I have disassembled one PB1400 battery pack and it contained eight Sanyo Twincell NiMH cells of size 4/3FAU (unsleeved measurements, excluding tab thickness were 66.7mm L x 17.4mm D). In the US, Digi-Key part number SY136T-ND for the solder tabbed version. But these are expensive as in $10.88 per cell! There is plenty of room for you to rig up some improvised spring loaded tubular holder for using eight economical AA sized (50.4mm L x 14.35mm D) NiMH cells of lesser capacity, length, and diameter. The battery pack internal label says 9.6V, 4000 mAh. Unfortunately I haven't come up with a way to fit 16 AA cells in a series/parallel configuration into the space available.
See
for a look inside. Those green plastic tubular sleeves each contain a pair of 4/3FAU cells in series. There is a Klixon thermal breaker, and both Polyswitch current limiter in series with what I think is a fuse all in series with the battery. The green PCB strip contains on the hidden side a DS2401 serial number device and a temperature sensor, each accessable to the PB1400 power manager. On the bottom of the battery assembly case is a brown colored insulated copper heat spreader which brings the green PCB strip close to the average temperature of all cells, probably for the use of the temp sensor that looks for the temp rise associated with charge completion. Your improvised battery holder should not excessively thermally insulate the replacement cells from that heat spreader. So any space filler material should go above the cells, not below.
I also recall a very tight squeeze but workable situation for button end and solder tabbed replacement AA cells, but this was for PB500 series batteries. In the PB1400 there will be 30 mm to spare in the length dimension if you use eight AA cells.
See
for a look inside. Those green plastic tubular sleeves each contain a pair of 4/3FAU cells in series. There is a Klixon thermal breaker, and both Polyswitch current limiter in series with what I think is a fuse all in series with the battery. The green PCB strip contains on the hidden side a DS2401 serial number device and a temperature sensor, each accessable to the PB1400 power manager. On the bottom of the battery assembly case is a brown colored insulated copper heat spreader which brings the green PCB strip close to the average temperature of all cells, probably for the use of the temp sensor that looks for the temp rise associated with charge completion. Your improvised battery holder should not excessively thermally insulate the replacement cells from that heat spreader. So any space filler material should go above the cells, not below.
I also recall a very tight squeeze but workable situation for button end and solder tabbed replacement AA cells, but this was for PB500 series batteries. In the PB1400 there will be 30 mm to spare in the length dimension if you use eight AA cells.