Lithium vs. AGM: How to Size a Marine House Battery Bank

By Paneltronics, Inc. | Trusted Authority in Electrical Power Distribution Since 1979

What actually changes when you move from lead-acid to LiFePO4, and how to size the bank you really need

A battery monitor turns the house bank from a guess into a number you can plan around.

The house battery bank is the fuel tank of every electrical system on board. It runs the fridge at anchor, keeps the chartplotter alive on a night passage and decides whether you need to start the generator before breakfast. Yet most banks are sized by habit: "the last boat had four batteries, so this one gets four too."

Lithium iron phosphate (LiFePO4) batteries have changed the math. In this article we compare them with AGM, show how to calculate your daily energy use and explain what the panel and charging system need to handle a lithium bank safely.

Rated capacity is not usable capacity

A 400 Ah bank does not give you 400 Ah. Lead-acid batteries (flooded, AGM and gel) should not be discharged much below 50% if you want them to last, so you can count on roughly half of the label. LiFePO4 handles 80–90% depth of discharge every day without losing life.

From the same 400 Ah rating, a lithium bank delivers almost twice the usable energy.

In practice this means a 200 Ah lithium bank can often replace a 400 Ah AGM bank, at about half the weight and space.

Step 1: calculate your daily energy budget

List every DC load, its current draw and how many hours a day it runs. Multiply to get amp-hours per day:

Lithium accepts high current almost to full, which cuts generator and engine run time.

Step 2: compare the chemistries

Lithium accepts high current almost to full, which cuts generator and engine run time.

Step 3: make the rest of the system lithium-ready

Swapping batteries alone is not enough. A lithium bank changes what the rest of the system sees:

• Charging profiles: chargers, alternator regulators and solar controllers must have a LiFePO4 profile, with no float or equalization stage.
• Alternator protection: lithium can pull an alternator to its limit for hours. Use an external regulator or a DC-DC charger.
• BMS disconnects: when the battery management system opens, loads and chargers drop instantly. The panel must handle that without damaging electronics.
• Fusing: lithium delivers very high short-circuit current. Use a Class T fuse at the battery.
• Monitoring: a shunt-based battery monitor at the panel is the only reliable way to read state of charge.

Is lithium worth it?

For boats that spend nights at anchor, run a fridge 24/7 or care about weight, lithium usually pays for itself over its life: fewer replacements, less fuel spent charging and more usable energy per pound. For a weekend boat that lives on shore power, a good AGM bank is still a sensible, lower-cost choice.

Planning a battery upgrade? Our team designs DC panels with integrated battery monitoring, lithium-ready protection and clear load management. Tell us about your boat and we will help you size the whole system.

Load	Draw (A)	Hours / day	Ah / day
Refrigerator	4.5	12	54
Chartplotter + instruments	2.5	8	20
LED cabin lights	1.5	5	7.5
Anchor light	0.3	10	3
Water pump	6	0.5	3
Phones, laptops, fans	3	4	12
Total			≈ 100 Ah

	AGM	LiFePO4
Usable depth of discharge	≈ 50%	80–90%
Cycle life	400–600 cycles	3,000–5,000 cycles
Weight (per 100 Ah usable)	≈ 130 lb	≈ 30 lb
Charging speed	Slows down above 80%	Near full current to 95%
Upfront cost	Lower	2–3× higher
Needs a BMS	No	Yes