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Solved problems · Electrical · Crew answer

Why isn't my start battery charging when the house bank is?

When one bank charges and the other doesn't, the relay that combines them during charging is usually not closing. Here's how to check its voltages, wiring and ground.

Symptom

The house bank charges from the alternator, charger or solar, but the start battery stays low, or the reverse. A combiner indicator never shows combined.

Likely cause

Many boats charge two banks through an automatic charging relay, voltage sensitive relay or battery combiner. It connects the banks only after one rises to a charging voltage, and disconnects them again when the voltage falls. If the charging bank never reaches the connect voltage, the relay has lost its ground or sensing lead, a fuse in its feed has blown, or the relay itself has failed, the second bank won't charge. A battery that won't hold charge can also look like a charging fault.

Stop now if

A relay, cable or battery is hot, smells, or the battery is swollen or hissing. Turn off charging sources and get help. Turn battery switches off and disconnect charge sources before working on battery wiring.

The fix, in short

  1. Measure both banks while charging. With the engine or charger running, measure voltage at each battery. If the charging bank reaches the relay's connect voltage but the other bank stays at resting voltage, the relay isn't closing.

  2. Know the connect and disconnect points. Most relays close at around 13 V or above after a delay and open again near float voltage. Victron's Cyrix-ct, for example, waits about 10 minutes at 13.0 V but connects within seconds at 13.6 to 13.8 V. Check your relay's own manual.

  3. Check the ground and fuses. Most relays need a negative wire to sense and switch. With power off, check that it's tight and clean, and that the fuses on both battery leads are intact.

  4. Check where the chargers connect. If the alternator feeds the start battery and the relay never sees enough voltage there, the house bank won't charge, and vice versa. Make sure charge sources are wired to the side the relay expects.

  5. Test the batteries themselves. Charge each bank fully on its own and let it rest. A battery that drops quickly under a small load may be failing rather than undercharged.

  6. Consider a DC-DC charger for lithium. A plain combiner isn't suitable between lead-acid and lithium banks. Use a DC-DC charger or the battery maker's recommended method.

Written by Sailor Cruiser crew. General guidance, not an inspection of your boat.

Equipment involved

  • Charging system: Automatic charging relay, voltage sensitive relay or battery combiner, alternator, shore charger, start and house batteries

Related guides

Sources

Background reading we checked while writing this answer. The wording above is our own.

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