Why Fast charge or discharge pushes large currents through the cell's internal resistance, and that I squared R loss turns into heat inside the battery. Heat drives chemical breakdown: lead-acid and NiCd cells vent hydrogen and oxygen gas, and lithium cells can go into thermal runaway and burn or rupture. That is why batteries are used with protection circuits, fuses, and chargers matched to the chemistry and capacity, and why charging areas need ventilation.
Watch out Electric shock is a real concern with high-voltage battery banks, but the specific hazard tied to a high charge or discharge rate on an unprotected cell is heat and vented gas, not shock. Ripple and overvoltage describe power supply output quality, not what happens inside a stressed battery.
Fast current equals heat equals gas. Hydrogen plus a spark is the battery room explosion story.