I'm not quite sure what you're asking so I'll answer as best I can.

There's 3 things you need to pay attention to with battery packs (other than the connector type that is), the type (NiMH, NiCd, LiPo etc), the voltage, and the current rating (mAh).

The type will only change HOW you charge it, what sort of charger you use etc. NiMH and NiCd can both use the same sort of charger (the one that came with your DX7 will work), LiPo needs a special charger.

The voltage will depend what you're putting it into. Receivers can use 4.8 or 6.0 volt packs (yes, including your spektrum receiver), and the voltage change will affect mainly affect how fast your servos react. There is a known issue with spektrum receivers that can cause them to reset (and crash your plane) at low voltages, so the 6.0 volt ones are strongly recommended. The DX7 transmitter requires a 9.6 volt pack, which is a different shape to the one in your planes. Plane & receiver packs are not interchangeable.

The current rating is the big one, especially with receivers. It refers to how much current can be delivered for one hour, eg: an 1800mAh pack (of whatever voltage) can deliver 1800 milliamps (mA, or 1.8 Amps) for one hour, or 900 mAh for 2 hours, or 3600mAh for 1/2 hour, or 7200mah for 15 minutes etc, at the specified voltage. Think of it as a "gas tank" for electricity... the bigger the mAh rating the bigger the "gas tank" and the more electricity you can store, and have available. Theoretically if you've been using an 1100mAh 6.0V pack in your plane and you replace it with a 2200mAh 6.0V pack you can fly your plane twice as long before you need a recharge. I say "theoretically" because there's always losses, so it works out to a bit less than twice as long.