The capacity is only 4000ma. A pair = 8,000mA.
Stock Yuneec batteries are 4,000mA but the system will accept 3 instead of 2 for a total of 12,000mA. If they were in good shape you could get 25 minutes form 12,000mA but as old as they are you’re lucky to see 21 minutes, and only then if your willing to deal with an extremely persistent voltage warning systems that triggers fairly quickly.
You can use only 2 Yuneec 4000mA batteries (8,000mA total) but flight time will drop to about 17 minutes with the same voltage warning irritant.
Instead of using 4,000mA batteries a little exploratory work should turn up a few 5000mA candidates, perhaps even higher capacity. Two good 5,000mA batteries (10,000 mA total) will consistently give you 21-25 minute flight times along with a much delayed (in measured time) voltage warning with a longer usable time period between the first low volt warning and the second, or whatever voltage you use for a “floor”.
Once you get to flying a 920 long enough to experience the voltage warnings, and deal with differences between stock and after market batteries, the voltage warning thing will make more sense than it does now.
Sure, you can use 4,000mA batteries if you want or if you can’t find 5,000’s. But if you can find 5,000’s the $ diff between the 4,000 and 5,000mA batteries is trivial for the flight time difference. You’ve stepped up to a rig that originally listed for $7,500.00+. Don’t skimp on the batteries.