The old days of worrying about getting your remote controlled airplane shot down by another pilot’s radio because of crossed frequencies is long past. Now, 2.4 ghz digital transmitters allow safe and secure flying as far as your eye can see. Yes, I fly radio controlled planes. My radio is a Spektrum DX-7 transmitter which I have owned since about 2008. The last couple of years I have noticed that the battery charge depletes quickly when used. It is time for a new battery but I don’t know that I want to spend $30 for a NiMH (nickel–metal hydride) battery pack when I can get a lipo (lithium-ion polymer) battery that cost a little less and will hold a charge all summer rather than just a few flights!
So, I want to upgrade to a lipo but my radio isn’t designed to support this type of power source. The DX-7 comes with an internal voltage regulator that is not rated for more than 12 volts. The NiMH battery is 9.6 volts and a 3 cell lipo is 12.6 volts. I could only charge the lipo to 80% capacity putting it at about 11.1 volts but even so, there would be a possibility that I could over-charge the lipo past that 80% and fry my radio. I searched the web and found a solution!
Using an inline voltage regulator, I can charge my lipo battery 100% (12.6 volts) and not worry about damaging my radio. It would reduce the voltage to a safe consistent 10 volts, well within the safe voltage requirements of the DX-7. Stepping down the voltage is accomplished by a simple plug in adapter that goes between the battery and the radio, easily fitting inside the battery compartment. The voltage regulator I found on eBay cost about 10 dollars and will work with most JR/Spektrum radios including my DX-7.
It is pretty straight forward (plug and play really) BUT there is a minor adjustment that must be made to the transmitter. The DX-7 has an alarm that is designed to go off when the battery reaches 9v. This helps remind the pilot that the radio power is low and should be charged before flying anymore. When converting to a lipo (Lithium Polymer), the low voltage alarm will go off at 9 volts as it was designed for the NiMiH pack but the lipo will be ruined if the power drains below 9 volts. The good news is that the DX-7 low battery alarm level can be adjusted (increased) to 10v. This is a good limit and will ensure that your radio will warn you to charge your lipo (which won’t be very often) before the battery gets near a dangerously low level. One important thing to mention here…. Charging the lipo must take place outside the transmitter. Don’t use the charging port on the radio. If you do by mistake, nothing will happen because the voltage regulator should restrict any energy to flow backwards through the diodes……. By the way, when the alarm does go off at 10v , it has been reported that actual pack voltage is 11.4 or 3.8 volts per cell.
Below is a very helpful step by step video by GreenMountainRC that will walk you through the process of changing your alarm level.