Battery (BT176) is "furry." Batteries can actually leak and destroy part of the motherboard, so replacement of corroded batteries is advised. Dalco's 3.6V 3-pin battery, part #46875, is an almost-identical part: the pins and size are identical, but it is rated for 60 mAH rather than the A4000 stock battery's 40 mAH. Later A4000s have a different type of battery, a flat "coin" cell. These later "cost reduced" A4000 models also have an external battery connector at the front left of the motherboard, which should take a 3.6V replacement battery with a wired connector. Correct operation would probably require removal of the original motherboard battery. The connector:
1 +----- -
2 +----- NC
3 + key
4 +----- +