Khulna, Aug 25 (UNB) - Acid, lead and other harmful chemicals from over 3,00,000 expired batteries, used in auto-rickshaws and motorised three-wheelers, are posing a serious threat to human health and environment.   There are at least 25,000 easy bikes and 11,000 auto-rickshaws in the city, according to Khulna Easy Bike Kallyan Samity and Easy Bike Chalok Samity. An easy bike uses five batteries while an auto-rickshaw has four.   These batteries remain usable for about eight months. This means, every year, an estimated 2,41,000 batteries are discarded.   Arman Munshi, an easy bike driver, said he sells six to seven expired batteries every year, each weighing 18-19 kg, to businesses at Sheikhpara Batterypatti at Tk 95 to Tk 100 per kg.