3. What is ripple (AC Content) and how can it be limited?
Output ripple is AC, superimposed on the DC output. Displayed on an oscilloscope it looks like the ripples on a pond produced after a stone strikes the surface. It is a constantly fluctuating voltage at a fixed frequency that is rising and falling around the charger nominal DC output voltage. Pure DC, which is exactly what a battery produces, is displayed as a flat line on a scope.
Some batteries won't tolerate any ripple whilst others ignore it and are largly unaffected by its presence. The battery manufacturers specify which cells can or cannot withstand ripple.
Ripple can be eliminated from a chargers output supply by incorporating a combination of inductance (a component which restricts the flow of ac current) and capacity (has the ability to store or release current). The correct combination of these two elements can reduce the output ripple to less than 50 millivolts peak to peak (CCITT).