[NSRCA-discussion] Lipo Stuff...

[David Lockhart]

In view of the recent discussions on lipos and chargers, I thought I've
offer a couple thoughts on some basics I have learned in the past 3 years
with many foamies, about 75 flights on a 40 sized modified MK Mattlas built
for electric pattern, 28 flights this past winter on an electric Abbra, and
many hours of reading, discussing, and emailing with people that have
demonstrated knowledge about electric power systems.  My opinions only -
your mileage may vary.

For Lipos, ESCs, chargers, balancers, and motors - read the instructions -
most are available online and most contain the information that if followed
would prevent 99% of the lipo problems I've read about and personally seen.
With specific regard to the Astro 109, it uses a different charging
algorithm than the Schulze, Orbit, Triton, or TP chargers - some special
"rules" apply to the 109 (instructions available online).  The online forums
contain a ton of information about darn near every charger (including the
109), balancer, and lipo on the market - read carefully and consider the
source.  There is also some excellent info in the Castle Creations Scribe
(at the Castle web site).

Get a good wattmeter and a good temp gun and use them, and understand what
the numbers mean.  It is very important to match the lipo, ESC, motor, and
prop as a complete system.  The lipo is obviously potentially the most
dangerous when abused, so size it the most conservatively.  Anytime you make
a change in the ESC settings, prop, mounting, or cooling, check the setup
with the wattmeter to see what the changes are, and conservatively fly the
setup and then check it again (wattmeter and temp) to see if/how much change
happened in the air.  Do not expect anything with electric power systems to
be linear - they rarely are - ie, an ambient 10F temp increase does not mean
the lipos, ESC, and motor will all run 10F warmer - they might actually run
cooler or substantially warmer.  Different props can be varied degrees of
stalled on the ground - amps consumed in the air do not always correlate
with amps on the ground.

Consider the possibility that lipo "C" ratings just might be influenced a
bit for marketing purposes, and many evaluations/ratings are based on lab
conditions which may have little or no bearing on conditions the lipo will
see in flight (which varies substantially in different planes and different
pilots).  Ambient temperature has huge effect on the voltage under load for
lipos - and the lipos can deliver substantially more amps and volts in warm
weather.  I am interested in long life from my lipos, and am going to follow
a couple basic ideas -

Discharge rate - ignore the burst ratings completely.  "Test" the continuous
discharge rating for accuracy - start with small props and increase prop
size until you are down to 3.5volts per cell - do the math and figure out
the C rate if you really need to know.  If you are not happy with the
available power, get a bigger battery, a more efficient motor/ESC/prop, or a
lighter plane.  Keep the discharge to 80% or less - determine this by
monitoring how much charge goes back into the lipo (ie, you don't want to
see more than 4000 mah going back into a 5000 mah lipo).  After a brief
period of rest after a flight, you should see a minimum of 3.7 volts per
cell, preferably 3.8 volts per cell.  If you are at 3.6 volts per cell or
less, consider decreasing flight time, amp draw, plane weight, or upping
lipo size.  You should never let the lipo go below 3.0 volts per cell (a
good Low voltage cutoff in the ESC won't allow it drop below 3.0 volts per
cell) under load or resting.  If it does, watch that lipo like a hawk and
know that the longevity of the pack has been compromised.

Temp - max power will come from the current lipos between 100 - 120F.  This
winter, I have preheated the lipos to 105F on the dashboard defroster and
picked up substantial power vs 60F.  And the lipos have come down at 95F
after being preheated.  If the lipos are above 130F after a flight, consider
more cooling, less throttle, less prop, lighter plane, shorter flights.  If
they are above 140F, seriously consider getting them cooler and acknowledge
you are likely decreasing the life of the lipo.  If they are above 150F, it
is very likely damaging the lipos - keep in mind, all cells do not see the
same cooling, and measuring the case on the outside cells isn't the whole
picture.  In my experience, heat generated by the lipo increases pretty
dramatically when discharged deeper than 80%, and the ESCs and motors get
hotter as well - I attribute most of the heat increase to reduced efficiency
of the system as the voltage of the lipo drops further into the discharge.
Maintaining high voltage plays a big part in keeping the lipos, ESC, and
motor cool.

I suspect good longevity will be the result if temps are kept below 130F,
discharge is less than 80%, and loaded voltage is 3.5+.  I also strongly
suspect balancing will not be an issue if the pack is properly constructed
of quality cells to begin with.  I've had very few balance issues, and none
when lipos were used within these parameters.  A balancer is not a cure for
a pack that consistency falls out of balance when charged or discharged - it
may prolong the life of the pack, but the weak cell is a sign of a pack with
increased risk and reduced life.

I consider myself a piker who is expanding their knowledge when it comes to
this electric stuff, but hopefully some of the above info is helpful in a
practical sense to current and future lipo users.  I'd be happy to entertain
questions and provide further brainwashing.......with the preface my answer
might be "I dunno....."  <G>

Regards,

Dave Lockhart
DaveL322 at comcast.net
Team JR
Team Castle Creations