Get a copy of Battery Amnesia or (better still if it is in your Extras folder) use Apple's own Battery Recondition software (designed for the era of NiMh batteries) and run the discharge/recharge cycle 5-10 times. If NiMh cells are taking a charge at all, they can often be brought up through cycling from a runtime of a minute or less to one of 60-90 mins, and maybe more. That's enough for most purposes in a 1400 these days.

NiMh cells are often way, way better to have in old machines than are old Lithium Ion cells, which mostly are just dead by now. A further advantage of NiMh cells is that it is reasonably easy to recell a battery, since they don't have a habit of exploding in the way that Lithium Ion cells can — though NiMh cells can literally melt metal in seconds if you touch them together the wrong way, and so they do need careful handling. You don't want to be jinging them around loosely in your pants pockets, believe me.