Marine Lithium vs AGM Batteries
Compare LiFePO4 and AGM boat batteries for usable capacity, weight, charging, inverter loads, cost, cycle life and marine installation.
| Factor | LiFePO4 | AGM |
|---|---|---|
| Weight per usable kWh | Usually much lower | Higher |
| Usable depth of discharge | Commonly higher | Usually managed more conservatively |
| Voltage under load | Relatively flat | Falls progressively |
| Charge acceptance | High when system permits | Lower as state of charge rises |
| Large inverter loads | Strong with suitable BMS | Possible but bank may need to be large |
| BMS required | Yes | No electronic BMS in the lithium sense |
| Alternator integration | Requires review | Often simpler in legacy boats |
| State of charge from voltage | Difficult due to flat curve | More informative at rest |
| Up-front cost | Often higher | Often lower |
| System complexity | Higher | Lower |
The important comparison is usable energy
A 400Ah LiFePO4 system may offer a larger practical working energy window than a 400Ah AGM bank, but the exact usable percentage should follow the battery manufacturer's limits and the owner's longevity strategy.
Why lithium can expose weaknesses in an old boat
Lithium batteries can accept charging current strongly. That can reveal problems that were hidden by a slower-charging AGM bank:
- alternator overheating
- undersized cables
- poor terminals
- weak battery switches
- inadequate fusing
- old shore chargers with unsuitable profiles
- inaccurate battery monitors
- excessive inverter current
An AGM-to-lithium conversion should therefore begin with a system audit.
Apply this guidance to your boat
Use this page as one input to a vessel-specific plan. Record the actual equipment, cable runs, charging sources and operating pattern aboard before choosing hardware or changing the installation.
- Measure the real load and its operating time instead of relying on a label or a generic boat profile.
- Check every proposed current against the battery, BMS, protection, cable and connection limits.
- Confirm the result against current manufacturer instructions and the rules that apply to your vessel.
Work out your own numbers
Lithium vs Lead-Acid Payback CalculatorWork out how long a marine lithium battery bank takes to pay back its extra cost against AGM or lead-acid, using your own quotes and datasheet cycle life.How to Compare Marine Lithium BatteriesIndependent framework for comparing marine lithium batteries sold in Australia without relying on advertising claims.
Related pages
Beginner's Guide to Marine Lithium BatteriesNew to lithium batteries on boats? Learn volts, amps, Ah, kWh, BMS, solar, alternators and marine battery sizing from the beginning.What Is LiFePO4 and Why Is It Used on Boats?Learn what LiFePO4 means, how lithium iron phosphate batteries work and why the chemistry is widely used for yacht and boat house batteries.Amp-Hours, Watt-Hours and Kilowatt-Hours ExplainedLearn the difference between amps, amp-hours, watts, watt-hours and kWh so you can compare 12V, 24V and 48V marine lithium systems.Marine Lithium Battery GlossaryPlain-English glossary of marine lithium battery, solar, inverter and boat electrical terms.Marine Lithium Battery Size CalculatorWork out the lithium battery capacity your boat needs from daily kWh, reserve, autonomy, voltage and charging capability.Marine Lithium Battery Authority ResourcesDirect links to Australian maritime, electrical, product-safety, privacy and battery-recycling authorities relevant to marine lithium systems.