11.1v indicates that the battery is flat.
I understand that the new Spring controllers have reset their cutout figure to 10.5v without issue.
Before replacing the battery ask:
What has changed recently? In other words, did your battery used to last but it doesn't now? If so, are you doing something different to before, such as not charging your battery as regularily as you used to?
How old is the battery? What size is it (amp hour rating)?
Our Shurflo pumps will draw between 3 and 5 amps per hour depending on what our controller settings are. If we are working our pumps for 5 hours working time, then we are going to use 25amps of charge from our battery. If we do this for 3 days an 85 amp leisure battery will theoretically last us 3 days before going flat.
However, if I have been hammering the battery like that I would recharge it every night.
When a nearly flat battery is left to stand, it will show a higher voltage on your tester and will drop voltage the moment it has any load put on it. Years ago, working a battery, letting it recover, and working it some more was a sure way of killing it off quickly.
Sometimes a battery's capacity also drops off. You can recharge a flat battery, shows fully charged on the tester or battery charger, but won't last long. In the case, best to replace the battery.
Most car batteries aren't 'sealed'. They have screw caps which enable you to top up the water. If you take all the caps off and find any chambers have low water, top them up - water must be just over the plates.
If I had to replace a battery, my personal choice is a leisure battery.
Spruce