Battery/speed controller questions

[Slick]

** Copy of my post on RCOnline**

I'm a newbie to this e-flight stuff so please bear with me.

I just completed my Lil Hornet e-plane. Went with Dave Thacker's equipment suggestions, Pico reciever and servos (little suckers) and a pixie 7P controller, 7 cell 750 Mah NiMH packs. Charged the packs with a Great Planes Electricfly peak charger. Both packs topped out around 9.5 volts.

Question 1: Is 9.5 volts normal for a 7 cell pack??

I ran one pack at 1/4 throttle to break the motor in a little bit. Everything seemed ok even though run time was a little shorter than I expected, figured it will take the packs a while to break in. Later on, while hooking up the servos and testing them out, the occasionally the motor would pulse off and on, at very low throttle. Then other times it would come on and run constantly at low throttle. The only way to stop it is to hold the engine cut-off button on my Hitec Flash4X transmitter.

Question 2: What gives with the motor coming off and on, etc.

Lastly, after testing flying on two packs I got very short run times before BEC cutoff, in the neighborhood of 2 min or so. I immediately put the batteries on my multitester and it read over 8 volts!!! According to the instructions with the Pixie it should be setup for 4.7 v cutoff.

Question #3: Why the early cut-off??

Sorry for going on and on. I'm really happy with the flight characteristics of the Hornet, just need much longer run times.

Thanks for your help.

[groo1887]

My 8 cell NiMH packs peak at around 13 volts. 9.5 volts sound very low for a 7 cell pack. Your charger might have false peaked. Next time feel the battery it should be warm. If its not, charge it again.

It sounds like you might either have a bad connection somewhere in your system. Make sure your making good contact all he way through. It might also be that your radio was glitching during the break in period.

Batteries always have a lower voltage while they are under load. The voltage that the speed control shuts off the motor is usualy reached while the batteries are under load. You measured the batteries while they were not under load and probably got a higher voltage because of that.

[pease1]

Q1) You want to see between 11.5 and 12 volts on a 7 cell pack - at least that's about what I get on my 7 cell NiMh's if they're not warm charge them again. Sometimes new packs false peak.

Q2) Unless you meter the voltage UNDER LOAD you're really not seeing the same voltage that the ESC is seeing. When you put a motor on the battery along with 2 servos and a receiver it's voltage is much different than with just a meter on it.


Q3) Short runs may have to do with the 2 problems above - mostly not getting a full charge. Also - be aware that NiMh packs don't really perform up to their full potential until they have been cycled 2-4 times.

Al

[ 07-17-2001: Message edited by: Allan Wright ]

[Slick]

Thanks for the advice Allan. I think the glitching didn't start until I added a capacitor between the lugs on the motor. Think I will revisit those connections and see if it helps.

As I remember it neither pack was warm when peaked. They may have been slightly above room temp, but not by much. I'll try re-peaking them and see if it helps.

Thanks again.

[GHMBO]

All the above is good advice. I have learned a lot about batteries from the Eveready web page http://data.energizer.com/ You might have a look. There is a lot of false info out there. We are using the batteries beyond the engineers design discharge rates and charge rates.