I was under the impression, if you are using a 2200Mah 20C battery, and a 2200Mah 60C battery, they will not last longer at all. The Phantom will only pull as much as it can handle, both will last the same time, but due to the extra weight of the higher C rating batteries it will not last as long.
Always used to be the case flying RC Helicopters. If your using a higher MAH rating then your battery will give you extra flight time, extra C rating will not.
As some batteries are better quality batteries than others it is possible one brand will last longer then another, but both batteries of the same MAH rating will last about the same time. Sometimes, I think batteries can be over, or underspec, and the MAH rating may not always be what the battery is stating.
But as a general rule the C rating does not determine the length of your flight time, but a higher MAH battery will last longer.
Are all your batteries the same MAH rating or different? A 2500Mah battery is going to last longer with flight time than a 2200Mah. The C rating does not determine flight time, only the Mah does, but once again also get heavier.
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Also the higher the C rating, the longer the charge time. If your using extra electronics, or using your Phantom in manual mode, where demands are higher, then a higher C rating will help, otherwise your just wasting your money.
Also what I have learned about Lipo batteries, what is written on the battery, may not be totally correct. Your battery may say it is 60C, but in real terms maybe lower than this or higher, same as the Mah rating, what you see is not always what your get.
Which may explain why some of your batteries some are lasting longer than others of your using the same mah rating. Just because it says it is 2500Mah battery does not always mean it is. You cannot always trust what is written.
I am into lasers and this is always the case, some are over spec, some under and same goes for batteries. You can have 6 different brands and all the same rating, but all may well run different lengths of times. it All depends on how accurate info written on the battery is, so do not assume if it is a 2200Mah thats what your getting, it could either be higher or lower then this. The same as the C rating. A C60 battery could in fact be lower, or higher than this. Often the cheaper batteries can inflate the figures to sell more and make them sound better then what they are, or make you think your getting more for your money, but could well be poor batteries.
Just don't trust what is written on the battery.