I must charge up my brain before discharging my mouth. :-)
Battery University had some interesting text about memory effects.
If your NiCd battery likes an occasional deep discharge (don't we all?) then perhaps your controller ought to apply one. In normal operation it would trickle charge in the day ready for night duty, but occasionally it might leave the lights on until sunrise. At which point it starts a full charge again.
IIRC, lead cells do not mind being kept topped up all the time, as they can become 'sulphated' if left discharged for a long time. They don't mind a deep discharge either.
NiCd cells are rated to about 500 or 1000 cycles, hence the garden lamp yearly cell replacement.
Lead cells can last for ages, as you can see from the many old jalopies driving around.
I'm sure the British Antarctic Survey did the comparisons for their monitoring stations, and chose lead acid cells as being the least maintenance and longest lasting. They are simple and adequate.
Other cells strive for higher energy to mass ratios, but you pay for that in fussier management. Lithium-ion cells have very high energy density, and very fussy management. A tenth of a volt overcharge can ruin them. I have some ordinary rechargeable lithium cells that I bought to replace non-rechargeable ones in a LED torch. I accidentally discharged them fully (pulling the LED off an LM3909 flasher does not reduce current to zero). To see if this was okay, I checked the data sheets and found that although they last for many shallow discharge cycles, they are only rated to about ten (!) full discharges.
So rough rule of thumb is that the more recent the cell technology, the more care they need to be managed.
I'm guessing you don't need the mass savings, higher price, or extra management requirements.
Besides, I can go into my local electronics store (Maplins), buy a cheap 12V lead-acid gel cell, and a solar panel specifically for keeping car batteries topped up, and a PIR motion detector. That's half the job done. The PIR thing runs off a 9V battery, so maybe buy an LM7809 regulator too. Get the PIR to trigger a 555, which then drives a power tranny or relay to switch on the lights. Oh and use an ORP12 photocell so it can tell night from day.
Was the A of E text I emailed you any help?
K.