Newsgroups: rec.models.rc
Subject: 5 cell Rx nicads (6 V) (was Re: <none>)
From: kaiser@elec.canterbury.ac.nz (Chris Kaiser)
Date: 26 Aug 92 14:54:08 +1200
Distribution: world
Organization: Electrical Engineering, University of Canterbury, New Zealand

Sorry about the previous lack of a subject, it was the first time I'd initiated a
posting with this News reader and by the time I realised where to enter it, it was
too late :-(

In article <1992Aug25.210421.16649@rtsg.mot.com>, svoboda@rtsg.mot.com (David Svoboda) writes:
> In article <1992Aug25.095210.486@csc.canterbury.ac.nz> kaiser@elec.canterbury.ac.nz (Chris Kaiser) writes:
> |Does anyone out there have any experience and/or comments about using 5 cell
> |nicad packs (i.e. nominally 6 V) with RC gear. The system I'm looking at using
> |it with comprises a Futaba R115 (I think - 5 channel FM) Rx and S143 micro
> |servos. I've been told by a Futaba NZ rep that 5 cell packs are bad news and
> |cause premature failure of components, however this guy had zero technical 
> |knowledge (you should have heard his explaination of why PCM was better than
> |FM which was better than AM :-) so I'm not sure how valid this is. The only
> |problem I can see is a higher current when a servo stalls, which may damage
> |it. Presumably the Rx has an internal voltage regulator, so it wouldn't be
> |affected by the higher voltage.
> 
> I use a five cell pack in my current airplanes.  I like the quicker servo 
> response, and that the servos seem to have more power.  I have had no problems.  

The servos *will* have more power! The higher voltage means higher current draw
by the servo motor, which gives more torque and a faster response.

> I got the idea from some pattern guys (the Dave Patrick crowd) who all use five
> cell packs.  They told me, however, that you should only use it on a PCM
> radio, and that it throws the receiver off on an FM.  I consider this to
> be balderdash, knowing that a PCM is just an FM with a signalling method, but

Yeah, if they were right and the battery voltage affected the FM frequency, then
even with a 4 cell pack the Rx would drift off as the battery voltage dropped. I'm
99% sure that Rx's have an internal voltage regulator, so the extra cell won't have
a noticable effect on them.

> I use PCM, so have not been able to test the theory.  (That particular pattern
> crowd has been wrong before--they are an empirical crowd which understands 
> little theory.)   Oh, my main radio is a Futaba 7UAP with the nine channel 
> PCM receiver.

I also have the Futaba 7UAP and have 2 PCM and 1 FM Rx's. I'll be using the 5 cell
pack with the FM one - I'll post if the plane "crash 'n burns" due to the extra
cell:-) Actually, it's more likely to burn literally with the 4 cell pack - the voltage
from a 4 cell nicad pack is marginal for latching on the relay in the electronic on-off
switch in my electric glider. If the relay doesn't latch then the soft start MOSFET takes
the motor current continuously and gets rather hot! This happened to me during a ground
test (luckily) - the MOSFET failed by short circuiting and I had to disconnect the motor
battery quickly to prevent the model catching fire (not easy since I also had to hold the
model and watch out for the prop). I shudder to think of the results if this happened in
the air...
 
> A downside of five cell packs with PCM is that you lose the ability of the
> receiver to tell when the voltage is about to fail and lower the throttle.
> Since the "knee" is at a higher level, by the time the receiver detects
> a low voltage, you have very few seconds of useable power left.
> 
> 		Dave Svoboda, Palatine, IL

Thanks for the info. The only problem I've been informed of so far (thanks to
Mike Goza) is that carbon tends to build up more quickly on the servo pot contacts
due to the extra voltage. Nothing that can't be cured by a careful application of a
little rubbing alcohol.

Chris Kaiser

Postgrad, Elec. Eng. Dept.
University of Canterbury
Christchurch, NEW ZEALAND

Newsgroups: rec.models.rc