Battery Types and Selection

Flooded Lead-Acid — The Workhorse You Know

Flooded lead-acid batteries have been powering boats for over a century, and they remain the most common battery type in the recreational fleet for one reason: they are cheap. A quality Group 27 deep-cycle flooded battery costs $100–$180, and you can buy them at any marine store, auto parts shop, or Walmart in any coastal town on the planet. When you're stuck in a remote anchorage and need a battery, flooded lead-acid is what you'll find.

The construction is straightforward. Lead plates sit in a liquid electrolyte of sulfuric acid and distilled water. During discharge, a chemical reaction between the lead plates and the acid produces electricity and converts the plate surfaces to lead sulfate. Charging reverses the reaction, restoring the plates and the acid concentration. This process also electrolyzes water into hydrogen and oxygen gas — which is why flooded batteries require periodic water topping and produce explosive gas during charging.

Usable capacity is approximately 50%. A flooded lead-acid battery rated at 100 amp-hours (Ah) should never be regularly discharged below 50 Ah remaining. Deeper discharges dramatically shorten cycle life. A battery discharged to 50% depth of discharge (DOD) on every cycle will deliver roughly 300–500 cycles before capacity drops below 80% of its original rating. Discharge to 80% DOD regularly, and you'll get 100–200 cycles — perhaps a single season. This 50% usable capacity rule is the single most important fact about lead-acid batteries, and it means you need twice the nameplate capacity to get the usable amp-hours you actually need.

Charging flooded batteries is slow. The bulk charge phase (roughly 0–80% state of charge) accepts current well, but the absorption phase (80–100%) tapers dramatically as the battery approaches full charge. A full charge cycle from 50% to 100% takes 6–10 hours depending on charger output and battery size. Flooded batteries also require a periodic equalization charge — a controlled overcharge at elevated voltage (typically 15.5–16.0V for a 12V bank) that boils the electrolyte to mix it and drives sulfate off the plates. Skip equalization for too long, and the cells become unbalanced, reducing capacity permanently.

Maintenance is real and ongoing. You must check and refill electrolyte levels with distilled water only — tap water contains minerals that contaminate the plates and destroy the battery. Terminal connections corrode from acid mist and must be cleaned and protected. The battery compartment must be ventilated because hydrogen gas produced during charging is explosive at concentrations above 4% in air. None of this is difficult, but it is non-negotiable. An unattended flooded battery is a dying battery.

AGM — Sealed, Maintenance-Free, and Tough

Absorbed Glass Mat (AGM) batteries represent the most popular upgrade from flooded lead-acid in the marine market, and for good reason. The electrolyte is absorbed into fiberglass mat separators between the plates rather than sloshing freely as a liquid. This construction makes AGM batteries sealed, spill-proof, and maintenance-free — no water to add, no acid mist to corrode terminals, and dramatically less hydrogen gas production. For boat owners who don't want to think about battery maintenance between haul-outs, AGM is the path of least resistance.

Performance advantages over flooded are meaningful. AGM batteries accept charge current faster — the bulk charge phase pushes more amps into the battery in less time, which matters enormously for sailors relying on limited engine run time or solar panels to recharge. A well-designed AGM bank can reach 80% state of charge in roughly half the time of an equivalent flooded bank. Internal resistance is lower, meaning less energy is wasted as heat during charging and discharging. AGM batteries are also vibration resistant — the compressed glass mats immobilize the plates, preventing the physical damage that kills flooded batteries in engine compartments or rough offshore conditions.

Usable capacity is still approximately 50%, same as flooded lead-acid. AGM shares the same fundamental lead-acid chemistry, so the 50% DOD recommendation for long cycle life applies equally. Some AGM manufacturers advertise deeper discharge tolerance, and while AGM does handle occasional deep discharges better than flooded (the plates don't shed material as readily), habitual deep cycling still shortens life dramatically. Expect 400–800 cycles at 50% DOD from a quality marine AGM battery — meaningfully better than flooded, but not transformative.

The cost premium is 2–3x over flooded. A Group 31 AGM battery runs $250–$450 compared to $120–$200 for a comparable flooded unit. This premium buys you freedom from maintenance, better vibration resistance, faster charging, and somewhat longer cycle life. For many weekend and coastal sailors, this tradeoff is worthwhile. For budget-constrained boat owners who are disciplined about maintenance, flooded batteries deliver equivalent energy storage at half the cost.

Charging AGM batteries requires correct voltage settings. AGM is sensitive to overcharging — the sealed construction means you cannot replace water lost to electrolysis. Most quality marine chargers and alternator regulators have an AGM-specific charging profile with a lower absorption voltage (typically 14.2–14.4V for 12V systems vs. 14.6–14.8V for flooded). Using a flooded battery charging profile on AGM batteries will overcharge them, causing internal gas pressure, venting through the safety valves, water loss that cannot be replaced, and premature death. Always verify your charger and alternator regulator are set for AGM before installing these batteries.

