I don't think you can do that.
The E520S is using proprietary battery. Meaning we have to use/buy "their" battery if we want to fly "their" drone.
If it were easy to hack it, Eachine would not have bothered to make it proprietary.
I also own a E520S for about 4 years now. Its battery still gives me 10 to 12 minutes flight time.
Is that not enough for you? Assuming you are looking for a longer flight time battery.
If you just want to spend less for extra battery, you are out of luck.
Sorry to hear that.
Maybe it just your bad luck getting 2 faulty batteries. As I said before, mine ( 2 of them ) are still giving me 10 to 12 min of flight time.
One question :
When you got "low battery" signal while flying the drone, did you land the drone right away and change battery, or did you ignore the signal and continued flying until battery was badly drained?
If you did the latter, that may be the reason why your batteries did not last.
1. At the end of each flight, do NOT charge the battery right away. Let it cool down first (10-15min), then recharge it.
2. Not a good idea to fly a drone in cold weather. Bad for the battery life span.
3. Store the batteries in room temperature.