I bought a Fujifilm Finepix S2980. As the AC power adapter is very expensive and hard to find (I could only find a mention of it in the manual), I would like to know if it is possible to build an AC adaptor? The main thing is converting AC to DC and supplying it to the battery connector terminals. It uses 4 1.2V 2100 mAh batteries. Does a little difference in Amperes make a difference?
2100 mAh represents the capacity of the battery. mAh stands for milliamp hours. It means the battery is rated to supply a current of 2100 milliamps for one hour (or 21 milliamps for 100 hours).
No, it's not that simple. There is very likely special charging circuitry in the camera itself. This circuitry is important in managing how the battery is charged. It doesn't just blindly connect the output of the charger to the battery.
Since it seems your knowledge of electronics is limited, there is a reasonable probability you will damage your camera or the battery by trying to make your own charger. At best, you can find what the charger for your camera is rated for. Look for a manual or find someone else who has one and look on the nameplate. The output voltage and current is usually shown. You can get a ordinary power supply that puts out the same voltage and is rated for the same current or more, and it should work electrically. You still have to find the right connector, which is often proprietary on cameras.
All in all, go buy a charger for your camera. A third party one is probably OK if from a reputable manufacturer and this camera is specifically listed as supported. Otherwise, you shouldn't have bought the camera without the charger if you wanted a camera and a charger.