Nickel Cadmium Batteries (NiCad) 1500mah (milliamp hours) to 2000mah.
The cells are made of layers of a nickel based material and Cadmium that flows between the layers.
The number of layers (rolls) is what determines the mAh rating, more layers, the greater the capacity. The layers are made of different nickel composite materials; they are distinguished by SCE, SCR and SCRC.
SCE type are older technology, the plates are very thin and can not handle much abuse.
SCR and SCRC cells use more durable plates.
A charged cell has Cadmium flowing between the layers, as the battery discharges the cadmium bonds to the nickel plates slowing the flow, as it is charged, the cadmium separates from the nickel plates, producing the electricity and the heat.

A "bad" battery is when the cells or even one cell has damage to its layers by getting too hot or is abused by vibration or impact.
A "leaking" battery is from vents located under the lid (positive terminal) a spring system vents off pressure when too much heat is produced.

When a NiCad cell heats up from charging or discharging, crystals form in the Cadmium and these crystals cause the cell to loose capacity.
Pulse chargers are designed to mix a discharge pulse (negative charge) with a charge pulse (positive charge) 1 negative to 5 positive, this breaks up the crystals to be dissolved.
Linear Chargers supply the battery with a constant uninterrupted supply of positive charge.
Auto Peak Chargers shut off by detecting the voltage rise and drop during the charge cycle, when a battery reaches full capacity, the voltage peaks and then begins to drop off; the charger detects the drop and shuts off.

False peak is when a fluctuation occurs for any reason from the power source or connections to the battery.
Trickle charging is a low charge rate that keeps the battery from self discharging, thus maintaining a full capacity charge, NiCad cells lose power naturally at a rate of about 1% to 5% a day.
Charging rates are a personal preference on adjustable chargers, some have ranges of 1 to 10 amps or higher, some give charts relating to the type of your cell and power source (AC or DC) personally I never exceed 5 amps.
Most non-adjustable chargers operate at 3 amps quick charge and approx. 1 amp trickle.

NiCad's will develop a memory, if a battery is not fully discharged when charged again; a false capacity will develop giving a shorter run time.  Discharging a NiCad the voltage should not go below .5 volts per cell (3 volts for a 6 cell pack) if voltage drops below .5 volts per cell, this can cause "Cell Reversal" and can cause permanent damage to the cell. Most auto chargers are programmed to stop at this voltage.
If you feel a battery has developed a memory, you will need to perform a series of charge/discharge cycles (Cycling the battery).

Nickel Metal Hydride (Ni-MH) Batteries have are a  higher capacity and longer discharge rate of the NiCad for the same cell size and voltage.
Capacity is 2000mah to 4000mah common, there are higher capacity cells and they have a price to match. Voltage is the same 1.2 volts per cell.
The vibration level and temperature has a higher factor in the life of this type of cell, thus charge rates are limited to 3 amps and lower for higher mAh batteries.
Ni-MH batteries do not create memory; they do not need to be discharged before charging and do not require cycling, unlike NiCad's, they can and should be stored charged.

Personally I prefer the NiCad over the Ni-MH.
Ni-MH do not have as much "punch" of power at the start and my run times average less than 10 minutes.
The Tower Hobbies brand are very reliable, well made and very inexpensive at $10 for the 6 cell 1500 and $17 for the 7 cell 1500