For the 400 use 2100mah 11.1 3cell thunder power or better. Something that can discharge 24amps continous. The 400 draws 24amps on the burst and about 19amps avg. You need something that will keep up with that. Using anything less than this will cause battery damage if you burst for more that 5 seconds. Thunder power has a 1900mah 11.1v but the differnece is extreme with only a 19amp continous discharge VS 24amps with the 2100 11.1v. The bat will cost you about 75-80dollas. I cost cutting alternative is to buy and attach your own cells togather but that is for very tech able people. You make a mistake and the battery is no good. I found a site called www.maxamps.com and they have a 2250mah 11.1v 33amp burst for sale 64.94 delivered to your door in a week. THere is a large and growing markett for LIPO and you can find all kinds if you look and the ONE TO LOOK FOR IS 2100mah 11.1v 3cell or 2250mah. You dont want to go any higher than 2250mah. Also make sure its 3cell. If it says 11.1volts it is 3cell. MOST IMPORTANTLY NEVER ARC THE BATTERY. NEVER LET THE BATTERY WIRES TOUCH EACHOTHER OR ANYTHING ELSE THAT IS METAL. THIS WILL DESTORY YOUR BATTERY INSTANTLY AND COULD CAUSE A FIRE. LIPO BATTERIES MAY REACT AFTER BEING DROPED, DENTED, OR UP TO 1 HOUR AFTER ARCing the battery, CAUSING FIRE OR EXPLOSION> IF this happens move the battery to a safe location for a couple hours. If the battery is not bloated or leaking 2hours later try charging in a safe location away from flamable property. The battery will never be the same if it does not catch fire or explode and you will notice a huge difference the next time you charge and fly. Hope I have helped. If ya have any more question in my realm of understanding and I will do my best to answer.