Systems & how-tos
AGM vs lithium batteries for Nordic cruisers
Compare AGM/Gel and LiFePO₄ house banks for Baltic and Scandinavian cruising — usable capacity, charging, cold weather, cost, and when each chemistry fits.
Overview
Choosing a house battery bank is one of the highest-impact systems decisions on a Nordic cruiser. You anchor without shore power, run refrigeration through summer nights, and face cold starts after winter lay-up. AGM/Gel and LiFePO₄ (lithium iron phosphate) solve the same problem with different economics and charging rules.
Start with your loads — see electrical budget at anchor — then pick chemistry to match charging sources and budget.
Usable capacity
| Chemistry | Typical usable depth | Notes |
|---|---|---|
| AGM/Gel | ~50% of nameplate | Deep cycling below 50% shortens life |
| LiFePO₄ | ~80–90% per BMS spec | Battery management system enforces limits |
A 200 Ah AGM bank gives roughly 100 Ah usable. A 200 Ah LiFePO₄ pack often delivers 160+ Ah usable — but costs more upfront and needs compatible charging.
Charging architecture
AGM tolerates standard alternators and many legacy shore chargers. Bulk/absorb/float profiles are widely understood.
LiFePO₄ needs a BMS and chargers that respect cell limits. Many Nordic refits addenda:
- Dedicated DC-DC charger from alternator (not raw alternator-to-lithium alone)
- MPPT solar with lithium profile
- Shore charger with explicit LiFePO₄ mode
Mismatch causes early BMS trips or shortened cell life — not a plug-and-play swap.
Cold weather and Baltic seasons
Nordic cruising adds constraints:
- AGM loses capacity in cold but is forgiving if kept charged over winter
- LiFePO₄ may restrict charge below ~0 °C unless the pack has low-temperature protection or heating
- Start batteries should stay separate — do not combine engine start with house lithium without engineered design
After haul-out, maintain charge or disconnect — see energy budget planning for winter notes.
Cost and lifespan (orientation)
| Factor | AGM/Gel | LiFePO₄ |
|---|---|---|
| Upfront cost | Lower per Ah | Higher per Ah |
| Cycle life | Hundreds of deep cycles | Thousands at partial SOC |
| Refit labour | Often drop-in with charger check | BMS wiring, charger upgrades |
| Resale story | Familiar to surveyors | Document installation and invoices |
On a boat you keep 5+ years with heavy anchoring, lithium total cost of ownership can win. For marina-heavy seasons with shore power, AGM may be enough.
Decision checklist
- Model daily Ah consumption at anchor (energy budget)
- List charging sources: alternator hours, solar W, shore power, generator
- Confirm charger compatibility before ordering lithium
- Keep start battery isolated
- Budget monitoring — shunt-based SoC beats voltage guessing on both chemistries
FAQ
Can I mix AGM and lithium in one bank?
No. Parallel mismatched chemistries causes charging chaos. Hybrid setups use separate banks with explicit combiner logic — design, don't improvise.
Will lithium alone fix my alternator problem?
No. Weak alternators and undersized wiring limit both chemistries. Fix charging first.
Next steps
Build your numbers in electrical budget at anchor or book advisory for a refit on a specific yacht.