Gel Batteries — The Specialist Choice

Gel batteries use a silica-based gelling agent mixed with the electrolyte to create a thick, paste-like substance instead of a free liquid. Like AGM, gel batteries are sealed, spill-proof, and maintenance-free. They produce even less gas than AGM and have excellent deep-cycle characteristics — gel batteries tolerate repeated deep discharges better than either flooded or AGM, with 500–1,000 cycles at 50% DOD in premium models. For dedicated deep-cycle house bank applications, gel has genuine advantages.

The critical limitation is charging voltage sensitivity. Gel batteries are extremely intolerant of overcharging. The gelled electrolyte can develop permanent voids and cracks if subjected to voltages even slightly above the manufacturer's specification — typically a very narrow window around 14.0–14.2V absorption voltage for a 12V system. Once the gel cracks, it cannot heal, and the affected area of the plate loses contact with electrolyte permanently, reducing capacity. This sensitivity means every charging source must be precisely calibrated, and generic "lead-acid" charging profiles will destroy gel batteries.

Gel batteries charge more slowly than AGM. The lower maximum voltage and the gel's higher internal resistance mean that charge acceptance in the bulk and absorption phases is slower. For a boat that depends on limited charging time — engine run time at anchor, for example — this slower charge rate is a real disadvantage. You'll run the engine longer to achieve the same state of charge compared to an AGM bank of equal capacity.

Market reality: gel batteries are rarely the best choice for new marine installations. AGM provides similar sealed, maintenance-free operation with faster charging and less voltage sensitivity. Lithium LiFePO4 provides dramatically better usable capacity and cycle life for the weight. Gel occupies a middle ground that doesn't win on any single metric for most boats. You'll still find gel batteries on older European-built boats (they were popular in the European marine market before AGM became widely available) and in specific applications like emergency lighting and float-service standby systems. If you're building a new bank from scratch, AGM or lithium will almost certainly serve you better.

Lithium LiFePO4 — The Performance Revolution

Lithium iron phosphate (LiFePO4) batteries have fundamentally changed the math of marine electrical systems. Every claimed advantage is real, and they are not exaggerated: LiFePO4 batteries deliver 80–90% usable capacity (compared to 50% for lead-acid), weigh one-quarter to one-third as much for equivalent usable energy, charge faster and more efficiently (round-trip efficiency of 95–98% vs. 80–85% for lead-acid), and last 3,000–5,000 cycles at 80% DOD — roughly 10 times the cycle life of AGM. A single 200Ah LiFePO4 battery provides more usable energy than a 400Ah lead-acid bank at less than half the weight.

The discharge curve is remarkably flat. A lead-acid battery's voltage drops steadily as it discharges — from 12.7V at full charge to 12.0V at 50% and 11.8V at empty. Equipment powered by lead-acid sees progressively lower voltage as the battery depletes. A LiFePO4 battery maintains approximately 13.0–13.2V across nearly the entire discharge range, dropping off sharply only in the last 10% of capacity. This means every device on your boat receives consistent, stable voltage from full to nearly empty. Lights stay bright, electronics operate at full performance, and the refrigerator compressor runs efficiently until the battery is genuinely depleted.

Every LiFePO4 battery requires a Battery Management System (BMS). The BMS monitors individual cell voltages, balances charge between cells, limits charge and discharge current, and protects against over-temperature, over-voltage, under-voltage, and short circuits. Without a BMS, lithium cells can be damaged by overcharging (thermal runaway risk) or over-discharging (permanent capacity loss). The BMS is not optional — it is a safety-critical component. Quality marine LiFePO4 batteries include an integrated BMS; if you're building a custom bank from individual cells, specifying and installing the correct BMS is essential.

Drop-in vs. custom installations. "Drop-in" LiFePO4 batteries are designed to be direct replacements for lead-acid — same case dimensions, same terminal configuration, integrated BMS. They're the easiest path to lithium: remove the old batteries, install the new ones, reprogram your chargers, and go sailing. The limitation is that drop-in batteries rely on internal BMS communication, which may not interface cleanly with your existing charging sources. Custom installations use individual prismatic cells, an external BMS, and purpose-built wiring to create a system specifically designed for your boat's electrical architecture. Custom systems offer better monitoring, more flexibility, and higher performance — but they require significantly more electrical knowledge to design and install correctly.

Temperature sensitivity is real. LiFePO4 cells cannot be charged below 0°C (32°F) — attempting to do so causes lithium plating on the anode, which permanently damages the cell and creates a safety risk. Most marine BMS units include a low-temperature charge cutoff that prevents this. Discharging in cold temperatures is less problematic, but capacity is reduced. For boats that winter in northern climates, this means the lithium bank either needs a heating system or must be isolated from charging sources when temperatures drop below freezing. This is a solved problem — heated battery blankets and BMS-controlled heaters are common — but it's a consideration that lead-acid owners never face.

