I have a Tamiya 7.2v 1700mah battery that has been charged for a couple of weeks now... I only shot a couple of hundred rounds with it. When I tested the gun at the field I used someone else's battery. It was a Sanyo 8.4v 1800mah Platinum, and simply left mine at home...

Am I supposed to discharge the battery before I put the gun away?

The guys told me to buy an 8.4v battery since my gun would "lack performance" if I used the 7.2v. I'm on a tight budget and I need to buy a charger too... so, should I really get the 8.4v battery AND a battery discharger??? And what difference does it make which one I use?

All stock AEG motors need 8.4v battery power to work at their best performance. There are batteries with higher voltage like 9.6v or even 12v that are used for upgraded guns with more powerful motors, or just to speed up more the regular motors. If you use the "more punch than 8.4v" batteries with a non-upgraded gun you'll damage it's motor.

In general,

• Voltage related to the battery's power and a higher voltage results in higher rate of fire.
• mAh is simply the battery's capacity.

Most batteries are of the Ni Cd kind and have to be fully discharged, through use or using a discharger, before they are recharged again. This results in better performance of the battery, they last longer and reduces any possibility of the so called "memory effect" that reduces it's capacity. I'd recommend using a discharger (about 10€). I am also of the opinion of buying the right battery for your gun. I have the official Tokyo Marui 8.4v MiniSD which is specifically for my gun (a Tokyo Marui G36c). It's a small capacity battery (600mAh) but it gives me no worries.

I bought my charger from an electric equipment shop for about 25.00 €  if I remember correctly.

You might want to check out the good on-line airsoft retailers to see more options on chargers.

Journeyman's Tokyo Marui G36C

That information really helps out. I had thought all along that voltage would only affect the rate of fire and not actually increment the speed of the BBs, but I needed to confirm with an expert.

So, I really, REALLY don't need to by an 8.4 battery unless I want to shoot faster... AND I need to get a discharger to prolong the battery's life and ensure it works right, for a longer time.

You also mentioned that a small 600mah battery didn't give you any trouble... which brings me to my next question...

How many shots can you get off your 600mah battery before it starts running out? Furthermore, from what I understood, a 7.2v 1700mah battery will shoot as many rounds as an 8.4v 1700mah battery since what determines the length of battery is the mah number.... so, after you answer how many shots you get off the 600mah, then I can just multiply that by 3 to know how many shots I'd get off my battery.

With my 8.4v 600mAh battery which powers up the EG1000 motor of my Tokyo Marui G36c, I can shoot approximately 10 standard mags (50 shots each), so this adds up to about 500 bbs. I don't use hi cap magazines so I try to pick my shots and that's why it doesn't bother me that much that the bb count is somewhat low.

UPDATE
<<< After paying closer attention, I noted that my G&P 600mah battery has shot over 1000 rounds without running out using an ICS M4A1 >>>

Mind that temperature affects the total amount of shots your battery can provide for a given capacity so, for example, on a hot day I might be able to shoot more than 500bbs with my battery.

Check out this article for more in depth info:
http://www.airsoftretreat.com/chal-battery.asp