Helicopters need batteries that can deliver high current. If you use cheap batteries, your helicopter will probably not fly well. The Sanyo CP2400 and RC2400s are the best choices for beginners on nonmicro helis because they are reasonably priced and can discharge at high rates. NiMH batteries are usually higher capacity but deliver less current than Nicad batteries, so they are better suited for duration flying.
One exception is the Sanyo HR-SC which is a NiMH battery specially built for high current drain which performs slightly better than the CP2400/RC2400 (HR-SCs must be used immediately after charging for best results).
NiMH batteries require about 5 charge/discharge cycles to reach full capacity. They work best immediately after charging - if you allow them to sit and cool off they will not work as well. Both Nicad and NiMH battery packs should be slow-charged on the first charge to ensure all cells in the pack will reach a full charge.
If this is not done, the cells in the pack may be at different states of a charge and the pack may never fully charge properly. If you notice a big drop in capacity in your battery packs after a few months of use, this is probably due to the cells in the pack having different states of charge. If this happens, you should try to "rebalance" the cells in the pack by slow charging them at a constant current of C/20 for 24 hours. This will usually restore the pack to full capacity. This should only be done for Nicad and NiMH packs, and not for other battery types such as Li-ion or LiPoly. Your batteries will last much longer if you do not allow them to become too hot, because the primary cause of battery failure is the deterioration of the separator which is accelerated by higher temperatures. Therefore, you should allow your battery packs to cool a little after use before charging them again.
The batteries known to work well are:
Corona / Logo 10/16/20 / ECO 8/16:
Sanyo CP1700SCR (NiCad 1700 maH, 5.5 milliohms)
Sanyo CP2400 (NiCad 2400 maH, 4.5 milliohms)
Sanyo RC2400 (NiCad 2400 maH, 3.2 milliohms)
Panasonic HHR300SCU (NiMH 3000 maH)
Panasonic RC-3300HV (NiMH 3300 maH, 5.0 milliohms)
Sanyo HR-SC (NiMH 2600 maH, 4.0 milliohms)
Gold Peak 3300 (NiMH 3300 maH)
Gold Peak 3700 (NiMH 3700 maH)
Micro helis (Piccolo and clones, Hornet, etc but not T-rex or Zoom):
Sanyo HR-AAAU (NiMH 720 maH, 30 milliohms)
PowerEx AAA NiMH
HECELL AAA NiMH
(milliohms is a measure of internal resistance; LOWER IS BETTER)
If you are buying batteries not on this list, then you should ask the manufacturer for the internal resistance of the cells. If the batteries do not have an internal resistance as low as the cells on this list, then it is probably not suitable for helicopter use.
If you intend to fly aerobatics, do NOT use commercial battery packs. Most of these packs use a flat springy metal to connect the battery terminals and the springy metal will melt at high (50-60+) currents. Be sure to use your own inline-soldered battery packs if you intend to do hard aerobatics with your helicopter.
Lithium-polymer batteries are NOT recommended for helicopter beginners. They are fragile and incur damage easily, and when they are damaged they can IGNITE up to ten minutes later.
There was a story posted on RC Groups about a guy who crashed his plane powered by Li-Pos, and he put it in his SUV. Several minutes later someone told him his SUV was on fire. He posted pictures of the totaled SUV, and the interior was completely burned.
NOTE: The 17mm-long N20 motor used on some micro helicopter tails (some Piccolo models, all Hummingbirds, etc) will burn out quickly (5-20 mins) if the motor is run on 8 cells. Be sure to use only 7 cell battery packs to extend the tail motor life.