I recently bought a cheap quadcopter (rc powerforce) and was dissapointed when I realised how short the flight time is (roughly 7 minutes.) Is it possible to buy multiple batteries and rotate them to increase flight time or will this fry the quadcopter itself? Also, is it essential to buy the same brand of battery, or can I buy cheap bulk ones? (I don’t want to blow my house up). Thanks for any help.
Hi
Unfortunately with most quad-copters the fight times are relatively low, adding more batteries will not work as it adds more weight, the only way is to charge several batteries and when one gets flat, swap it for a fully charged one and fly again. If you do ever upgrade to a proper RC quadcopter, depending on the type you get you can expect flight times of around 10 min for racing drones, and up to 40 min for larger photography drones. We have plenty of guides on this forum so if you ever need any help, myself or one of the other members will be happy to help 
Hi, yes you can. Choose batteries having more or less same mAh (same weight or a little more) and exactly the same voltage.
I guess your quadcopter is running brushed motors, meaning they get old and die faster than brushless motors. Therefore it isn’t advise to make to many rotations without respecting a pause to make the motors cool down.