I don't know about BTY as a brand, or whether they are known to make good batteries.
I bought BTY 2500mah batteries on Ebay Item number 390223264207 from 4udiscount724 . Very good price.
When I received them, I was surprised how light they are - only 13 grammes each. Other Uniross and similar batteries I already have weigh about 26 grammes each.
When a battery is charged and discharged for the first time, the capacity may be much lower than after being 'exercised' a few times. Particularly with 'unbranded' batteries.
I have two battery computers. They charge and discharge batteries until the power delivered on each charge levels off and the batteries are in a state where they deliver as much charge as they ever will.
I have so far tested 8 of the 2500mah batteries. After charging and discharging until their performance levelled off, they delivered the following capacities:
314mah, 293mah, 280mah, 298mah, 319mah, 294mah, 352mah, 300mah.
i.e. Nothing like the claimed capacity of 2500mah. Less than one eighth of the claimed capacity.
I have emailed the seller telling them my findings.