Extract taken from one of my battery topics.
A flat battery is any battery continuously registering below 12v, this same battery will continue to give power but is being irepparably damaged in the process. So although you will still be able to take power from the battery, do not be surprised when the battery doesn't last
Now I have not tested this but a 12v pump may be able to operate with a couple of volts less say 10.5v, (pushing it,) this then has the affect of the struggle you hear, from your pump. And the depletion, of the remaining current and voltage continues. Until the voltage, and current, are no longer able to run your pump.
The battery’s I would recommend for the Trolley, Trailer, and van mounts are all the same and I would recommend the leisure battery for your equipment.
All batteries are manufactured to provide a certain number of deep discharge cycles.
The conventional leisure battery should give a cycling life of between 200-300 deep discharges.
A Gel battery will give between 400-500 cycles and an AGM battery 600-800 cycles.
There are manufacturers that sell leisure batteries that deep cycle up to and over 1000 deep discharge cycles
Batteries fail in cycling life due to a permanent encapsulation of PbS04 on the plates (lead sulphation)
The capacity loss is more rapid with these battery types, the deeper that the battery is discharged, the quicker lead sulphation builds up. Upon recharging the battery, not all of this sulphation is converted back to lead (PB0). The result is a gradual loss in capacity.
Another major reason for premature battery failure is due to the oxide shredding from the plates. Standard lead plates are bound only with acrylic and polyester fibres, they do not serve as reinforcement of the plate. The cycling life of the more expensive battery is much greater than all other battery types because they are built using carbon fibre and this eliminates lead sulphation of the plates permanently. Upon recharging all the original capacity will be replenished. Carbon fibre acts as a mechanical reinforcement fibre to the lead grid and paste reducing shredding.