Always use Brand makes of crystals. Cheap crystals = expensive crashes.
When you get a new crystal check it out on the Frequency meter / checker.

Li-Polymer Batteries.   

Li-Polymer batteries are excellent for all forms of electric flight but there is a price to pay.
These batteries MUST be charged "By the book" or they WILL bite back.  Here are a few tips when charging :-

NEVER leave them unattended.
Always remove them from you aircraft when charging.
Always use the correct charger.
Always set the correct number of cells / charge rate.
Put the battery on charge in a pyrex dish.
Never short circuit any battery.
Check the battery every 5 mins to make sure it is not overheating.
Do not use a battery that has started to swell.
Ensure correct polarity when charging.
Always read the instructions to your charger & batteries.
Do not charge them in your car.

Batteries have caused total loss of Aircraft, houses, workshops and cars (Ask Sparky).
You have been warned.