01
Sizing on the best trip, not the ordinary one
A bank sized for the once-a-year fortnight in the Whitsundays is money sitting idle for eleven months. Size for the ordinary trip and manage the exceptional one with the engine.
Coastal cruiser · two aboard · 12 V · one engine
38 ft cruising yacht Lithium Battery System
Two people aboard, coastal passages with a fortnight at anchor at a time, a 12 V system, one engine, and a fridge that is the largest single consumer on the boat. This is the profile we see most often in Australian cruising fleets, and the numbers below are the starting point we would argue for.
Typical load profile
Fridge, 45% duty54 Ah
Freezer36 Ah
Lighting12 Ah
Instruments + plotter11 Ah
VHF + AIS standby11 Ah
Fans10 Ah
Devices11 Ah
Water + shower pump4 Ah
Fridge duty cycle assumed at 45% in Queensland summer, which is where most owners underestimate. Windlass, autopilot on passage and the inverter are intermittent and covered by the reserve rather than the daily figure. Row figures are rounded; the calculator sums these rows to 149 Ah.
What we would fit · 400–600 Ah
Budget build
$6,400
400 Ah drop-in LiFePO4, 12 V
30 A DC-DC charger
Lithium-profile 40 A mains charger
Shunt monitor with Bluetooth
Re-terminated bank cabling and fusing
Premium build
$11,800
600 Ah with CAN telemetry to the plotter
60 A DC-DC plus alternator temperature sensing
60 A lithium mains charger, galvanic isolation
Full battery monitor with logging
New sealed and vented compartment, certified
Installed prices, Australian east coast, September 2026, including labour and certification. Cells are roughly half of either figure — the rest is the charging system, and it is not the part to save on.
On this hull specifically
Battery installation mistakes for this boat type
02
Underestimating the fridge in summer
The duty cycle owners quote is usually the one from the temperate month. In a Queensland summer at anchor it is closer to 45%, which on this boat is a third of the daily draw.
03
Reusing 25-year-old cable runs
The original sizing was for a lead-acid bank that could never deliver these currents. The cable, the fusing and the terminations all need to be reconsidered, and this is where corners get cut.
04
One big bank in a compartment built for two
A single 600 Ah case is often the wrong answer on a 38-footer. Two banks fit the space, balance the trim and leave you sailing home if one BMS disconnects.
Other hulls