Here is what I received from Opus support:
Dear. Sir.
I think it's time to replace the product's battery.
The symptom is probably due to the old battery.
The battery replacement cost is US$ 39.8
It should be repaired in Korea. There is no other way.
If you send it to our service center in Korea, we’ll check it
After you send the product, reply to your address, phone number and tracking number.
Please send opus except for SD card or leather case.
TheBit co., Ltd.
Room no. 504, Yucheon Factopia, 30 Jeonpa-ro, Manan-gu,
Anyang-si, Gyeonggi-do, Republic of Korea (14086)
Tel : +8228620485
Attn : A/S manager