# Battery Charge and Discharge time for constant voltage charge battery, and how to know if its fully charged

In my Project I used below battery and a battery back SMPS.

Battery Type:- Sealed Lead acid , Volt:- 12V, Ah:-7AH Charge parameters:- Constant voltage charge with voltage regulation Standby use 13.6V-13.8V Cycle use 14.1V-14.4V Max initial current 1.4A

SMPS Input 85Vac to 264Vac , 50Hz Ouput :- 13.8V DC Below image from SMPS datasheet

I read the SMPS datasheet , but there is no information of charging and discharging of battery timing.

Load I have connected a load of 20W at +v and -V terminal.

Q 1) I setup everything, Connected the battery, put a multi-meter across battery terminal, voltage on display slowly slowly increasing. At what level I know that my battery is fully charged and I should disconnected it to being overcharged.

Q 2) About Battery Backup time.

 a) If we use formula  Tbck= (Vout* AH)/(op Watt) , i got 4.2hrs. Is it    Right calculation

b) If I use other formula with Battery spec (see attached image)


Consider 3HR rating of 5.2AH. Current= 5.2AH/3HR = 1.73A current it can supply till 3 Hrs. ( right ??) so after 3 Hrs how much voltage of battery will be
0 volt or 10.5V. Since my SMPS cut the battery if its lower than 10.5V.

Q 3) Charging time of battery

Since no information given in SMPS datasheet about charging current or else, can not figure out about charging time of battery. a) Battery is constant voltage regulation charge type and SMPS is charging the battery with 13.8V, then any method to calculate the charging time of battery.