Why Your Yamaha 40HP Outboard Is Not Charging the Battery
If your Yamaha 40HP outboard engine runs but the battery keeps dying, the problem is almost always in the charging system. On 2-stroke models, the magneto charging coil is responsible for generating the electrical power needed to recharge the battery while the engine is running.
Understanding how this system works makes troubleshooting much easier.
How the Yamaha 40HP Charging System Works
Unlike automotive alternators, Yamaha 2-stroke outboards use a magneto charging coil mounted under the flywheel. As the engine spins, the flywheel magnets induce electrical current in the coil, supplying power to both the ignition system and the battery.
When this coil weakens or fails, charging output drops significantly.
Common Causes of Battery Charging Failure
- Failed or weak charging coil
- Damaged wiring or corroded connectors
- Faulty rectifier/regulator
- Poor battery ground
Among these, charging coil failure is one of the most common causes, especially on older Yamaha 40HP engines.
Charging system problems often begin with early warning signs. Our guide on Yamaha 40HP charging coil symptoms explains what to look for before complete failure occurs.
How to Fix a Yamaha 40HP Not Charging Issue
Start by inspecting wiring and connectors. If everything else checks out, replacing the charging coil is often the fastest and most reliable fix.
👉 Direct Replacement Option:
This Yamaha 40HP 2-stroke charging coil (replaces OEM 66T-85520-00) is designed to restore proper charging output and stable electrical performance.
Conclusion
A properly functioning charging coil is essential for battery life and reliable engine operation. Replacing a worn coil can immediately solve most charging-related problems on Yamaha 40HP outboards.