Hitachi EX150 Battery Relay Wiring Explained

[MikePhua]

   


Overview
On the Hitachi EX150 excavator, the battery relay acts as a safety switch—isolating nearly all electrical circuits when the machine is not running, except for the ignition circuit. It also enables alternator charging once the engine is running.
Wiring Configuration

• The battery’s positive terminal connects to one large terminal on the relay—this also includes a red fuseable link that feeds the key ignition switch.
  
• The other large terminal serves the alternator/starter circuit and is connected via a white wire, typically going to the alternator’s output or starter.
  

User Confirmation
One technician confirmed this setup after tracing and testing:

• Red wire → key switch (left side of the relay alongside the battery + cable)
  
• White wire → alternator output (right side of relay, alongside starter feed)
  

Additional community feedback supports this arrangement, noting that 24V appears correctly across both large relay terminals when the engine is running. A brittle connector wire was identified and repaired, restoring battery charging.
How It Works

• Ignition On: The red fuseable link energizes the relay coil, allowing battery power to flow from the alternator/starter circuit and energize the vehicle’s electrical system.
  
• Engine Running: The alternator generates voltage (around 26–28V), feeding back through the relay to charge the battery and supply electrical circuits.
  

Quick Checklist for Wiring & Troubleshooting

• Confirm red link goes to ignition circuit.
  
• Confirm white link ties to alternator and starter output.
  
• Ensure both batteries are grounded properly.
  
• Check for corrosion or damage in fuseable links and connectors.
  
• Measure voltages:
  • Battery (Engine OFF): ~24V
    
  • Battery (Engine RUNNING): ~26–28V
    

Small Story
One user found their EX150 wasn’t charging. Upon inspection, the alternator output wasn’t reaching the relay due to a brittle connector. A simple splice repair restored continuity and the machine began charging normally—highlighting the importance of secure relay wiring.

Summary

• Red fuseable link + battery positive feed the key switch circuit.
  
• White link provides alternator/starter output through the battery relay.
  
• The relay safely disconnects circuits when off and enables charging when running.
  
• Inspect and secure wiring for reliable electrical operation.