Why isn't my boat battery charging while the engine is running?
My yacht's diesel starts normally, but the domestic battery monitor still shows discharge with the engine running. After an hour the fridge is still pulling the batteries down. How do I check whether the alternator is faulty or the charge isn't reaching the batteries?
Answer
Your battery may not be charging because the alternator is producing no power, the charging circuit is interrupted, or onboard loads exceed the available output. First measure voltage directly across the battery posts with the engine stopped, then running above idle, to distinguish a charging fault from slow charging or a misleading display.
Check voltage above idle
Set a digital multimeter to DC volts, with its leads in the voltage sockets. Disconnect shore charging and, if practical, isolate solar charging at its proper switch so another source cannot mask the result.
- Engine stopped: a rested, charged 12V lead-acid battery usually reads about 12.6 to 12.8V; around 12.2V indicates roughly half charge, depending on temperature and battery type.
- Engine running: conventional alternator charging commonly raises battery voltage to about 13.8 to 14.6V, depending on the regulator, temperature and charging stage.
- No rise: voltage that stays around 12.2 to 12.6V and falls under load strongly suggests little or no charge is reaching that battery.
Idle speed may be too low. Some Hallberg-Rassy installations specify full alternator charging capability at approximately 1,300 engine rpm, or 1,500 rpm with Yanmar engines; other installations differ. Use the control's neutral-throttle facility as instructed, secure the boat and keep the propeller disengaged.
These figures apply to conventional 12V systems; double the voltages for 24V. Lithium batteries and electronically controlled charging need their specified settings. Battery voltage alone is a poor guide to lithium state of charge.
Inspect the alternator drive and wiring
Stop the engine before touching belts or wiring. Remove jewellery and keep tools clear of exposed positive terminals: battery short circuits can cause burns and fire.
- Check the alternator drive. A missing, loose or slipping belt prevents useful output. Black dust, glazing and squealing suggest slipping; set tension to the engine manufacturer's specification rather than overtightening it.
- Check the charge-warning lamp. On many conventional installations, it illuminates with the ignition on and goes out after starting. A failed bulb or damaged excitation wire can prevent some alternators from charging; modern arrangements vary.
- Inspect the charging connections. Look for loose battery clamps, corroded cable lugs, overheated insulation and damaged engine-negative connections. Check alternator-output fuses and battery switches against the wiring diagram.
Never disconnect a battery or turn its isolator off while the engine is running. The resulting voltage surge can damage the alternator and electronics.