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 batteries voltage it is about 11.4 and is enough to turn on the modem also in this state when I connecting a 16W car lamp it turns on with full light! so Why the modem is not works?! also I connected a Bench psu to my modem to test it by 10V it works correctly and max amp usage is 0.7A regardless on it label is 1.5A.
I confused! Do I need a voltage booster module? or problem is another thing!
Thanks very much
My modem consume label:

 

Try connecting it to a bench power supply, and varying the voltage around 12V to see if you can replicate the situation.
I would be surprised if it was pedantic about being precisely 12V, as it is likely to be regulated down to 5V and 3.3V inside the unit.

 

By bench power works fine until 9.5V

 

Could be that the modem takes short bursts of higher current that pull the battery voltage below the minimum required.
A large capacitor (e.g. 1,000µF or more) across the battery may help.

 

Could be that the modem takes short bursts of higher current that pull the battery voltage below the minimum required.
A large capacitor (e.g. 1,000µF or more) across the battery may help.

Thank you
I inserted 6800uF capacitor and just worked for 10 minutes! I think need more capacitors.

 

Thank you
I inserted 6800uF capacitor and just worked for 10 minutes! I think need more capacitors.

I used a voltage booster module and problem solved.