How often have you calibrated the battery?
All batteries have a limited life, usually referred to as charge cycles. MacBooks, for example, have a 3000 charge cycle I think, that means it can be charged 3000 times before it'll need replacing. Bear in mind that means full charge cycles, not part charges. As the battery is used, it starts to lose its full capacity, which is why laptops never seem to last as long over time.
With your Sony, if it's old and had a lot of use, it probably is time to just fork out for a replacement. However, I would try recalibrating (should be done once a month or so anyways) just to ensure the battery hasn't 'forgotten' it's full capacity. Charge your battery up until its full, and leave it plugged in for another 5 hours at least. This will get all the cells wizzing around nicely. Then use the laptop as normal until it completely dies. this doesn't mean you have to use it all in one sitting, just normally until it shuts itself off.
Because the laptop will probably hibernate at the last moment so you don't lose any work, leave the laptop with a dead battery for 5 hours, so it's completely empty. Then charge until its full again.
See if that makes any difference at all.
If not, then we know it's time for a new battery.
I tend to advise not to go for cheaper unbranded batteries. Although they are cheaper in the short term, the cells are of a lower quality, so the life of the battery will be nowhere near that of a genuine battery. Hence you'll probably go through 2 or 3 before a genuine one would have died. The genuine one will last a lot longer, both in a single charge, and in overall life.
Try to avoid buying a battery via eBay, or any other site where you are unsure of its status. In fact, it's even better to buy from somewhere where they are out of stock! Reason being, you want to try to avoid buying a battery that's been sitting unused and idle for a long period of time, as this also affects its overall life. When buying a battery, check the manufacture date to make sure it is only a few weeks or months old, rather then 6 months or a year. Batteries are expensive as the laptop manufacturer has you tied in to their non standard part, you want to make sure you are getting the most possible from your purchase!
Which way you want to go is up to you really. If you are happy with a cheaper battery for the meantime (you may only want to keep the system for another year or so anyway before replacing it) then go for anything available. If you are after a long term investment, it has to be a genuine original.
As for whether it will fry your laptop - a half decent battery shouldn't do this. I've never come across it in the past. I would avoid a completely unbranded battery though and go for one that at least has a brand name, even if it's not Sony themselves.
