iPhone 4s, my iPhone 99% of the time won't charge arms the battery is power-draining. Started happening at random, going from good - absolute hell while i was at work. One time out of the many times I've recently tried, it charged about 10% after hours and hours on the charger. tried 3 separate chargers (3 separate brands), tried draining 100% and charging back up etc. could the battery be shot? I can switch the battery out in a simple minute, but I don't want to go out of my way and buy a new battery at my shop if it won't work
Battery certainly could be bad, but there are some other common areas of failure.
If you shine a light into the 30-pin connector, check for any physical obstructions such as pocket lint, scraps of paper, I've even seen staples and gum. If you spot it, try to use compressed air to get it out.
Also look for any burnt (blackened) pins which could explain it.
Lastly look for signs of liquid. Liquid LOVED getting into those docks. Signs of liquid include corroded (green) pins, or tripped (red or pink) liquid contact indicator. The liquid indicator is in the center, under the connector plate.
Barring any of those things, it's most likely either the battery or the charging circuitry.
You didn't mention which iOS version you are running. With iOS 9 you can have a look at which application is using the battery the most (in Settings/Battery).
In general I would recommend a full reinstallation of the whole phone in such situations. But as the 4s is rather old so is probably the battery, which would explain the behavior you are seeing. Going for a new one sounds like a good idea (especially if you can replace it yourself).