The NiCd cells in my Axis packed up last year. I e-mailed Simrad to ask for battery pack price, but didn't get a reply. Very carefully, with a fine hacksaw, I cut the end off the battery (the end with the contacts). Inside, there are 8 AA cells, sealed in rigid foam which needs digging out. I bought 8 NiMh cells (2200mAh) with solder tags from EuroBatteries, wired them together, and inserted them in the case. Then glued the top back on with Araldite. New lease of life for £20, and much better battery performance than the old NiCds.
As you say, there's nothing to be lost by taking the old one apart. Rebuilding it is, if I'm totally honest, a bit of a fiddly job. I seem to remember there's some sort of thermal resistor in there as well, to stop the cells overheating during charging. Just make a few sketches of what you find as you dismantle it, and which bits are connected.
pvb - thanks for your advice so far but I'm afraid I have another query. I notice that the original battery unit is 700mAh. If I substitute it with 2100mAh as you suggest should I assume will it take 3 times as long to charge. I use the charger that came with the Axis 200 and there's no indicator light to show when charging is complete. Would you know if its possible to over-charge the "new" NiMh batteries if I leave the unit on charge for longer than necessary?