Wet cells: The de facto standard for high capacity batteries in boats for many years has been the basic wet cell lead-acid battery. These are somewhat simple in design. Plates are submerged in a liquid acid electrolyte contained in a plastic box. The cells are freely vented to the outside of the case. Wet cells are the least expensive type of battery and readily available even in remote locations. They are reliable and usually have a life span of three to five years. Wet cell batteries contain toxic chemicals which if released can be harmful to the vessel and crew. They also give off toxic and explosive gases when charging. This requires a good installation to prevent damage to the vessel. Although wet cells do have a good service life it is not as long as some other types of batteries. They also require a bit more in maintenance. Because they are freely vented some water is boiled from the electrolyte when charging. This water needs to be monitored and replaced from time to time.
Wet cells have limitations on their charge acceptance rate. What this means is you can only charge them at a certain rate and if you try to put too much current into them too fast they can be permanently damaged. Wet cells should also not be discharged more than 50% of their capacity as this will also damage their plates. That said these batteries are cost effective and available almost anywhere in the world making them a decent choice for cruisers planning distant voyages.
Gel cells: Gel cell batteries are similar to wet cell with the exception of having a gel electrolyte instead of liquid. They work best for slow discharge rates. This make them better suited for house banks although some are designed as dual purpose meaning they will work for both slow and rapid discharge rates. As they are sealed they do not require the same ventilation as wet cells (they have a valve to regulate over pressure should over charging. This is often referred to as “valve regulated”). This also means they do not require checking fluid levels and adding water from time to time.
Charging regulation is critical and if charged too rapidly they can be damaged. Advances are being made in gel cell technology that are allowing faster charging rates however it is still important to make sure they are charged correctly. Gel batteries can withstand vibrations and G forces better than wet cells making them a good choice for applications where heavy vibration or impacts are expected. A high speed motor yacht might be an example of this. They are more expensive than wet cells but still reasonably priced. There lifespan tends to be a bit longer than wet cells if properly cared for.
AGM: Absorbent Glass Mat (AGM) batteries use a fine fiberglass mat that is capable of absorbing and holding the electrolyte. Unlike the gel cells, because the electrolyte is in the glass mat, it is part of the battery structure. Like gel cells, they are valve-regulated to control over-pressure. If damaged they will not leak hazardous chemicals. AGM batteries generally can take a charge or discharge faster than gel or wet cell batteries. The weakness of AGM batteries is they do not like high temperatures and over-charging can cause damage to the battery. Care needs to be used to monitor the battery during charging to avoid over-heating or over-charging. AGMs tend to have a longer lifespan than wet or gel cell batteries if properly charged and installed. Although AGMs are more expensive, the extra cost should be offset by a longer life and more efficient charging.
Lithium-ion: The newest item on the battery market are lithium-ion batteries. These are high-end and high-performance batteries. They are lighter, can be charged faster and discharged deeper than any of the other types. At first they would seem like the perfect choice. However, this great performance comes at a cost not only in initial purchase price but also in the charging systems required to get the most out of these high-performance batteries. Generally speaking, lithium-ion batteries will produce twice the power at half the weight of the lead-acid type batteries. They can be discharged to nearly 100 percent without damage and can be rapidly recharged. Charge control is very important to these batteries, not only to prevent premature failure but also to prevent possible fires. We have all heard about the hoverboard and computer fires as a result of charging failures. Thankfully, most manufactures now have their own built in charge regulators making these batteries safer. If considering lithium-ion, it is strongly recommended to get professional assistance to select and install the batteries. This technology is still somewhat new and needs to be approached carefully to avoid wasted time and money.