There are many issues regarding zapping.
If done the right way you can increase the performance of a battery. By how much, that is questionable and very objective. I only go by what our equpment tells me and more important by what the people that use our batteries report to us... However it depends at what you are looking for. Some people just zap to get higher voltage reading right after the zap and this will fade out after a few cycles. If the process is done right, you can actualy improve the overall performance of the battery.
To zap the right way you need to know several factors including the quality of the cell, the IR, and electrode potential. If you know these you can select different zapping levels (energy) to take advantage of the endothermic properties of the cell and alter the internal compsition. This can cause minimal to substantial changes.
Good companies like ProMatch, SMC, Axxis Racing, perform continous R&D to find better ways to zap and have designed and built their own equipment. Zapping s a very generic name...because it can be done in different ways.
Some people say that zapping at higher voltages is better than at lower volyages, but the issue is that you are transfering large amounts of energy at a very fast time (less than 300 miliseconds) Voltage is not realy important as what you are transfering (zaping) is related to joules (or watts per second). It just happens to be that it is easier to design and build a high voltage zapper than a low voltage one...
Is it safe to use...well as long as you know what you are doing it is safe, but precautions need to be taken to avoid accidents. With zappers like the DCX3300 yo do not have to worry as the energy levels are very low compared to what professionals use.
We can take a battery at room temperature and get it up to 170'F within 45 seconds... that shows you the amount of energy that can be delivered. Also, afuuly discharged battery can be zapped multiple times (less than 30 seconds) and thereafter if you discharge the cell you can see a very high and continous voltage for about 25-35 seconds and then it just drops to 0 immediately... This is part of the design process and we do this to see how zapping actualy affects the batteries. We destroy clopse to 100 cells ever so often just experimentnig...Off course when we do this we damage the cells, but only through continous experimentation we find better way to zap and get the batteries to perform better.
I hope this will help you.
Some people call zapping voodoo magic, but we know that there is much more to that. Knowing the way a battery is built and how it reacts to changes in energy is the only way to figure out the best way to zap a battery and obtain better performance out of them