ThinkPad R51 Intermittant battery charging

[medons]

My ThinkPad R51 periodically does not charge the battery when I plug the AC adapter in the morning. I usually just leave it in Standby overnight and plug it in at work each day. Is it doing this on purpose to cycle the battery, or something else, is this normal? I've noticed this about three or four times in the last three months. Last night I used it plugged in and the battery was down to >70% and no apparent charging, and this morning it's at 57% and still not charging. One time I rebooted and it seemed to start charging after that. I don't need a full battery now, but if I did, say for a plane flight, I don't know how to force it to charge up the battery.