I have owned a set of the Porter Cable 18V tools for some time that originally came with 18V NiCD battery packs. I've been diligent about running the batteries down all the way before recharging, so I'm still getting good runtime from the original batteries 8 years later.
Umm, no. That does nothing useful. NiCd do not have a memory effect outside of specific NASA testing that caused the exact same discharge level every time, in satellite use with solar recharging no less. What people more often mistakenly report as "me too" memory effect observations are just worn cells that need a discharge and recharge cycle to get them back to the rest of their (now worn) remaining capacity.
If anything it is worse for cells to drain all the way as they get older because the weakest cell in the pack will get closer and closer to being reverse charged, reach 0V and the user keeps using the tool so current through that 0V cells begins to reverse charge it. 2nd common problem was dumb chargers that overcharged and cooked the cells, though obviously you do not have a dumb charger or you'd never have gotten 8 years out of them, unless you barely ever used them.
[qote]However, it seems like PC released the exact same battery pack in a lithium-ion version a few years ago, and those batteries would fit my tools.
So 18V is 18V right? Even if the chemistry is different, the tool itself doesn't know the difference? [/quote]
Yes, the voltage is fine even considering that both battery types reach a higher peak charge voltage which ends up nearly the same. Your confidence and confirmation that they mechanically fit the same is the issue as far as function goes.
Would the factory charger (meant for NiCD) batteries be capable of charging a lithium battery? (I know PC sells a multi-chemistry charger for this purpose, but I'm trying to see if I can "upgrade" my tools to use lithium batteries at the lowest cost possible)
No, you definitely cannot use a NiCd or NiMH charger for Li-Ion. Some Li-Ion generation chargers are backwards compatible but most definitely no older chargers are forwards compatible with Li-Ion.
The two most cost effective options are buy more NiCd packs or buy a new tool *set* including tool, battery(s) and charger. In the short term you could just buy the compatible charger seperately. Sometimes you can find *anything* cheap on ebay or amazon if you are patient, but as far as average pricing goes, by the time you buy batteries and charger separately, you come close enough to the price of a new kit including a tool, to go ahead and get the tool too.
On a side note, what Porter Cable now calls a 20V battery, is the same 18V nominal voltage. They've just marketed it at higher voltage like Dewalt did while both have the same volt range as their older packs marketed as 18V whether Li-Ion or NiCd (or NiMH too for that matter).