Unless your batteries are damaged, there are some techniques you could employ to significantly increase your backup time (how long the battery powers your appliances before it goes flat).
Integrate a battery equalizer
Uneven charging and discharging are reasons why you might not be getting the most out of your batteries. When you have batteries connected in series and any charges to 100%, all others stop charging even if they are just about 70% charged. When discharging, if any goes flat then all others too stop discharging even if they are still at about 70%.
This uneven charging and discharging is because of the batteries not being 100% identical (even if they are of the same brand and specs) and it builds up over time. This not only cheats you out of battery time but also could cause the batteries to wear easily due to over-charging and over-discharging
What a battery equalizer (Battery Management System) does is to ensure that the batteries are charging and discharging at the same rate so you have optimal charging and discharging.