it will depend on these and some other factors (wire & switch
resistance, load imposed by the flight pack etc). </FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>What you then see is that the failed pack starts to
come up in terminal voltage fairly quickly, causing the "charging" current to
ramp down. Over several minutes it will typically drop down to around 40
to 50 ma. It will again depend on the flight pack loads, state of
charge etc. how much it comes down, but the key point is that it does not
represent any kind of catastrophic current drain from the good pack to the bad
one. The extra load is roughly equivilent to wiggling the throttle
servo a whole bunch more than you should. The trick to survival is
checking both packs under load before each flight. You will probably not
notice the failed cell otherwise. You might go all day with no harm, you
might, maybe plant it if you have the failure occur early in the day, fly a
bunch, not check it and drain the good pack at a slightly higher than normal
rate all day. I bet you already knew this, but I figured it might be worth
mention to anyone thinking about trying this. I've used it very
successfully for years on the bigger birds. Using regs again though, just
for the consistent response all day long.</FONT></DIV>
<DIV><FONT face=Arial size=2>Ed</FONT></DIV>
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<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: black"><B>From:</B>
<A title=ronlock@comcast.net
href="mailto:ronlock@comcast.net">ronlock@comcast.net</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A title=discussion@nsrca.org
href="mailto:discussion@nsrca.org">discussion@nsrca.org</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Monday, January 24, 2005 1:39
PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: any merit in running dual
battery packs ?</DIV>
<DIV><BR></DIV>
<DIV>I wanted redundancy in the power system for my pattern birds, but without
ANY new potential points of failure. My confinguration is:</DIV>
<DIV>Two 700mah 5 cell batt packs. Each pack direct to a
switch, direct to Receiver.</DIV>
<DIV>No regulators, diodes, etc. </DIV>
<DIV>I'm more concerned about connector, switch, and solder
joint failures in the pack, than failure of a battery cell. This
config gives me redundancy in those areas.</DIV>
<DIV>When things go normally, I have 1400mah available with only a slight
weight gain over a single larger pack, switch & wiring.</DIV>
<DIV> </DIV>
<DIV>I understand cells more often fail open, than short. But in
case of a short, (the worst case for this config) the good pack must fly
the plane, and charge the "bad" pack for duration of the
flight. After research & discussion with others, it's my belief
the good pack will tolerate the load of charging the bad one (which won't
be at a very high rate) and finishing a flight. </DIV>
<DIV> </DIV>
<DIV>A potential failure point is me. (darn humans!)
My duties as crew chief include checking both batts before every flight,
and turning on both before flight.</DIV>
<DIV> </DIV>
<DIV>Later, Ron Lockhart</DIV>
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Original message -------------- <BR>
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<DIV>
<DIV>In a message dated 1/24/2005 7:19:07 AM Eastern Standard Time,
hitesh@salt.ac.za writes:</DIV>
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style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: blue 2px solid"><FONT
style="BACKGROUND-COLOR: transparent" face=Arial color=#000000 size=2>
<DIV><FONT face=Arial color=#0000ff size=2><SPAN
class=095360912-24012005>Hi,</SPAN></FONT></DIV>
<DIV><FONT face=Arial color=#0000ff size=2><SPAN
class=095360912-24012005></SPAN></FONT> </DIV>
<DIV><FONT face=Arial color=#0000ff size=2><SPAN
class=095360912-24012005>Now that we all using high powered digital
servo's with incredible holding power etc - is there any merit in running
2 battery packs, say 1 Ah each as opposed to 1 high capacity
pack thereby eliminating the single point failure ? If I did want to
run 2 packs, is a diode necessary to prevent 1 pack from possibly charging
the other if 1 pack were to go bad ?</SPAN></FONT></DIV>
<DIV><FONT face=Arial color=#0000ff size=2><SPAN
class=095360912-24012005></SPAN></FONT> </DIV>
<DIV><FONT face=Arial color=#0000ff size=2><SPAN
class=095360912-24012005>Cheers,</SPAN></FONT></DIV>
<DIV><FONT face=Arial color=#0000ff size=2><SPAN
class=095360912-24012005>Hitesh</SPAN></FONT></DIV></FONT></BLOCKQUOTE></DIV>
<DIV> </DIV>
<DIV>Hitesh, a 1500 to 2000ma battery is all you need. There's little
advantage to redundancy unless you are planning to fly more than 5-7 flights
(res) on any given day. But then again, I use NiMH on the airborne and only
Sanyo packs, which have proven extremely reliable for several years now.
As such, a redundant power source isn't really necessary in pattern
models and you can avoid some unnecessary weight build up. </DIV>
<DIV> </DIV>
<DIV>What has been done in large models is to add another battery
and switch harness to an unused channel on the receiver, which
provides adequate redundancy. The extra weight on such models isn't as
critical as it is in pattern models. Both switches "on" at take-off
please.</DIV>
<DIV> </DIV>
<DIV>Of course, if you just have to have the extra battery and you have
enough weight margin and room, the above is one way to do it. There are
other ways, but this is simple and reliable.</DIV>
<DIV> </DIV>
<DIV>MattK</DIV></FONT></BLOCKQUOTE></BLOCKQUOTE></BODY></HTML>