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76.ta
12-12-2005, 03:27 AM
Finished my extra Stiffy yak the other day and was waiting for a break in the weather to fly it. Take off was about ten feet. I added no trim whatsoever. After about thirty seconds of flying, the motor cut out and I found myself in a bad way but managed to bring it in with no damage. I took the battery (s) and speed control from another plane and never had any problem with them. Here is the information and I would appreciate any suggestions to cure this problem. The motor (new) was not at full throttle when it cut out. I was trying to do knife edge so I was putting in some rudder.

KOKAM batteries, 15 C discharge, 3 cell light poly…first one I tried 1500 mah, second one—2000 mah.

Hacker (http://www.rcuniverse.com/buynow/keywordclick.cfm?bid_id=172) A20 20L, outrunner, 11x4.7 prop, astroflight meter said max amps pulled was 21 at around 200 watts.

4 h-55 servos (I checked for binding)

Phoenix speed control 25 with the following settings:

9 volt cutoff
moderate overcurrent threshold
soft brake no delay
auto calibrated throttle
standard advanced timing
hard cutoff
soft start

As soon as I got to the plane I felt the motor, speed control and battery and none of them was past slightly warm. When I got home, I recharged the batteries and held the plane while I played with the throttle and hit my snap roll button and ever so often the motor would shut off again but the motor, speed control or battery never got past a slight warm feeling..
It was kind of random.
I would advance the throttle and it would cut off or I would be moving the control surfaces one or two at a time and advance the throttle and it would cut off. Also, I could let the motor run at full throttle on a fully charged battery and after less than a minute, the motor would cut off without moving a control surface, which is definately not right.
Thanks,
Roy
H.P.Pilots
Any advice would be welcome.

Spaceclam
12-13-2005, 01:49 AM
simple. the kokam batteries, although quaility cells, drop their voltage under load a lot more than other brands of batteries, the 2000mah kokams are famous for that, but the 1500s are not so bad. with the smaller battery, your maximum pull of 21A comes close to the 15c limit of 22a, and the cells can drop to as much as 2.4v/cell under a heavy load, which is 7.2v for a 3 cell pack. so when you pull that much from your battery, the voltage drops and your ESC thinks your battery is dead. with the larger battery, you are encountering the same thing. although you are not as close to your maximum current limit, the 2000 mah cells drop considerably more than the 1500 mah cells. i will bet that if you took the 1500mah pack and flew it around at half throttle it would not cut off on you like that.

76.ta
12-13-2005, 01:57 AM
Spaceclam,
I came to the same idea today as I watched my meter .The cells, drop their voltage under load and the ESC thinks the battery is dead. What kind and size Batt would you use? I Really don't Like the idea of the motor cutting off but I don't want a real heavy batt.
Thanks,
Roy

Spaceclam
12-13-2005, 02:04 AM
then dont use kokams. my best experiances have come from thunderpower and polyquest. the rule of thumb i go by is to set your maximum current draw to no higher than 80% of the maximum discharge rate. use a non-kokam cell, follow the rule and find a battery with the that will give you the flight time you want. i would say a 2000 mah battery would be nice. that's 6 min at full power, and most cells now are 20c.