Hello KC Wu,

We are sorry to hear that you were not completely satisfied with the run time of the 2 combined batteries when used in this lawn mower. 50 minutes when using tow combined batteries would be normal. This unit is advertised as running Up to 70 minutes of run-time* with 4 fully charged 4.0 Ah USB Batteries. It is important to adjust your mowing techniques to make the most out of the battery life, here are a few mowing tips to help achieve the optimal run time.
1) When cutting thick grass, reduce walking speed in order to allow for a more effective cut and a proper discharge of the clippings.
2) Only mow 1/3 of the total grass length at a time - no more. For a healthy lawn, always cut off one-third or less of the total length of the grass. The average lawn should be approximately 1 1/2 to 2” (3.8 to 5 cm) long during cool months, and between 2" to 3 1/4" (5 cm to 8.3 cm) long during hot months.
3) Mow when the grass is dry. As this is an electric unit, mowing wet/damp grass is a safety issue. However, ensuring the grass is dry will also help the grass cut cleanly and discharge appropriately. Wet/damp grass carries more weight and will mostly stick to the blade/motor shaft and will not appropriately discharge or cut easily.
4) Make sure the battery is cool before use. When the battery will not be in use, we suggest storing it indoors, in a cool dry area - away from direct sunlight. Leave the charger unplugged and the battery disconnected from the charger unless it needs charging.
We hope these tips will help. We would be happy to carry on this conversation to offer some additional suggestions, if you are willing, contact Greenworks at 1-888-909-6757 (1-888-90WORKS). All we ask that you have access to the unit so that we can complete troubleshooting with you and gather necessary unit information.