I could easily have spent a fortune replacing the motherboard, the memory, the processor etc when it all came down to 2 dollar battery. What's more is these batteries are easy to get. What you need is a CR2032 available at most hardware stores like home depot and I've even seen them sold in Safeways. Of course every reputable computer lithium stock store should stock them. Just check to make sure that your PC uses a CR2032 before you go to the shops!
I run a PC repair business in Vancouver. I always tell people who call with clock problems to check their motherboard battery first before calling me round. For most people it's easy to lithium facts check and easy to replace. I don't want to make money from fixing something so simple.
The price of an external laptop battery may vary from brand to brand. But they are quite affordable and very compact device. They can be recharged with an external charger so you don't have to look out for an electric supply to charge the external battery.
Why lithium bettery stock is this important? If you have a relatively new PC the only problem you will get are error messages relating to the clock on start up. No big deal. If your PC is older then a low battery could be a nightmare. I had a PC which once every few months refused to boot. Rather than take the PC in for repair or start replacing parts myself the first thing I did was look at the BIOS settings. It was soon obvious that the BIOS settings were corrupt (wrong bus speed). I put a volt meter on the battery and sure enough it was reading 2.2 volts.
Briefly relating back to overall size... many lithium 18650's will differ ever so slightly in length (measured in millimeters). This is mainly due to an added protection circuit. The circuit helps to prevent the battery from over-discharging, since Li-ion cells can easily become damaged if their power is drained below a particular voltage. The protection circuit also prevents the cell from being over-CHARGED as well. These cells are very sensitive. They must be handled carefully, gently, and as described above, must never run-down too far, or be charged over and above their threshold. They can actually explode if severely mistreated!
Third, it is quite important to fully charge the battery every time, also to run the battery to be empty every time. The reason is the battery will lose charge day after day if the battery in your notebook is a nickel metal hydride. Therefore, your battery will dies a certain days later and you need to order a new one. Your work will be affected and money will come out of your pocket. Shortly, you can keep your battery in a good condition for a long time if you do as what have mentioned above.