Battery bank

The house battery bank stores energy for domestic and navigation loads; a separate start battery provides high current to crank the diesel. Traditional flooded lead-acid batteries require venting and water. AGM batteries are sealed lead-acid with less maintenance but still lose capacity when repeatedly deeply discharged. Lithium iron phosphate (LiFePO4) provides more usable capacity, less weight and high charge acceptance, but must be protected by a battery-management system (BMS) that monitors cell voltage and temperature and can disconnect unsafe charging or loads.

Amp-hours (Ah) describe charge, not energy by themselves; at 12.8V, 400Ah is about 5.1kWh nominal. Usable energy depends on chemistry and operating limits. A shunt measures current into and out of the bank so a monitor can estimate state of charge. The enclosure, restraints, terminal protection, fuses, busbars, cables and charging sources are part of a safe installation even though their costs sit mostly under electrical.

Loss of the house bank can remove autopilot, AIS, radar, communications, pumps and lights. Excessive voltage drop makes apparently charged batteries unusable. Lithium brings a different failure mode: a BMS may intentionally disconnect the bank, so essential navigation and alternator systems need a planned response. A short across a high-capacity bank releases destructive current. Reliable capacity and redundancy matter more than the advertised Ah number.

Service flooded batteries with water and clean/protect terminals; equalize only when the manufacturer permits. A bad cable or shunt setting can imitate a bad bank. Replace lead-acid cells as a matched bank rather than mixing markedly different ages/capacities. In a modular lithium bank, manufacturer guidance may permit a cell/battery replacement after state-of-charge matching, but first identify why it failed.

Changing AGM to lithium requires an energy design: BMS, main protection, load/charge contactors or compatible integrated BMS, alternator current/temperature control, shore/solar programming and a start-bank strategy. Drop-in lithium can be appropriate, but “drop-in” does not mean the old alternator and charger are automatically safe.

• Covered by a standard survey — chemistry, approximate age from labels/invoices, voltage, visible cases/terminals, restraints, venting, main switches and basic charging voltage. Some surveyors apply a short load but not a full capacity test.
• Not covered — 20-hour capacity testing, per-cell lithium data, BMS event logs, alternator heat/load behavior and detailed charge programming. A marine-electrician capacity/system test ($500–$1,500) is worthwhile before purchase when the bank is a material part of the price, lithium integration is unclear or endurance claims drive the cruising plan.

• The broker or owner — “Send purchase invoices and exact model/serial for every battery, BMS, shunt and charger.” “Provide the current BMS settings/event history and a recent controlled capacity test.” “Show how charge and load disconnects work, how the alternator is protected, and how the engine starts after a house-bank shutdown.”
• The surveyor — “Identify actual chemistry/capacity, inspect restraint and terminal protection, record resting and charging voltage, and flag whether the installation needs an electrician-led lithium audit rather than assuming the labeled Ah is usable.”

Check case dates and invoice dates. Look for swelling, cracks, acid, corrosion, loose restraints and exposed positive terminals. With a monitor, compare current against known loads and see whether state of charge behaves plausibly. Start the engine and switch on inverter/other heavy loads while watching voltage. For lithium, connect to the manufacturer app if authorized and record cell-voltage spread, temperatures and alarms. A pristine display does not prove capacity; only a measured discharge test does.