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Your Reviewer

My Credentials
So who's doing this reviewing then?

Well I've been building and flying or driving radio controlled models for over 40 years and during that time I like to think I've built up a reasonable amount of knowledge.

I'm also a qualified electronics engineer who has worked in radio frequency, analog, digital systems and software for more than three decades. In fact I designed and built my first RC set back in 1969.

For the past nine years I've also been involved in the design and manufacture of some rather sophisticated engine technology and UAV flight control systems.

So, chances are I've been there, done that and have a huge pile of tee shirts to prove it.

Right now I'm heavily into 3D flying and enjoy all aspects of the RC hobby. I may be old but I don't feel it.

In the Pipeline

Here's just a little bit of what's to come on this site...

RC explained: Demystifying terms such as PCM, PPM dual conversion, single conversion, full-range etc., this feature will explain it all.

Cheap Chinese Engines: Just how good are those cheap Chinese glow and gas engines that sell for half the price of their "brand-name" equivalent? I put several to the test.

Build your own radio gear?: Back in the old days, building your own RC gear was not uncommon and now the arrival of 2.4GHz has made it practical again.

 

A DIY $8 Lithium Transmitter Battery (Part 4)

INSTALLING THE RESISTOR

Now comes the tricky part, installing the resistor that fools the transmitter into thinking that our new 2-cell lithium battery is actually an 8-cell NiMH one.

You will need an 8.2K Ohm resistor -- the smaller the better. Almost any electronic hobby/component shop will have these and they cost just cents a piece.

If you can get a surface-mount type the same size as the 5.1K unit already in place then it's possible to simply piggy-back the extra resistor directly across the existing one. However, it's more likely you'll end up with a resistor like the one pictured below:

Note how the ends of the wire are bent so that they contact the tinned areas of the copper circuit board we prepared previously. Be sure to generously tin the resistor wires before attempting to solder it in place.

To install the resistor it's probably easier to solder the bottom wire to the copper first -- since that copper area is larger than the top one.

Once the bottom leg is attached you can adjust the position of the top leg until it sits neatly atop the other area of tinned circuit board you prepared previously. A quick application of the soldering iron and the solder already on the resistor's wires should melt onto the tinned area of the circuit board, providing a good electrical and physical connection.

You can see in the picture above that I also slid a piece of tape under the resistor so that it won't touch the actual circuit board once we reassemble the transmitter.

Again, you should inspect your soldering with a magnifier to make sure that you haven't accidentally got solder shorting to any nearby components or bits of the circuit-board.

Now you can put the back of the transmitter on again -- we're done poking around inside!

Now put the new battery back in the battery compartment and stuff some foam at each end to stop it moving about. You'll notice in the picture above that there's a spare lead and connector floating around. I use this for charging.

When you turn your newly modified transmitter on now, instead of beeping and displaying a voltage in the mid 7's, it should show around 10V or so, as in the picture below:

This shows that our modification is working. Notice that it no longer beeps either!

With this modification, the low-voltage alarm will kick in when the lithium pack gets down to about 6.6V. That gives more than enough time for landing safely and is high enough that the low-voltage protection built into the battery won't kick in and remove all power.

Charging

There is a diode in the transmitter which protects the battery from being accidentally reverse-charged. Unfortunately, this diode also prevents a regular charger from working with lithium pack so it's recommended that you charge the pack by plugging directly into your charger.

Note the extra lead on the pack I built -- this is a standard servo/battery connector soldered in parallel with the lead that goes to the transmitter itself. I just use this lead for charging.

What charger should you use?

Most of the low-cost 4-button field chargers coming out of China have a Li-Ion setting but you can also use the LiPo setting as well. It pays to limit the charge rate to 0.5C (which is about 1.2A for these cells) for maximum life.

You'll notice that they charge just like a LiPo.. starting out at maximum current but slowly dropping off as they near full-charge.

Field Testing

After fitting my own freshly charged 2S lithium ion pack and making the above modification, I turned my 9X on at 9am in the morning and left it running with the FlySky 2.4GHz module installed. The voltage display initially showed 10.2V but within an hour it had fallen to 10.0V.

Three hours later, it had only dropped to 9.9V (remember that the low-voltage alarm doesn't kick in until 8.8V) and I went out flying for two hours.

Despite leaving the transmitter on the whole time, the voltage still read 9.9V at the end of the day.

When I used 600mAH NiCads in my 9X, I was lucky to get two hours of use before needing to recharge. With the new setup, it would seem that I'll possibly four or five times that at least.

Because the internal circuitry of the 9X is still working at 5V, the range and other aspects of operation have been unchanged by this modification. Just a note though... if you do install a 2S lithium battery, you won't be able to use a regular FM module in your transmitter. The FM modules really do need the full 9.6V that an 8-cell pack produces.

One thing I have noticed already is that the Corona 2.4GHz module which used to run quite warm with an 8-cell pack, now runs cool. That's mainly because the voltage regulator inside isn't having to waste anywhere near as much power in the form of heat.

Summary

This has proven to be a very worthwhile project.

If you're considering doing the same but are a little daunted by the prospect of soldering that resistor onto the circuit-board, find someone who has more experience and the right soldering iron to do it for you. Most clubs have someone with a background in electronics who can probably help.

I hope you have as much fun and enjoy the results of this project as much as I have. If you have any questions or require clarification of any step, feel free to contact me.

And, if there's a demand, I'll look at creating similar project pages for other popular transmitter types that will allow them to also use these excellent value lithium batteries instead of an 8-cell pack.

And of course, if you feel moved to donate to the continued operation of this website (and more DIY projects) then please feel free to use the "Donate" button below :-)

If you found this information useful then please consider making a small donation towards the operation of this website.

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The Blog

Updated: 20 Sep 2012
Here's a blog that will keep you informed just what's going on behind the scenes at RC Model Reviews and also tells you a little more about myself.

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