Correcting the AKAI RL-KL710 standby light's charging voltage

When the rolling power blackouts started in January 2008, I invested in two standby lights from AKAI (sold by Builders Warehouse). These lights have two fluorescent tubes powered by a 6V 4.5Ah battery. When working as it should, the battery powers two tubes for 4 hours and one tube for 10 hours. Initially, it worked really well, but as time went by, the standby time got shorter and shorter, eventually topping out at only 30 mins on one tube.

After opening it up and checking the charge voltage, it was quickly evident what the problem was. First, the battery was of a seriously low quality, but secondly (and more importantly) the battery was being over charged by a large margin. This simply would not do, so I set about correcting it.

On the right, you can see a part of the charging circuit. The 3.6V zener diode feeds into the base of the S8050 transistor via a resistor-diode pair. The transistor is configured in common-emittor mode, directly charging the battery via the collector.

Unfortunately this configuration charges the battery to 8V, which is far above the manufacturers specification.
Transistor and Zener diode
Turning the PCB over, the CBE pins of the transistor are neatly next to each other. To lower the charge voltage, I could either use a different zener diode (which I didn't have) or use a potentiometer as a voltage divider to accurately tune the voltage. I decided on the latter.
Using a sharp craft knife, cut the track leading to the base of the transistor and carefully clean the track coming from the zener so that it can be tinned and soldered. This will be connected to the one leg of the pot (a trimpot in my case).
I bent the trimpot legs down and tinned them for easy soldering onto the PCB.
Here you can see the trimpot soldered in place. The trimpot acts as a voltage divider between the stable voltage delivered by the zener diode and the emitter. The center-tap is connected to the base of the transistor.
Finally, connect a 1k ohm load to the output (i.e. where the 6V battery will go). Adjust the charge voltage to 6.9V. On a 1k ohm load, this will ensure a minimum 3mA trickle charge current to the battery, which is the internal self-discharge rate of the DELTEC BK6-4.5.

After tuning the output voltage, I connected a new DELTEC 6V 4.5Ah battery to the lamp and discharged it down to 6V. I then plugged the lamp into mains so that it could begin charging the battery. I also hooked up my trusty PicoScope to the bettery to plot the charging curve. After 10 hours of charging, it stablised at around 6.85V.