Cost Per Cycle — The Math That Changes Your Mind

The sticker price of a battery tells you almost nothing about its actual cost. What matters is the cost per usable kilowatt-hour per cycle — how much you pay for each unit of energy across the battery's entire life. This calculation reverses the apparent cost advantage of lead-acid and reveals why lithium is the cheapest option for anyone who uses their batteries regularly.

Example calculation for a 400Ah 12V house bank. A flooded lead-acid bank (four 100Ah Group 27 batteries) costs approximately $500, provides 200Ah usable capacity (50% DOD), and delivers 400 cycles. Total usable energy over its life: 200Ah x 12V x 400 cycles = 960 kWh. Cost per kWh: $0.52/kWh. An AGM bank of equivalent size costs approximately $1,200, provides 200Ah usable at 600 cycles. Total usable energy: 1,440 kWh. Cost per kWh: $0.83/kWh — AGM is actually more expensive per cycle than flooded despite the longer life, because the purchase price premium is larger than the cycle life gain.

Now lithium. A 200Ah LiFePO4 bank (which provides the same 160–180Ah of usable capacity as the 400Ah lead-acid bank) costs approximately $1,500–$2,000. At 3,000 cycles and 80% DOD: 160Ah x 12V x 3,000 cycles = 5,760 kWh. Cost per kWh: $0.26–$0.35/kWh. Lithium delivers each usable kilowatt-hour at roughly half the cost of flooded lead-acid, and at a fraction of the weight. The upfront cost is 3–4x higher, but you buy lead-acid batteries 5–8 times over the lithium battery's lifespan.

The breakeven point depends on usage. A weekend sailor who cycles the bank 50 times per year reaches the breakeven point in approximately 3–4 years — after which the lithium bank is saving money on every cycle for the remaining 15+ years of its life. A liveaboard who cycles daily (365 cycles/year) reaches breakeven in under 2 years. A seasonal sailor who uses the bank 20 times per year may take 6–7 years to break even, at which point the value proposition is still strong but less compelling if cash flow matters more than lifetime cost.

What the math doesn't capture is the value of reduced weight (critical for sailing performance and stability), consistent voltage to electronics, faster charging (less engine run time, less fuel, less noise at anchor), and zero maintenance. For cruising sailors, the fuel savings alone from reduced engine charging time often cover a significant portion of the lithium premium within the first year.

Choosing the Right Battery for Your Boat

Battery selection is not a question of which chemistry is "best" — it's a question of which chemistry matches your sailing profile, your budget, and your willingness to maintain the system. The right answer for a weekend harbor sailor is different from the right answer for an offshore cruiser, and both are different from what a marina-based liveaboard needs.

Weekend and coastal sailors who spend most weekends on the boat with regular shore power access between trips are well served by AGM batteries. The maintenance-free operation means the batteries don't suffer from neglect during the work week. The moderate cycle count (400–800 cycles) is more than adequate for 50–100 cycles per year. The faster charging helps recover the bank during day sails when solar is the primary charge source. If budget is tight, flooded lead-acid works perfectly well for this use case — but you must commit to checking water levels and terminal condition regularly. Cost: $800–$1,500 for a typical house bank.

Coastal cruisers who spend extended weekends or weeks aboard, anchor frequently, and rely on the house bank for refrigeration, lighting, instruments, and charging devices benefit significantly from lithium LiFePO4. The higher usable capacity means a smaller, lighter bank provides equivalent or greater energy. The faster, more efficient charging means less engine run time at anchor — often cutting daily charging from 2–3 hours to 30–60 minutes. The flat discharge curve keeps the refrigerator running efficiently all night. The 3,000+ cycle life means the bank lasts a decade or more. Cost: $2,000–$4,000 for a typical house bank, but the reduced engine run time saves fuel and noise that add up over seasons.

Liveaboards and long-range cruisers who depend on their electrical system every day, often in remote locations without shore power, should seriously consider lithium LiFePO4 with a properly designed solar and alternator charging system. The daily cycling that destroys lead-acid batteries in 1–2 years barely registers on a lithium bank's 3,000+ cycle capacity. The weight savings — often 50–100 kg (110–220 lbs) for an equivalent energy bank — matters for ocean passages. The efficiency gains compound daily. The cost-per-cycle analysis overwhelmingly favors lithium at this usage level. Cost: $3,000–$8,000 for a complete lithium system including BMS, smart alternator regulator, and properly configured charging sources.

The start battery is a separate decision. Regardless of house bank chemistry, many experienced cruisers maintain a dedicated AGM start battery that is isolated from the house bank and kept at 100% charge by a DC-DC charger or echo charger. The start battery's only job is cranking the engine — it needs to deliver high current for a few seconds, not deep-cycle capacity. AGM excels at this: high cranking amps, vibration resistance, sealed construction, and no maintenance. Keeping the start battery separate and fully charged means you can always start the engine, even if the house bank is depleted. This is not luxury — it's a fundamental safety practice.