Trickle-chargers kill Nicads! The only way to ensure full charge is to leave it on for 14 hours (and if you forget to unplug it, maybe several days!). Consistant overcharging causes 'memory' effect, and can permanently damage the battery. The solution to this problem is to use a peak-detecting charger, which cuts off automatically when the battery is fully charged. Without the danger of overcharging, much higher charge rates can be used. Full charge can be safely achieved in less than an hour.
Unfortunately, some peak-detect chargers will not work through the transmitter's charging jack. Inside the transmitter there is a diode in series with the jack, which allows charging current to flow into the battery, but not out (this prevents damage due to shorting out the jack, or using incorrect polarity). Modern peak-detect chargers usually read battery voltage during short pauses in the charge cycle (for greater accuracy) but the diode doesn't allow this. Also, the 0.7V forward voltage drop across the diode might fool some chargers into thinking that battery voltage is too high.
To make your transmitter compatible with peak-detect chargers, you just need to bypass the diode. However, this also bypasses the protection features. Therefore, I suggest wiring a 2A fuse across the diode. Don't try to charge the battery at more than 1.5A, or the fuse might blow. I recommend keeping the current to 1A or less (at this rate, a 600mAh battery should peak in about 40 minutes).