There are two circumstances under which you will need to use a receiver battery:-
1) All glow powered planes require an independent power supply to energise the radio receiver.
2) Electric motor powered planes have the option of eliminating a separate receiver battery either through the use of an Electronic Speed Controller (ESC) with a built in Battery Elimination Circuit (BEC) or by using a separate BEC. Both are driven from the main Lipo Battery used to power the planes motor. A normal receiver battery can be used if preferred but this is unnecessary weight for the plane to carry.
So, to all intents and purposes, we are really talking to those of you who have decided to go down the glow engine route.
Most receiver batteries today are Nickel Metal Hydride types (NiMHs), either four or five cells connected in series. Each cell carries 1.2 volts so four in series gives:-
1.2 + 1.2 + 1.2 + 1.2 = 4.8 volts
Five cells in series gives:-
1.2 + 1.2 + 1.2 + 1.2 + 1.2 = 6 volts.
Standard NiMHs have a relatively short voltage retention span and need to be recharged prior to every flying session for guaranteed performance. Nowadays, there are a new generation of “long life” cells that retain their charge over many months. They are a little more expensive than the standard type but well worth the additional investment.
NiMHs come in a range of current capacities from a few hundred milliamp hours (mah) up to 2000 to 3000 milliamp hours (mah)
Your trainer will perform totally adequately on a four cell NiMH battery. My suggestion would be a new generation four cell with a capacity of around 2000 mah which should provide several flying sessions between charges. Interestingly, most Lipo battery chargers are also capable of charging your NiMH batteries through a separate programme. The charger instruction manual will give you full details on how to charge correctly.