Lead-acid batteries come in three basic types and many varieties. Starting batteries like those found in cars have many thin, porous or spongy plates that deliver occasional spikes of high current, down to about 80% of capacity in a short time, immediately followed by a fast and full recharge. Deep cycle batteries used as house batteries on boats have fewer solid lead plates that deliver less current over a longer time and can be discharged down to about 20% of capacity before slow recharging. Hybrid batteries are a compromise that delivers high current for starting and can be discharged down to about half of capacity. Batteries designed for marine use have stronger cases than automotive batteries and their plates are more rugged and more firmly attached. When buying a battery, heavier is always better and the best tend to be black all around with few or no labels or markings.
Wet
cell
Flooded lead-acid batteries, called wet cells, are the most common. They have liquid electrolyte, added through caps on the top, which should just barely cover the lead plates at all times. Some automotive wet cell batteries are maintenance free, having a valve that prevents loss of electrolyte by recombining the gases produced during recharging. Although fine for starting cars, these valve regulated lead acid (VRLA) batteries are less often found in boats because the electrolyte cannot be topped off if any is lost through total discharge, overcharging, or less than optimal recharging. Gel cell
Gel cell batteries have a thick, viscous electrolyte that cannot spill if the battery tips or cracks. Like VLRA wet cells, they are sealed, making it impossible to replace electrolyte that will be lost through gassing if the rate of charge is not precisely correct. Gel cells charge at a lower voltage than flooded cells and can be ruined if used with a wet cell charging system.Absorbed
glass mat
Absorbed glass mat (AGM) batteries have fiberglass mats between the lead plates to hold the electrolyte and prevent spills. These mats make AGM batteries the least susceptible to vibration. They charge at nearly the same voltage as wet cells. AGM batteries are sealed and cannot be topped off if electrolyte is lost during recharging. They take more current, during charging, which can overload the charging system but also means that they charge faster. Both gel cells and AGM batteries can discharge at a higher rate than wet cells, thereby reducing the need for battery capacity. Gel cells also charge slightly more efficiently than wet cells and AGM batteries charge significantly more efficiently. While unsealed wet cells can leak off up to 1% of their charge every day, a VRLA battery loses only about 2% a month.
VRLA
batteries
A valve regulated lead-acid (VRLA) battery is a maintenance-free, sealed lead-acid battery with an immobilized electrolyte (either gel or absorbed in glass mats) and a safety valve system. VRLA batteries offer improved safety, vibration resistance, and two to three times longer life than flooded types.They do, however, have many drawbacks. They do not tolerate deep discharge well, making them less suitable for house batteries. Taking them below 50% of full charge, and especially below 30%, will greatly shorten their lifespan. Overcharging can dry up the electrolyte, which is impractical to replace. Undercharging leads to sulphation that reduces amp-hours permanently. They also experience voltage sag under high loads such as an inverter, a winch, or an engine starter. As house batteries, VRLAs can have a lifespan as low as one to three years if not carefully managed.
Golf cart
batteries
A good alternative to marine deep cycle batteries is golf cart batteries that are best obtained at warehouse clubs. They have heavy plates, well attached to tolerate the bumpy motion of a cart, and they will survive three or more times as many discharge cycles as a typical marine battery. They provide 6 volts, having only three cells, and so two must be wired in series. This is no problem if you change the cable connections and then cruise with the A-B battery selector set to “both” (as many recommend doing anyhow) but otherwise you might need another battery tray. Warehouse club golf cart batteries cost only one-third as much as marine deep cycle batteries, per amp-hour, over their lifetime.Lithium
batteries
Lithium batteries are of two types, only one of which is suitable for a sailboat. Neither lithium-ion (Li-ion) nor lithium iron phosphate (LiFePO4 or LFP) is inherently better; they excel in different areas, with LFP offering superior safety, longer lifespan (more cycles), and better thermal stability, while standard Li-ion boasts higher energy density for devices needing compact power, like phones and EVs, although with a higher risk of fire. On a boat, LiFePO4 is what you want.Lithium batteries in a deep-discharge house battery bank need a special charger and voltage regulator that takes them to a higher peak voltage, around 14.6 volts for a 12-volt battery, unlike lead-acid chargers that stop at around 12.7V. It uses a constant current, constant voltage (CC/CV) algorithm and has no trickle or float charge that would damage the cells over time. Unlike other battery types, lithium batteries can and should be brought up to 100% charge and run down to 0% charge (but not below that).
A lithium battery will also have a battery management system (BMS), which is a circuit board that monitors the battery cells, both individually and in series, for safety and to optimize performance. The BMS measures the voltage of every cell and typically also monitors current and temperature. It provides several protections:
- Overcharge protection. If one cell gets too high during charging—typically around 3.6–3.65 V for LiFePO4—the BMS can stop charging. This can happen even though the overall battery voltage appears acceptable.
- Over-discharge protection. If one cell falls below the safe minimum voltage, the BMS disconnects the load before that cell gets damaged.
- Overcurrent protection. A 100-amp BMS, for example, may allow 100 amps continuously but disconnect if the load substantially exceeds that limit.
- Short-circuit protection. A sudden very large current causes the BMS to disconnect cells instantly using high-current electronic switches called MOSFETs. This protects the battery; however, the resulting voltage spike (ie, load dump) can harm the alternator and other electrical equipment. It should be thought of as a safety fuse or circuit breaker, not as a system for ongoing maintenance.
- Temperature protection. Many BMSes stop charging when the cells are below freezing because charging LiFePO4 below about 32°F/0°C can damage them. They may also disconnect for excessively high temperature, especially at the alternator.
- Cell balancing. If one cell gradually reaches a higher state of charge than the others, the BMS can bleed a small amount of energy from that cell so the cells remain balanced.
The BMS protects the battery and only the battery. It is more likely to fail than the battery itself and so it is wise to carry a spare.
It would be a mistake to simply swap a lithium battery into a boat without also redesigning the entire electrical system with particular attention to the alternator, voltage regulator, and charger. A LiFePO4 battery should not be used for engine starting unless the manufacturer rates it as a starting battery. This is because engine starting draws high current—up to 100 amps or more—that is likely to trigger the BMS, cutting power just when you need it. The wet cell charging battery will then need its own separate charger.