UK Heli Log

The convenience of a lipo battery in your transmitter speaks for itself. The problem is many modern transmitters aren’t designed to take the higher voltage that lipos supply. Most transmitters are designed for use with NiCad or NiMh batterys which are 1.45v per cell fully charged, where as lipo batteries are 4.2v per cell fully charged. Stock NiMh TX batteries are 8 cell which equates to 11.6v fully charged by comparison aftermarker TX Lipo batteries are 3 cell which is 12.6v fully charged, that’s a 1.6v difference. Also lipos have a tendency to sit at relatively high voltage for longer unlike NiMh batteries which drop from peak voltage very quickly.

Currently there are plenty of people are using 3 cell TX lipos in there DX7′s and DSX9′s and have had no problems and maybe they never will? I’m guilty of doing this with my DX7 for months now with no problems to date. It’s no secret that the internal circuits of most transmitters is around 3v, which means the TX is using a voltage regulator to stepdown the voltage. Hence the higher the input voltage (from the battery) the more stress you are putting on the TX voltage regulator. So maybe your TX wont go pop immediately but the lifespan of the voltage regulator is probably shortened, either way it doesn’t fill you with confidence.

No one wants to open up their TX’s and mess about with unsoldering and re-soldering PCB components and in the process invalidate the warranty and/or destroy a perfectly functional transmitter. So how about stepping down the lipo battery voltage before it enters the TX…sounds good to me.

The Modification

Diodes have a property call a forward voltage drop, in normal use it’s a side effect rather than something that is specifically capitalized on, however that’s exactly what I’m going to do. By using two 1N5404 rectifier diodes in series (see photo above) on the positive side of the TX battery cable you will effectively have approximately (2 x 0.75v) = 1.5v stepdown. This will bring your lipo down from 12.6 to 11.1 which is a nice healthy voltage for your transmitter and close to what it would expect from a NiMh battery. The 1N5404 is a rectifier diode and designed for power supply/rectification application and is rated for 3A hence it should be fine in this application as the TX is only drawing approximately 300mA (this varies from TX to TX, normally it’s stated in the manual).

One thing to note is the diode stepdown can not be part of your charging circuit otherwise your lipo charger will complain about the voltage or in the worst case you might have a “little” fire on your hands The battery I used was a Outrage XPTX 11.1v 2500mAh (see photo) which has a TX plug, a standard rc connector and a balancer plug (Thunder Power) all pre-wired which turns out to be very handy.

Below you can see the redundant rc connector which is used to charge the lipo hence isolating the stepdown.

I urge anyone who is considering attempting this to do there own research in order to better understand what they are getting into. Here are a couple of links that may help:

http://www.kpsec.freeuk.com/components/diode.htm

http://www.fairchildsemi.com/ds/1N/1N5406.pdf

Summary

Don’t know yet too early to tell…seems fine so far