2.1 AGM vs. Lithium - What is better?

Particularly for frequent drivers and for tradesmen, a lithium-ion battery should always be the first choice. We mainly use non-flammable lithium iron phosphate batteries.

The lithium battery is ideal if you have little space on board and do not want to add unnecessary weight. For larger applications (10 kW or more) it hardly goes without lithium battery.

If they do not drive so often (<20 times a year) and you do not mind the extra weight on the boat, they can confidently fall back on a cheaper AGM battery. This is especially recommended for smaller engines and if they generally do not need much total capacity. It is important that you use a traction battery for e-motors.

The most significant advantage is a lower price, especially when entering into electric mobility or with a tighter budget, this is very appealing. Note are the storage and charging in the AGM batteries and the lower depth of discharge.

There are very many different types of batteries. However, really relevant for use as a boat battery are only a few. We can recommend two different types of batteries that are currently most commonly used on boats.

They all have one thing in common: they are absolutely maintenance-free, leak-proof and safe to handle. Apart from that, they can be installed in any position. Nonetheless, there are some striking differences, from which certain advantages and disadvantages result.

Lithium-ion batteries - familiar from the automotive sector - are on everyone's lips. Lithium-ion and lithium-polymer batteries are combined under this term. With lithium-ion batteries weigh only about 30% of the lead-acid batteries (depending on the type different) and also have an extremely high cycle stability and depth of discharge!

The further developments of lead-acid batteries are gel batteries, in which the acid is encapsulated in a gel. Meanwhile, even more popular are AGM batteries, in which the acid is bound in a fiberglass fleece. Both battery types they are significantly more cycle resistant than a wet battery.

The gel battery is higher in cycle resistance and has a lower self-discharge per month. The AGM battery can deliver higher currents for a short time and is therefore also suitable as a starter battery and is slightly cheaper than a gel battery. Both types are extremely heavy due to the large amount of lead in the battery.

There are large differences within the battery types, especially in the AGM and in the lithium-ion batteries. Some models of AGM batteries can be discharged even 80% (Aquamot) or 100% (Victron Energy). However, the service life should be taken into account. 

For a long time, the lead-acid battery was also used on boats. The disadvantages are obvious: it is heavy, not leak-proof or maintenance-free, and in addition it has a low service life and capacity. However, the biggest advantage of lead-acid technology remains that it is comparatively inexpensive. This technology has been further developed to reduce these disadvantages.

In principle, of course, any battery can be completely discharged, but we do not recommend this. Batteries that are not suitable for this lose particularly strong life or are subsequently irreversibly damaged. There are also usually differences in the charging and discharging currents and other properties, so a comparison between different products may be worthwhile for you. You can always find the product-specific details with the respective articles or in the product comparison.

When calculating the necessary capacity, we always use the manufacturer's recommended maximum discharge depth to determine the effective usable capacity of the battery. Based on this, the battery is then selected so that you can really drive as far and long as you want.

The energy density of batteries has increased continuously in recent years. Energy densities of up to 300 Wh/kg are now possible for lithium-ion batteries. Typically, however, these are lower at around 120-160 Wh/kg. Lead batteries, as well as AGM and gel batteries have a significantly lower energy density of about 25-30 Wh/kg.

This explains the higher weight of lead batteries at the same capacity compared to lithium-ion batteries.

Batteries can not keep up with gasoline in terms of energy density, however. Gasoline has an energy density of just under 12,000 Wh/kg. Effectively, then, an electric drive always runs on less energy than an internal combustion engine, even though an electric motor is up to 10 times more efficient.