If the battery is discharged (too much load on it) and wants to be charged up at 10A, the controller will go into overcurrent mode, i.e. reduce its output voltage to the point where the current is 3.6A. It could be that the battery is just plain faulty. So first of all, disconnect any load from the battery, if the voltage starts to rise, you have fixed the fault (too much load). Then disconnect the battery, does the controller's output voltage reach 26V or there abouts?, if it does then the battery is faulty.
If both the previous tests fail, you could try to reset the output voltage, (its unlikely to work!!). Down at the bottom of the PCB there is a little blue multiturn pot. This almost certainly adjusts the output voltage via the "screw "head on it. So put a tiny blob of paint/ink/scratch on the screw so you can identify its present position, relative to its case. Make a sketch of this, because you might have to go back to this position again. Twiddle the screw and monitor the controller voltage, if you can get it back to 27.6 V, this is about right for a 24V lead acid battery.
Frank