Lithium, GEL, AGM battery: which type for which use?

On board or on your RV, batteries play a crucial role. AGM battery, GEL battery, lithium battery… Historical technology, sealed lead batteries are gradually giving way to more modern technologies. But not all of them are equal, depending on the desired applications and the budget. Discover the advantages and disadvantages of each type of battery for each use.

Sealed lead-acid battery: a standard at the end of its life

Capable of providing a high intensity of energy at start-up, sealed lead-acid batteries are also the most widespread and less expensive technology. They therefore have the advantage of being easy to find, but quickly show their limits in terms of performance. Lead-acid batteries are indeed limited in the amount of current they are able to deliver. As an example, a 37 kg battery offers a current of 800 amps. If you need more power, you will have to add more batteries, and therefore more weight and space. This can be particularly problematic in the RV industry, where an increase in vehicle weight can result in a change of category… In addition to this poor weight/energy ratio, lead-acid batteries offer a low amount of energy storage and require regular maintenance. For all these reasons, lead-acid batteries are considered an end-of-life technology, despite their attractive price. For the general public, these batteries are worth about 220 euros for 150A

Gel batteries: ideal for emergency batteries

To operate, a battery needs plates and an electrolyte: it is between the current and the plates that the current is created. On historical batteries, this electrolyte is liquid, which is not optimal for a boat that is not meant to be stable. Thanks to a gel electrolyte, gel batteries are very resistant to vibrations. Gel batteries have been used in the industry for almost 40 years and have the advantage of a very low discharge rate, which makes them excellent candidates for all emergency applications. For example, they can be found in hospital generators, thanks to their lifespan of 10 to 20 years. Gel batteries can be tipped over, can easily withstand a list on a boat and offer 30% more storage capacity than a standard battery. They can be completely discharged and do not require any particular maintenance. Of course, all these advantages have a cost. Count about 470 euros for a 150A gel battery. Their only drawback? Their recharge is considered slow, because the battery is not adapted to high charge and discharge currents.

AGM battery: the compromise between gel and lead

AGM stands for “Absorbed Glass Mat”. As the name suggests, AGM batteries have a gel electrolyte and a glass fiber separator between the plates. A hybrid version of the lead-acid and gel batteries, AGM batteries have become increasingly popular in the marine sector over the last 10 years. They offer an excellent compromise between these two technologies: more efficient than lead-acid batteries and less expensive than gel batteries. For the same weight, an AGM battery offers almost twice the power of a lead-acid battery (18kg for 800A). They also have a longer life (self-discharge rate between 1 and 3% per month). On average, AGM batteries are 30% more expensive than lead-acid batteries, but they are relatively easy to find on the market. On the other hand, it should be noted that some models do not support total discharge. It is thus advised not to exceed 80% of discharge, in order not to damage it. Price: 330 euros for 150A.

Lithium batteries: excellent performance, but high price

Although they have been around for about ten years, lithium batteries have really been democratized in the last 5 years. Based on lithium ion ferric phosphate technology, they have an energy density three times higher than lead acid batteries, for the same dimensions. This excellent power/volume ratio makes it possible to have a lot of current in small spaces. For example, it is possible to replace 4 gel batteries with a single lithium battery. The weight saving allows to double the energy capacity: a crucial point in the world of transportation!

The other big advantage of lithium batteries is that it is possible to use all the energy stored in the battery. On a traditional 100A battery, it is indeed impossible to use more than 50% of the battery, at the risk of not being able to recharge it. On the opposite, a lithium battery is capable of discharging itself to 100%! Moreover, they recharge twice as fast as traditional batteries. This is particularly interesting for boats, which can reduce the running time of the generator, for example. For all these reasons, lithium batteries have the wind in their sails… For those who can afford it. Even if their price tends to decrease from year to year, lithium batteries cost about 1500 euros. This is a significant investment, but it should be seen in the light of the lifespan and the comfort of use. As a reminder, the life of a battery is correlated to its number of cycles. While a lithium battery lasts 500 cycles, other types of batteries hardly exceed 250 charge cycles.

What are AGM and gel batteries?

