01
Believing the fridge’s rated amps
The compressor’s plate says 5 A. Its duty cycle in February at anchor is the number that matters, and it is closer to half than a third.
Overnighter · two to four aboard · 12 V
Half cabin Lithium Battery System
The half cabin is where boats start spending nights on the hook and where lead-acid banks start dying young. A fridge running overnight, cabin lights, a plotter at dawn, and a single outboard that charges when it is running and not otherwise.
Typical load profile
Fridge, 40% duty40 Ah
Plotter and sounder12 Ah
Cabin and anchor lights8 Ah
VHF standby and stereo8 Ah
Pumps and device charging7 Ah
One night at anchor, with the fridge doing most of the work. Two nights with no engine time is where 100 Ah of AGM fails and 100 Ah of lithium is merely tight.
What we would fit · 100–200 Ah
Budget build
$2,100
100 Ah drop-in LiFePO4, 12 V
25 A DC-DC charger from the outboard
Lithium-profile 20 A mains charger
Fused house distribution
Battery box, ventilated and restrained
Premium build
$4,400
200 Ah LiFePO4 with low-temp protection
40 A DC-DC charger
200 W panel on the hardtop with MPPT
Shunt monitor at the helm
Certified install to AS/NZS 3004.2
Indicative installed prices, September 2026, east coast. Solar on the hardtop is the single best dollar spent on this hull: it turns a one-night boat into a weekend boat.
On this hull specifically
Battery installation mistakes for this boat type
02
Charging lithium off a 60 A outboard directly
Most outboards below 150 hp have small, internally regulated alternators. A DC-DC charger set to 25 A is the difference between a charged bank and a rebuilt stator.
03
No monitor
Lithium’s flat voltage curve means the helm voltmeter reads 13.2 V from full to nearly empty. Without a shunt you are guessing, and the guess is wrong at 3 am.
04
One bank doing both jobs
Starting the outboard from the house bank is a warranty and a BMS question. Keep a small lead-acid cranking battery.
Other hulls