LiFe Receiver Batteries for SPA Airplanes
 Part 2 - Modifying the Charge Switch and Generic
LiFe Battery Pack for in-Plane Balance Charging and Checking

Jim Johns

Links to other articles in this series:
Part 1 - Why change to LiFe Receiver Batteries
Part 3 - Building Charge and Test Cables

OK, so you've decided to try a LiFe receiver conversion, but don't want to spend a bunch of money on A123 pack until you're sure you want to commit to this technology. Below I'll show you a simple way to modify any generic LiFe receiver battery pack to allow onboard balance charging and checking.

Modifying Your 3-Wire Charge Switch

If you plan to balance charge in your airplane, you'll need a 3-wire charge switch. Some aftermarket models only use two wires which won't work for this application - you need 3 wires. You'll either need to modify your own switch or purchase one. NoBS Batteries and Electrodymanics sell both the modified switch and the charge cable if you'd rather go that way.

If you decide to build your own, go to this webpage - http://hangtimes.com/a123rxsetup.html - and scroll down to this article *Basic Combo Switch for Single Lead Balance Charging. It will show you exactly how to modify your existing 3-wire switch. This shouldn't take more than 5 minutes, as getting at the connector plugged into the receiver will be the hardest part if it's mounted in the plane. Just lift up the plastic tongue holding the signal lead - ORANGE on JR or WHITE on Futaba - and pull out that wire. Fold it back and insulate it with electrical tape or heat shrink tubing as shown.

Here's on of my switches for reference.

  • The upper photo shows an unmodified ED Ultra II Charge Switch I bought at Perry in 2015.
  • The lower photo shows just the lead that plugs into the receiver of the same switch before and after modification. The stock configuration is on the left and the modified switch with the signal wire removed from the connector, folded back on itself and covered with heat shrink tubing is on the right.

Sources:

Here's the standard charge switch if you want to make your own:
http://www.electrodynam.com/rc/usw/index.shtml

If you'd rather not make your own, here are two places you can purchase them.

NOTE: ED usually has a booth at the Perry, GA Southeastern Model Show in March and prices are lower there than shown on the website.

Modifying a Generic LiFe Battery Pack
Let's modify a generic 2S LiFe receiver pack, such as the 1600 mah Cheetah pack shown here, to allow balance charging and testing of each cell through the charge switch.

These are the supplies you'll need.

  • LiFe receiver battery pack.
  • 6" servo extension - style or brand doesn't matter.
  • Heat shrink tubing.
Here I've removed one wire from the male end of the servo extension which is the end that would plug into your receiver. Refer to the section called "*Basic Combo Solution Switch Mod for Single Lead Balance Charging" on the web link below to see how to remove this wire from the connector.
http://hangtimes.com/a123rxsetup.html.

Separate this wire from the rest of the extension - untwist it in this case - and clip it off as close as possible to the other connector.
This is a close up of the connector end of the wire. Each side is different, so notice the configuration.
Here's the additional wire being inserted into the battery pack connector. Note that the top side shown in the photo immediately above is UP in this photo. The wire won't lock into the connector properly if it isn't. Press it all the way in until it clicks. Give it a gentle tug to be sure it's secure. If not, repeat until it is.

You can see the added twisted around the original red and black wires at the top of this photo.

I've removed the CENTER connector of the small white JST XH balance plug. This is a little more difficult than removing the wire from the servo extension because you have to PRESS DOWN on the pin itself to be able to remove it from the plug. The ONLY reason for doing this is to allow me to put heat shrink tubing around the wire for insulation. You can skip this step if you'd rather not tackle it and use electrical tape. It won't be as pretty, but it will work fine.

Regardless of color, the center wire of this balance plug is always connected between the two cells of a 2S pack. This allows for balance charging and measuring each individual cell of the pack.
I've stripped back the insulation just a bit to expose the bare wire.
The added wire is stripped, wrapped around the balance wire and soldered. Cover with a piece of heat shrink tubing, shrink it, then reinstall the connector into the plug. Be cautious of orientation or it won't lock in place.
Here's the completed pack after modification. Notice the green heat shrink tubing over the solder joint. This pack is going into my psychodelic green, pink and black Daddy Rabbit.
Time for the acid test - does it work or not.

This is how you'd normally check a LiFe battery, with the JST XH balance plug connected to my battery checker. The meter is reading the voltage of the first cell connected to the negative lead of the pack - note the small "1S" just above the 3 in the voltage.

Note that the voltage is ~3.3V which is the nominal storage voltage for LiFe batteries whne not in use.
Now to see if our mod worked. Here the battery checker is connected to the receiver output of the battery pack which will be connected to the charge switch. The meter shows the voltage of the second cell only - the one directly connected to the red lead - note the small "2S" just above the 3 in the voltage.