Piezo transducers are I think an irrelevance here - the idea is applying a high frequency current to a suphated lead-acid battery to help break down the lead-sulphate crystals - I don't know if this really works but you'd need to modulate the charging current directly at these high frequencies so a very fast MOSFET switch is probably needed, something like 50ns switching times at several amps or such.
The timer units can generate upto 8MHz from a 16MHz clocked Arduino, the period being a multiple of 2 or more 62.5ns periods (8, 5.33, 4, 3.2, 2.67, 2.29, 2MHz). Such a signal would need to be boosted through a high speed MOSFET driver chip to a high speed MOSFET switch (you need a MOSFET capable of carrying the current needed without overheating, but with as low a gate capacitance as possible to achieve this).
void setup ()
pinMode (9, OUTPUT) ; // use pin 9 for timer1
TCCR1A = 0x82 ; // setup programmable frequency and duty cycle mode
TCCR1B = 0x19 ; // fastest clocking, full clock rate
ICR1 = 0x0001 ; // set TOP - the counter cycles from 0 to this value then wraps again, here 0 to 1 to 0 to 1 etc
OCR1A = 0x0000 ; // set threshold, should be ICR1's value integer divided by 2
TCNT1 = 0x0000 ; // set starting count to 0
Will give 8MHz on pin 9. For 5.33MHz you'd change ICR1 to 2, for 4MHz you'd change ICR1 to 3, OCR1A to 1