Mecer 720W inverter

I send my mecer 720w inverter to Mustek Cape Town branch. They say the Inverter is not faulty and the technician said he will do software update. You can even do a software update. When i received the inverter again it even worse at 75% battery life the inverter makes the noise for low battery and last and hour. I now used a old 720w inverter and work perfect for 4 hours.