I'm not really good in English, especialli writting, so I just use short sentences, OK?
My Dell laptop got the same problem with you and I really worry ablout that, but I found the way to solve that stuff.
I took my laptop to the Dell Distributor in Viet Nam. It's FPT Center. They also have the Servicing Center.
The warrantor checked my laptop. He turned it around. He used a screwdriver to open it. Immediately I could see the BIOS battery, it's round and easy to pick up. He picked it away to check it, after 5 minutes he returned a told me that the BIOS battery is still good, no problem with it.
This is interesting. After that I asked him about the problem "Time-of-day clok stopped". He told me that: "That's because you always take the laptop battery out when you use it (I mean the LAPTOP BATTERY, long, hard.. and you must lnow that :P), and your laptop need the power from your pin to help it supply the energy fot the BIOS". You know, he was right. I always unplug my Laptop Battery and use the power supply (throw a cable which used to charge the laptop Battery when it's empty) because I just don't want to use the laptop battery too much (it's more convenient when I use the power suppy directly because I don't need to charge the battery when it's out of energy :D).
That guy put the BIOS battery in it right position again. Turn my laptop on, and it's working :D