The thing that kills NiMH cells is overcharging, ie charging after the voltage has peaked. Do that a few times and your total charge cycle life has reduced noticably.
Sadly, even a top line transmitter such as the DSX9 does not appear to have any smarts in the charging circuit, the manual just says 'charge for 16 hours'. This of course does not account for the battery only being partially discharged, thus overcharging it on a regular basis. (later ed: Since I wrote that, the Hitec Aurora 9 and Spektrum DX8 have come on the market with intelligent onboard charging)
My suggestion is to use your nice computerized charger (that you have anyway for your LiPo flight packs) that can properly peak detect for the charging. However, trying to do this via the socket in the side of the radio does not work. I tried it on my DX6i and could not get a connection that the charger was happy with, and have been advised it does not work on the DSX9 so I am not even going to try.
The problem is that the plug on the JR pack is made to fit the circuit board socket and nothing I have comes even close to fitting it. Besides that, repeated unplugging of that socket will cause the solder joints on the circuit baord to fail, after a time. Of course, they will fail intermittantly, when you are flying!
To prevent this and change to a plug that my charger is already set up for, I decided to butcher a servo extention and use those plugs to facilitate proper charging.
- The completed modification of the packs lead. I used a 10cm servo extender but did cut the leads so that when it is plugged together it is the same length from pack to plug as it originally was.
If it became longer it would be harder to fit the folded wire into the compartment.
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- Plugged in and inserted into the radio. The servo plug lies very neatly
sideways like this so one has to make sure that the bit of wire left on the
pack is long enough to allow this position. For a few seconds I
contemplated some sort of rigid mounting for one or both halves of the
socket so that the pack would 'plug in' but quickly realized that while
possible it is just an extra fiddle to do.
- Here is a view of the charge in progress. The charge rate is 250mA simply
because that is what I use for my other NiMH packs. Once it detects the
peak it will fall back to about 25mA trickle which will not harm the pack.
I normally unplug as soon as the charger beeps when it peak detects.