3 to 4.5 volts AC cycling every 15 minutes, or a DC polarity reversal every 15 mins? I am thinking that you are reffering to the latter in which case a 555 (depending on accuracy required) and a DPDT relay to do the polarity reversal. Can you give any additional infor to help us understand your needs.
sorry its really rough, im on the wifes PC, so no schematic capture progs here. hope this helps
oops, sorry this is alternating the 13.5 volts, your relay would be connected to the 3 to 4.5v supply rather than the 13.5 other than that its exactly the same. Can you tap the 3rd cell in the battery pack for your 4.5v or is it a sealed unit?