The desulfator is giving an output of 54v accross C6 with charger. 47v without charger.We could not find a hydrometer to measure the specific gravity.The change in PWM duty cycle percent is a good fix. You should replace the pot with a fixed resistor for reliability's sake.
It didn't occur to me before to recommend a power switch, which it needs. The easiest way would be to simply break the circuit at R3.
Don't expect quick results. Depending on the size of the battery, the output of your desulphator and the extent of battery sulphation, It may take a month or more before the battery is restored.
One good way to track the process is to measure and record the specific gravity of each cell during the process. Heavily sulphated cells will have a much lower specific gravity, even down to 1.0 - which is plain water. As the desulphator works on it, you should slowly see the specific gravity increase to around 1.255-1.265 - that is, if you have a trickle charger connected across C4.
Another way to track the progress is the peak voltage output of the desulphator. You can measure the peak voltage across capacitor C6. When the battery is heavily sulphated, the peak voltage output will be rather high. As the plate sulphation is removed, the peak voltage output will decrease, depending on how short/large your wires from the board to the battery terminals are.
Note that during the desulpation process you should subject the battery to a heavy discharge cycle, and then re-charge it - every four to seven days. Otherwise, dendrites of lead may form between the plates creating a short, and make the battery unusable. The heavy discharge cycle will burn up dendrites that are forming.
The battery we are trying to restore is a 50AH car battery which was out of use for about a year.
We have subjected the battery to a deep discharge. Is dendrites formation
an outcome of desulfation process??