My IBM THINKPAD is losing time. From 100% charge several days between use the PC clock looses more than 60 minutes. My question is should I replace the main battery or is this a sign of a different battery inside going bad. If it is not the main battery should I send it to the shop for replacement ? Thanks in advance for your help.

That would be the CMOS battery. They are tough to replce on most laptops.
Check here; http://www-307.ibm.com/pc/support/site.wss/document.do?lndocid=MIGR-4YQNTP
And http://www-307.ibm.com/pc/support/site.wss/document.do?lndocid=MIGR-50627

If you connect to a network, you could just get your laptop to synchronise the time with a local or internet time server every time you log on!

Do not use on network strictly stand alone. Leaning more toward the CMOS battery. Just have to find my user manual for my model R40 Thinkpad.