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<DIV><FONT face=Arial size=2>Mike:<BR>Unless the batteries were put away in a
fairly low state of charge or unless they were connected to some accessory that
slowly drained them, they should not have self discharged so much as to damage
themselves just over winter. Were they connected to any type of voltage
regulator / failsafe switch setup for example? Some, not all, exhibit
enough OFF leakage current to completely drain a pack that size within a couple
of months. Others could drain it a fair amount over several
months. Depending on brand/model of a failsafe type regulator, you could
see anywhere from 30 or so micro amps (uA) to 7.5mA flow between
battery and regulator (NOT the receiver side) when the regulator is off.
Something like a Jaccio Smartswitch performs pretty well over extended OFF
times, but multi-months could result in a noticeable drain, combined with the
packs own self discharge. I would say that if you had the batteries
disconnected and left fully charged, they should have been OK and if not, they
are suspect. Odd coincidence they would both drain so completely
though.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Before tossing them, it might be a good idea to see
if something in the charging circuit is to blame. Assuming that the cutoff
levels are set right, a poor connection between the battery and the voltage
sensing circuitry in the charger could "trick" it into an early cutoff during
the discharge portion of the cycling. There is some resistance in every wire and
connection point and whenever current is flowing in either direction
(charge/discharge), there is some voltage drop across that path. Ideally,
there would be no drop for the most accurate sensing of the actual battery
voltage, but for example, assume some dirty/worn connector contacts and some
long charger leads and you wind up with a combined resistance of 1
ohm. At the discharge rate of 650 mA, the voltage drop along that path is
0.65V, so assuming a 4 cell pack, you "lost" about 0.163V per cell of cutoff
voltage, i.e., the discharge would have ended early. The higher the
discharge rate, the worse this gets - at 1A, the 1 ohm assumed lead/connector
resistance would drop 1V, hence .25V / cell in a 4 cell pack. The
fact that it seems to have improved some when you lowered the cutoff voltage
from 1.1 to 1.0 / cell at the same time that you increased the discharge rate
could make sense. For my example of 1 ohm total R in the leads,
0.25V / cell - 0.163V / cell yields 0.087V. Subtract that from the
0.1V delta in the cutoff voltage you set and you effectively had the discharger
cycle seeing 0.013 voltage lower per cell, on average. </FONT></DIV>
<DIV><FONT face=Arial size=2></FONT><FONT face=Arial size=2></FONT><FONT
face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>You should change only one factor at a time to get
a better picture of what's going on. If you keep the discharge rate at 65A and
try it again at 1.0V / cell cutoff, you should expect even better than you saw
at the 1A discharge rate. BTW, other factors vary - higher discharge rates
also affect the real output of the pack. Also don't forget that there's also
internal cell resistance that adds into this, just as the wire/connector
resistance does. Some NiMh cells have a fairly high internal resistance and
won't perform well on higher discharge rate capacity checks.
Net-net: The closer you run the disharge cycle to the average current use
of the pack in flight, the more accurate your results. I wouldn't go any
higher than 250 mA if you want to closely approximate what happens on a per
flight basis in a typical pattern bird. The shorter the charger leads and
the better the contact is in the connectors, the more accurate the results as
well. BTW, IF you do have the high resistance problem in the charger
leads and IF you are also charging at a fairly high rate, you are also losing
something in perceived capacity. Early cutoff can happen here as
well.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>I hope this helps.</FONT></DIV>
<DIV><FONT face=Arial size=2>Ed</FONT></DIV>
<BLOCKQUOTE
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: black"><B>From:</B>
<A title=precisionaero@comcast.net
href="mailto:precisionaero@comcast.net">Michael Cohen</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A
title=nsrca-discussion@lists.nsrca.org
href="mailto:nsrca-discussion@lists.nsrca.org">NSRCA Mailing List</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Thursday, February 23, 2006 9:48
PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> [NSRCA-discussion] NiMH
Question</DIV>
<DIV><BR></DIV>
<DIV><FONT face=Arial size=2>I am cycling my batteries and getting ready for
the flying season. On my initial 5 cycle run at my NiMH batteries, both
my 1650 mAh packs cycled very low. One pack that I have not used since
end of November only cycled up to 850 mAh and the other pack I have not used
since end of September only cycled up to 1000 mAh. My charger is set to
5 cycles, charge and discharge are set to .65 amps with a cutoff voltage at
1.1 Volts per cell. I am now trying 1.00 amp charge/discharge with a 1.0
volt per cell cutoff and it is looking better, but I have some
questions:</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>1) Did I damage the batteries by letting
them self-discharge too low? I know NiMH batteries have a higher self
discharge rate than NiCD batteries, but I am not aware letting the NiMH
batteries run down could cause damage.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>2) Is this a normal behavior? Maybe
NiMH just need a little more juice at the beginning of the
season.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>3) Is the cold garage a
factor?</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Thoughts?</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Mike Cohen</FONT></DIV>
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