About a year and a half ago, I purchased an Acer TimelineUltra M3. Initially, I would get pretty good battery life and could use the laptop for 5-7 hours before it died. However, about 6 months ago I lost the charger that came with it and purchased an off brand one. After that, battery life was never as good and lasts less than 3 hours. It also takes more than 3 hours to get 100% charge. I am now forced to bring my charger with me everywhere I go. Considering that I have used this charger for almost half a year, would buying the official Acer charger restore my battery life to (more or less) what it originally was or is it too late for that?
Rechargeable batteries, whether NiMH or Lithium-ion have a limited number of charge/discharge cycles (typically 500-1000) before they begin to lose capacity. I believe the dropoff can be fairly rapid.
It sounds like the off-brand charger may have been the wrong voltage. Measure it with a voltmeter if you have one to compare it to what it should be.
As to the replacement battery, you can buy a new aftermarket battery on eBay fairly cheap, but it will have lower capacity, and likely a much lower number of cycles; or you can go to a place like BatteriesPlus, pay three times as much, but get a battery (e.g. Ray-o-Vac) that will be at least as good as the original, and will also last a lot longer. I'd go cheap only if battery life is not important to you.