Purchase a battery from a reputable dealer/manufacturer. The "quality" of a battery isn't easily recognized. Outside of maker's marks, voltage capacity and current rate, there's no much to tell you if one battery is "better" than the next.
Recognizing "quality" in airsoft guns is fairly easy - metal or high-polymer construction, quality gearboxes & motors, type of wiring harness used, fit & fitment, and builder's name.
Batteries, on the other hand, are chemical storage piles. What you're paying for is the quality, reliability, and performance LIFE of those piles. The chemicals that make up the piles require a minimum level of purity to function, and a high level of purity to function reliably over a long period of time. You can't see the chemicals, so you can't gauge their quality, but this quality is reflected in the price for the piles.
The chemical piles in cheap batteries will almost always have impurities mixed in. They will hold a charge, but they may not hold them over as long a period of time. They may self-discharge at a faster rate, or consume energy faster in the field (not lasting the day). Their rechargeable lifespan may not be as long as a quality battery pack. They may overheat quicker (because of the impurities), and they may use cheaper materials for the battery housings (leading to leaks). Really cheap batteries have been known to explode or ignite on fire while charging.
So, although you are free to use whatever battery you want, from whoever you want to get them from, just be aware that cheap batteries - while able to perform in most cases - do not come without inherent risks.
VFC SCAR-L - VFC SCAR SSR - KJW Hi-Capa 5.1 - CAW M79
The Three Sisters - WE G39 E/K/C
Originally Posted by kalnaren
Stalker stays where he is.
His BB's fly across the country to hit their target.