E.g. 3500mAH 25C max 放電 does not mean using 87.5A 放電 can have 3500mAH. Need to look at the battery capacity spec. for using how much A 放電. 25C is just for max. safe discharge rate, not to specify the battery capacity.
E.g. GP 2500mAH AA 電. If you use 0.5A放電 and 2A放電 you will get big difference in capacity (much less when using 2A放電).
Using electronic load equipment can test the battery capacity using different C放電.