It's likely your mowing conditions, not a bad battery. Here's the troubleshooting order:
- Check your grass. Mowing tall, wet, or dense grass cuts runtime drastically. Mow more frequently when grass is dry.
- Check your height. The highest cutting height setting uses less power. If you're scalping the lawn, you're draining the battery.
- Check your pace. A steady, moderate walking speed is most efficient. Running it full-tilt burns power.
- Check the battery contacts. On the mower and the battery. Clean any debris with a dry cloth.
If that didn't work: The battery itself may be degraded. Ensure it's charged with the correct 40V charger. If it still only lasts 10 minutes under normal conditions, the battery likely needs replacement.