AGM (Absorbed Glass Mat) and gel batteries are both examples of VRLA (Valve Regulated Lead-Acid) batteries. They are also known as SLA (Sealed Lead-Acid) batteries. These abbreviations refer to the safety valves in the battery’s lid. In common parlance, the term gel battery is used to indicate both AGM and Gel batteries. AGM batteries may also be called “membrane”, “starved electrolyte” or “dry” batteries.

What are the differences between AGM and gel batteries?

VRLA batteries come in two distinct versions: AGM batteries and gel batteries. In the gel battery certain elements – often specific mixtures of silicon – are added to the battery acid, transforming the electrolyte into a gel-like substance that cannot leak. By “drilling” channels in the gel, gaseous oxygen moves from the positive plate to the negative plate. Here it encounters hydrogen gas and recombines into water, releasing energy. Gel batteries have excellent capacity, but their slightly increased acid resistance makes them less suitable for use as starter batteries. They are highly resistant against excessively deep discharge, retaining function even when the battery is discharged to 20% of its nominal capacity.

In AGM (Absorbed Glass Mat) batteries the electrolyte is held in place by a separating membrane. This is why these batteries are also known as membrane batteries. The membrane consists of a fiberglass mat and functions as a sponge. This function is based on the capillary properties of the membrane. AGM batteries can be produced using very thin separating membranes, leading to low internal resistance. This means that high output power can be achieved with batteries of relatively low size, making this type of battery ideal for a wide variety of applications.

The Differences Between AGM, GEL and FLOODED Batteries

Even though inside all AGM, GEL and flooded batteries contain lead acid, the internal construction of the battery divides them into their respective categories.

Absorbed Glass Matte or “AGM” batteries are the latest and greatest in lead-acid batteries. An AGM battery uses a separator consisting of fiberglass between the plate and wrappers to hold the electrolyte in its place with capillary action. Combining the lead plates, electrolyte, and fiber glass separation fibers in a confined space, AGM batteries create a “physical bond” by way of capillary action. Similar to how water creeps up a towel when it is put in a bathtub. This capillary action holds the liquid inside the glass matting, making the AGM Battery “spill proof” if it is ever exposed. Due to the tight packing of an AGM battery, it is also the most impact resistant, and boasts the least internal resistance. The lower internal resistance increases the output voltage, decreases charging time, and reduces losses to heat as power flows through the system. AGM Batteries then bring the trump card to the table, they are maintenance free. Premium AGM batteries recombine the gases produced internally, back into liquid. This recombination makes the AGM battery maintenance free. No acid leaks, no mess while charging, no corrosion on surrounding parts. You plug in these batteries and walk away. AGM batteries can do anything that flooded and GEL batteries can do, just better.

Flooded or “wet cell” batteries are the most commonly used batteries on the market today. Flooded batteries come in the widest variety of shapes and sizes due to their widespread usage in a multitude of industries and applications. Flooded batteries again use lead plates, a sulfuric acid electrolyte, and plate separators but that is where it stops. Usually flooded batteries are not sealed, and do not recombine the gases to liquids internally. Instead, these gases are vented externally. Internal gases produced are released directly to the environment. Through these same vents can flow acid, steam, and condensation, leading to maintenance. Flooded batteries do require maintenance, in the form of water, to routinely replenish lost electrolyte through the vents. Lead plates start to deteriorate when they touch the atmosphere, so if you fail to maintain your batteries, they will corrode and fail. Flooded batteries hold very good rates of charge for the price, but require more work. Unfortunately due to the internal construction, flooded batteries have the weakest internal construction, and some very high internal resistance statistics.

GEL cell batteries are also sealed just like the AGM battery listed above. That is where the similarities end. A GEL battery uses a silica (sand) to turn the sulfuric acid into a jelly like substance. This jelly is then used as the electrolyte. Great care must be taken with GEL batteries not to expose them to high amperage situations. High amperage situations can literally ‘SCAR’ the jelly inside of a GEL battery, creating a pocket. These pockets allow the plates to begin corroding, leading to premature failure. GEL batteries should not be used for fast charging/discharging, or high amperage charging/discharging situations. Use the other types listed above for these high amperage situations. GEL Batteries are slightly stronger in regards to internal construction than a flooded battery, but pale in comparison to the physical strength of an AGM battery.

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