Many players want to run their AEG's with Lipo batteries (Lithium Ion Polymer) to increase their ROF and to pull harder springs.
A lot of AEG's now claim to be "Lipo ready". And technically most AEG's are Lipo ready, because you don't need to change anything internally to run a Lipo battery in them. A stock JG should run a Lipo and without blowing up (unless it's a lemon). But there are things you need to know about what Lipo's do to your AEG.
LiPoly are the newest type of rechargeable battery used in cell phones, laptops and digital cameras. Lithium Polymer batteries have twice the capacity at about half of the weight of NiCad or NiMH cells. They require a special computer controlled charger to avoid catastrophic overheating during charging which can cause them to burst into flame. AEG LiPolys come in either 7v or 12v packs. 12v packs require that your AEG have high quality wiring, motor and gearbox that can handle high-speeds.
LiPoly batteries are more complex due to their chemistry. LiPoly batteries that are fully discharged can cause an odd chemical reaction which causes the metals to heat up. 'Fully Discharged' LiPolys can catch fire (which is what happened with some early computer laptops). So it's advisable to always keep LiPoly's partially charged, or fully charged. And never, ever store and forget them somewhere flammable (like a closet next to your clothes or a garage next to cans of green gas.)
And a lot of new players want to know how do they make their AEG's "Lipo ready" and what exactly that means. There are two parts to being Lipo Ready: 1) Electrical and 2) Mechanical
ELECTRICAL
Lipo batteries output at much higher voltage (11.1v which is much higher than stock batteries that are 8.4v) and amperage. The wiring and motor of most high quality AEG's are good enough to handle the higher voltage and heat. So there is no need to replace your wiring for all but the cheapest AEG's. Though players who want to pimp their ride may want to buy low resistance wiring which more efficiently conducts current.
DEANS
Because of the higher voltage, your wiring and motor will heat up much faster and so you must be careful that you don't overfire. Melting may occur in the solder around the connections to the motor and with Tamiya type wiring connectors. This is why many switch to DEANS/DIN connectors which are made to handle higher power current.
If you choose DEANs, you will have to convert all of your connectors to DEANS (as any Tamiya connectors will be your weak point) on your AEG and on all of your batteries. The downside is that most players have Tamiya connectors, so if you need to borrow a battery from a friend, you'll probably need to have a DEANs to Tamiya converter.
MOTOR
Because of the higher voltage, your motor will spin faster. Much faster. So your motor will wear out (burn out) sooner than normal. So you may want to buy a high-speed upgrade motor. These motors have reinforced parts for higher RPM's and have internal ball bearings to allow them to spin with less friction.
FUSE
Lipo's output much higher amps than NiCad and NiMH batteries and you need to buy higher amp fuses (or remove your fuse) or you will burn them out quite often. Most stock AEG have 20A-30A fuses. You should upgrade to a 40A or higher fuse with Lipo setups. You can find these in autoparts stores.
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MOSFET (metal–oxide–semiconductor field-effect transistor)
This is an optional but recommended upgrade. Lipo's higher voltage and amps can cause the metal contacts in your trigger to partially melt due to electrical arc'ing (sparks). MOSFETs are solid state switches that bypass the higher voltage battery current directly to your motor, shunting lower current to your trigger to prevent melting. Some more expensive switches can also function as resettable fuses and programable selective fire controllers (allowing you to fire in 3-shots bursts)
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MECHANICAL
Because Lipo setups run at much higher RPMs your gearbox will cycle much more often (almost twice the speed that they were originally designed for). So your gearbox parts will wear out twice as fast. Get used to it and be prepared to inspect your gearbox more often.
REINFORCED GEARS
Tokyo Marui, DE, BE and sub $100 AEG's do not have reinforced gearsets and are not 'Lipo Ready'. JG, KWA, CA, G&G, ICS, G&P, STAR, etc. have steel gears which can withstand higher speeds. But nothing lasts forever. All gears eventually wear down and fail. At higher RPMs they will fail that much sooner. So at the first sign of wear, buy a replacement reinforced gear set.
Be sure to buy original gear ratio or torque up ratio sets with Lipo. Do not install high-speed gears! Speedup gears were designed to modify the gear ratio in stock motors to increase ROF. A speedup gear set with a Lipo will only cause your gearbox to spin even faster and that may be too fast for your mag and hop-up to feed properly.
HELICAL GEARS
Helical gear sets have more teeth that mesh much tighter and so can handle higher stress by spreading it across more teeth. But the tolerances are much tighter. You must have excellent shimming for them to work. And you must change your piston to a half-teeth type.
FULL TEETH PISTON
Most stock pistons come with a single metal gear tooth at the very end of the gear rack. This tooth experiences the most stress because it's the last tooth your gearbox pushes on when your spring is at its most compressed. With Lipo's, when you pull the trigger, your gears start to spin much faster. You get crisper trigger response but you will also experience faster wear on the beginning piston teeth, sometimes stripping them.
Many Lipo users have switched to full teeth pistons where the metal teeth extend across the entire row. The downside is that metal on metal creates more wear and stress to your gears, so they will wear out sooner.
BUSHINGS
Common wisdom is that you should replace your bushings with metal bearing bushings. These will reduce friction and stress on your gears at higher RPMs. I'm a bit of a contrarian about this. I've never seen nylon bushings fail. With springs lower than 350fps, I don't believe you need to change to metal, because nylon is similar to teflon and has very low friction.