This is a known bug in the 126.96.36.199 firmware that was fixed in the 188.8.131.52 firmware release. The bug is caused by ADC reads not working when the UART is closed which always returns a 0. If upgrading to v184.108.40.206 or newer is not an option,, there is a potential work around you which is to use a recurring timer to open the UART, read the ADC, and then close the UART. This will save more power than keeping the UART continuously open but will still use more power than desired. Unfortunately this workaround will introduce another issue from a bug in the 220.127.116.11 firmware which will cause the BL600 to rollover after roughly 12 days of use. The work around for this bug is to reset the module after 10 days of use.