My guess would be that the original designs charged at between an initial C/2 or C/4 rate (taking 2 to 4 hours to charge) then switched either to a slow trickle rate or stopped charge altogether. The trickle rate (If any) is probably between .01C to .03C where C is the spec'd capacity of the original Apple battery.
When a series parallel configuration is scrapped and a single string substituted, consider the following possible issues, which suggest some measurement and analysis and/or some experimentation:
1. The internal resistance of the single string (of modern cells) might be higher than the original two string series parallel design. This could make a voltage monitoring gas gauge misbehave and seeing more voltage drop earlier in a discharge, declare empty too soon. In an extreme case like using just one AAA string, there might be so much drop that the PowerBook would declare empty immediately. Internal resistance varies between battery cell vendors, and some vendors feature alternative designs within the same physical package dimensions, one with lower resistance but less capacity, and another with higher resistance but increased capacity.
2. One string rather than two would absorb the entire rather than approximately half of the initial charging current. Unlike fast charge RC and power tool chargers, the PowerBooks probably charge slowly enough that doubling the current by cramming all of it into a single string made up of cells of about one half the original specified total pack capacity would not fry the cells. But the voltage rise again due to internal resistance might cause the voltage monitoring gas gauge to declare nearly full too soon. Then it would take a much longer time (like as much as a day) to really top off the battery at the trickle rate. In an extreme case like using a single AAA NiCd string, if the capacity is too small the initial charge current might overheat then vent the cells, or declare the cells full charged much too soon, possibly immediately. When the white stuff has mostly vented from the original cells, they start behaving like really undersized new cells, and eventually when all the white stuff vents they act as nearly open, no connection (unless they short first).
3. One string rather than two will see and have to absorb all, not half, of the trickle charge current. The trickle charge current contributes a lot to the eventual appearance of that white stuff, but is tolerated if it kills the battery much slower than just daily cycling of the battery does. If you charge then unplug the AC adapter no problem, but if you mostly operate and store with AC adapter plugged and powered the new single string might meet an eventual venting death twice as fast than a two string series/parallel recelling (this might be several years).
So if you can match (or somewhat exceed) the original amp hours capacity with a single NiCd string that physically fits and the internal resistance is low enough, no problem. If you find batteries of slightly increased capacity but use only half of them, experimenting may be necessary to see if both charge and discharge performance is acceptable. Depending on time spent on charge, some diminishing of long term life might occur in a single string recell. If you go way lower in capacity than the original design, even as a temporary bench test, the initial charging current might be too much, and heating followed by alkali steam venting might soon follow unless the PowerBook charger is smart enough to detect an over-voltage, declare full charge and go into a protect mode.
When a series parallel configuration is scrapped and a single string substituted, consider the following possible issues, which suggest some measurement and analysis and/or some experimentation:
1. The internal resistance of the single string (of modern cells) might be higher than the original two string series parallel design. This could make a voltage monitoring gas gauge misbehave and seeing more voltage drop earlier in a discharge, declare empty too soon. In an extreme case like using just one AAA string, there might be so much drop that the PowerBook would declare empty immediately. Internal resistance varies between battery cell vendors, and some vendors feature alternative designs within the same physical package dimensions, one with lower resistance but less capacity, and another with higher resistance but increased capacity.
2. One string rather than two would absorb the entire rather than approximately half of the initial charging current. Unlike fast charge RC and power tool chargers, the PowerBooks probably charge slowly enough that doubling the current by cramming all of it into a single string made up of cells of about one half the original specified total pack capacity would not fry the cells. But the voltage rise again due to internal resistance might cause the voltage monitoring gas gauge to declare nearly full too soon. Then it would take a much longer time (like as much as a day) to really top off the battery at the trickle rate. In an extreme case like using a single AAA NiCd string, if the capacity is too small the initial charge current might overheat then vent the cells, or declare the cells full charged much too soon, possibly immediately. When the white stuff has mostly vented from the original cells, they start behaving like really undersized new cells, and eventually when all the white stuff vents they act as nearly open, no connection (unless they short first).
3. One string rather than two will see and have to absorb all, not half, of the trickle charge current. The trickle charge current contributes a lot to the eventual appearance of that white stuff, but is tolerated if it kills the battery much slower than just daily cycling of the battery does. If you charge then unplug the AC adapter no problem, but if you mostly operate and store with AC adapter plugged and powered the new single string might meet an eventual venting death twice as fast than a two string series/parallel recelling (this might be several years).
So if you can match (or somewhat exceed) the original amp hours capacity with a single NiCd string that physically fits and the internal resistance is low enough, no problem. If you find batteries of slightly increased capacity but use only half of them, experimenting may be necessary to see if both charge and discharge performance is acceptable. Depending on time spent on charge, some diminishing of long term life might occur in a single string recell. If you go way lower in capacity than the original design, even as a temporary bench test, the initial charging current might be too much, and heating followed by alkali steam venting might soon follow unless the PowerBook charger is smart enough to detect an over-voltage, declare full charge and go into a protect mode.