Marine Battery Types & Charging Tips

Most boats under 50 feet in length have 12-volt electrical systems. Yet many experienced boaters don’t fully understand the batteries they have on board or indeed how their batteries and charging systems work.

Marine batteries are designed and built for specific applications and their internal structure will reflect their use—starter, deep-cycle, or dual-purpose—as well as their limitations. For instance, a battery designed for starting your boat engine will typically have more internal plates closer together, providing more surface area to give that higher, one-time discharge required in powering a starter motor, but will not be as good at the long, steady discharge that deep-cycle batteries, with thicker active plates and higher antimony concentrations give. Deep-cycle batteries can be discharged from 50 to 80 percent and recover easily, while starting batteries don’t like to be discharged more than 50%.

Chemical Types:

Marine batteries are available in four chemical types: Flooded, Gel, Absorbed Glass Mat (AGM) and Lithium Iron. Regardless of chemical type, they’re rated by energy output, generally expressed as ampere hours, and categorised by how many charges (cycles) the battery is likely to withstand in its lifetime. The output and lifespan will typically dictate the price. The use, chemical type, and the number of batteries best suited to your boat will depend on the kind of boat, how you use it, and your budget.

Flooded Batteries: Wet or flooded batteries following Plante’s design are the most common and least expensive kind of battery found on boats. They use a reservoir of liquid sulphuric acid to act as a pathway between lead plates. This electrolyte produces hydrogen and oxygen when the battery is being charged, requiring vented battery boxes and compartments to let the gas escape safely outside the boat. Due to the heat and outgassing produced during charging, flooded batteries require periodic inspection and topping up with distilled water (if they are not sealed, and this is possible). They self-discharge at a higher rate (6 to 7% per month) than AGM gel or lithium iron batteries, and thus require routine charging when not in use for longer periods; over the winter for example. Wet cells must be installed in an upright position (which can be difficult to maintain on boats) and do not tolerate high amounts of vibration (also prevalent on boats). The good news is flooded batteries handle overcharging better than gel and AGM batteries. Wet-cell batteries are also significantly less expensive to purchase.

Gel Batteries: The “gel” is a combination of sulphuric acid, fumed silica, pure water, and phosphoric acid. The gel is quite viscous and prevents leaks if the battery is inverted or the case is damaged. Charging does cause a small amount of hydrogen and oxygen to be generated at the plates, but the pressure inside the cells combines the gases to create water. This also keeps the battery from drying out due to charging. Gel batteries charge at a lower voltage than flooded or AGM batteries, requiring a vessel’s charging system to be very carefully regulated to prevent high voltage overcharging.

AGM Batteries: Absorbed Glass Mat batteries feature glass mat separators saturated with acid electrolyte between the battery’s positive and negative plates. During charging, pressure valves allow oxygen produced on the positive plate to migrate to the negative plate and recombine with the hydrogen, producing water. AGM batteries have better shock and vibration protection than wet or gel batteries, and are virtually maintenance-free. AGM batteries also have lower internal resistance, allowing greater starting power and charge acceptance, and quicker recharging than other types of deep-cycle batteries. AGM batteries can accept the highest charging current, up to 40% of the amp-hour capacity of the battery, compared to about 25% for the flooded type or 30% for the gel—meaning they recharge faster. Long life, a low 3% self-discharge rate, and outstanding performance make AGM batteries excellent dual-purpose batteries for boaters who require quick starting power and reliable deep-cycle ability.

Lithium Iron Phosphate: Lithium Iron phosphate is the latest technology to arrive on the market and has many beneficial features. They have a super long life cycle, achieving 2750 cycles at 90% depth of discharge (DoD) that jumps to an even more impressive 3500 cycles at 75% DoD. A lithium iron battery with 61.2Ah of useable power has the equivalent energy of a 120Ah lead acid flooded battery yet is a third smaller in size and a quarter of the weight. This enables them to be transported much more easily and safely. Lifos, a leading brand of lithium iron battery also boasts a Bluetooth app that connects directly to your smartphone that keeps you informed of the battery performance and key performance indicators at all times. It is worth noting that lithium iron batteries are considerably more expensive to purchase, however if you consider the massive increase in available cycles and therefore longevity, the cost per cycle is by far the lowest out of all the technologies.

Battery Tips

• Shallow discharges lead to a longer battery life.
• 80% discharge is the maximum safe discharge for deep-cycle batteries.
• Don’t leave batteries deeply discharged for any length of time. Leaving batteries in a discharged state will cause sulphation and lower their capacity.
• Charge batteries after each period of use.
• Never mix old batteries with new ones on the same charging system. Old batteries tend to pull down the new ones in the battery bank to their deteriorated level.
• Keep batteries in acid-proof storage boxes secured with tie downs.
• Clean terminal connectors regularly to avoid loss of conductivity.
• Maintain fluid levels in wet-cell batteries.

Whichever technology you choose, caring for and maintaining your marine battery will enable you to get the maximum performance, longevity and piece of mind when out on the water. Your choice of marine battery should be based on what function the battery needs to perform. Will it need to start the engine, run all the systems and equipment when the engine is not in use or both? Seeking the best advice is very important and we would always advise speaking to one of the BBL Battery experts when deciding which battery will perform best and suit your individual application. BBL Batteries have been supplying marine batteries for approaching 50 years and have the knowledge and experience to help you make the right decision. They also stock all the technologies discussed in this blog and supply a choice of premium quality marine batteries from the best brands sourced globally.