Warm Ballast, I like that name mate "warm ballast". I'm guessing that's another name for SLF?
Nickle Cadmium (NCD or NiCad) batteries are different to all other battery types, including others that appear similar such as Nickle Metal Hydride (NMH) and Lithium Ion (LI or L-Ion).
I am a bit thin in regards to the actual science behind the reason, however:
The reason that NiCad batteries need to be discharged fully and then recharged fully is because the battery has a tendency to 'remember' its most recent state of charge. This state of 'memory' reduces the batteries capacity to hold charge, a condition that gets worse every time the battery is cycled (charged - discharged - charged) without being fully discharged. I have often witnessed this cumulative effect halving a NiCad batteries holding capacity even though the battery charger indicates it is fully charged.
There are special machines that will cycle NiCad batteries from fully charged to deeply discharged over and over again - over a two or three day period for example - which has the effect of 'breaking down or breaking through' this memory effect and restoring all but a fraction of the original capacity.
As for the actual science, I believe that it is something to do with an incomplete chemical reaction within the battery that accumulates over time, but all of my reading/learning/research on the topic was to do with the effect, its causes and its cures, not the actual science.
NiCad batteries shouldn't be charged if only half discharged, whereas NMH & L-Ion batteries can be 'topped up' when only partially discharged without a problem.
Cheers,
ABX