back-up

Hi I build a backup battery to use on my FTTH modem when power loses. I used 3 batteries of following image in serial. I using a 3cell 10A BMS and charge them by 12.6V. For first 1 Hour modem works correctly but after it modem just run into restart loop. but in this state when i measuring...

Having repaired an old Kambrook KD36 timer gifted to me by a friend, I'm sharing my circuit schematic and notes for anyone who might find them useful. The timer had been rendered unserviceable due to acid leakage from the two back-up batteries. The leaking acid had damaged some areas of the PCB...