Warranty on phones is a big issue. Voiding it could be disastrous in cases and when your warranty is void, it means you will no longer be able to claim your insurances the right way or ask for a repair or replacement within the free service limit.

Know your warranty rules properly. Is water and mechanical damage covered? Burn in the phone are generally not included in warranty. Using a third party charger could be the major difference that causes the phones to get burnt. The manufacturer specifically manufactures a charger to fit the battery capacity and the terminals of the phone. Using another charger than the one provided means that you may be supplying excess or lower current and charge than normal. While lower than usual charge will consume more time for battery charging, the higher ones will burn your phone's circuits and can cause permanent damage.

Always use the chargers provided and don't use cheap alternatives. Charging via USB is safer but slower.

(AW- Anil)