Current = Voltage divided by resistance.

If battery voltage is 12 volts and the typical resistance of the human body from hand to hand is about 1 Megohm then the current that flows is 12 microamps if my math is right.

More if you have sweaty hands but nothing near what it would take to cause heart fibrillation.

Refer to http://www.allaboutcircuits.com/vol_1/chpt_3/4.html for what lethal currents might be.