Some warmth is normal. Excessive heat is dangerous. Stop charging immediately if it's too hot to touch.
Use only the specified 21.5V, 1.5A charger. Other chargers can cause fire. Charge within the specified temperature range. Do not charge above 40°C (104°F).
If that didn't work: A damaged battery pack can overheat. Inspect for physical damage or leaks. Do not use a damaged pack. Do not service it yourself. Only the manufacturer or authorized service should handle damaged batteries.
Heads up: Never expose a battery pack to fire or leave it in direct sun. Liquid ejection from an abused battery causes burns. Flush with water if contact occurs.