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Choosing batteries: AGM, gel and LiFePO4

AGM, gel and LiFePO4 compared: usable capacity, charging, BMS and choosing a safe bank.

The inboard engine of a funeral boat in Venice

Choosing batteries starts with separating the jobs: a starter battery (a short, very high current for the starter motor) and a service battery (steady, moderate currents over many hours). These two tasks should not be combined in one battery. The service bank is sized from an energy budget — the daily consumption in Ah plus a reserve for autonomy and for limiting the depth of discharge. It is the depth of discharge (DoD) and the number of cycles that decide the real service life and running cost.

Author: Maciej Baniewicz · Boat & Yard editorial team

Methodology: The material was prepared from the sources given and from practical service rules; the parameters have to be confirmed in the manual for the specific manufacturer or model.

AGM (Absorbed Glass Mat) is a lead-acid battery with the electrolyte held in glass mat: maintenance-free, resistant to vibration and able to accept a relatively high charging current. In practice about 50% of the capacity is used, to keep a sensible service life (typically several hundred cycles). A gel battery has the electrolyte in gel form, tolerates deep discharge, but needs a lower charging voltage and current — overcharging destroys it quickly. Both types are heavier and have a genuinely smaller usable capacity than the nominal figure suggests.

LiFePO4 (lithium iron phosphate) offers far greater usable capacity (safely 80–90% DoD), many more cycles (thousands), lower weight and higher charging efficiency, plus an almost constant voltage under load. It is thermally safer than other lithium chemistries, but it needs a BMS to manage the cells and disconnect the battery when limits are exceeded. The drawbacks are the higher initial price and sensitivity to charging in sub-zero temperatures — below 0°C (32°F) charging LiFePO4 is harmful and has to be blocked.

Moving from lead to lithium means checking the whole charging infrastructure. An engine alternator not designed for lithium can be overloaded, because LiFePO4 accepts a very high current — alternator regulators or current-limiting DC-DC converters are used for this. Mains chargers and solar controllers must have a lithium profile with the correct voltages. Lead charging profiles (with a high equalisation voltage) are unsuitable and dangerous for lithium.

Usable capacity is the key to any comparison. A 200 Ah AGM bank really gives you about 100 Ah, while 100 Ah of LiFePO4 gives about 80–90 Ah at far lower weight and many more cycles. That is why, despite a higher price per nominal Ah, the cost of lithium per kWh delivered over its whole life is often lower. For a rarely used cruising yacht AGM is often enough and cheaper to install; for intensive use lithium usually pays off.

Safety and monitoring are compulsory. Fit a main fuse right at the positive terminal of the bank, a battery switch and cable cross-sections properly sized for high currents. The best knowledge of the bank's state comes from a monitor with a shunt counting actual amp-hours, not from voltage alone — with LiFePO4 voltage is a poor indicator of the state of charge. Ventilate lead batteries (they give off hydrogen when charging) and secure every bank rigidly against moving when the boat heels.

When choosing batteries, use the manufacturers' documentation, which publishes cycle tables against DoD, permitted currents and temperatures, and the required charging profiles. Do not mix batteries of different types, capacities and ages in one bank. Remember the standards and rules for electrical installations on boats (for example the ISO/ABYC guidance on protection and wiring). A properly designed bank with monitoring means years of predictable energy on board.

Material prices — products and options

Indicative prices in EUR (incl. VAT), as of July 2026. Products widely available in Europe; prices depend on the shop, the pack size and consumption. The links lead to example shops that ship across the EU. Dollar amounts are converted at 1 EUR = 1.1622 USD (ECB, 7 September 2026) and rounded — treat them as a rough guide, not a price in a US shop. Imperial values in brackets are conversions of the metric figure.

Service batteries (AGM / gel / LiFePO4)
A separate starter battery
Charger / controller with the profile matching the chemistry
A BMS (for LiFePO4) and a temperature sensor
DC-DC converter for charging from the alternator
A main fuse, a battery switch, large cross-section cable
A battery monitor with a shunt
Mounting brackets/enclosures and ventilation of the battery box

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