What shall I think of when buying batteries?

Lead-acid batteries

Economically, you probably get the most amperage hours per dollar, with standard marine or leisure lead-acid batteries from well-known manufacturers like Exide or Varta. They are usually 75 to 80 Ah. If you buy larger lead-acid batteries, they are more expensive per Ah, and they are much heavier and more difficult to get into the boat.

In addition to starter batteries, there are leisure batteries and various special batteries such as gel and AGM batteries.

A starter battery is made for fast charging and discharging, but is poor for deep loads. The leisure battery is made to withstand deeper discharges, down to 45-50 percent of the battery capacity at the expense of the starting current capacity when it is very cold. A regular marine battery has a starting current of over 500 A at -30 ° C. It's enough to start the most engines.

Gel & AGM batteries

Gel and AGM batteries can withstand even deeper discharges and more cycles, but they are more expensive batteries. However, you get more Wh (watt hours) from such a battery. So the cost per Wh you can use is not much higher, in some cases, it can be cheaper with a AGM or Gel battery.

You do not usually need a special starter battery, but you can have lead-acid batteries of the same type throughout the entire battery bank. Then you can also switch so that the starter battery, which never gets deeply discharged, can become a consumer battery after a season. In this way, you extend the life of your battery bank.

Lithium batteries

Lithium batteries are natural in your cell phone nowadays and  are more and more common in vehicles. If you are looking at Gel or AGM batteries, you should consider purchasing lithium batteries instead. The price difference is smaller than before and the quality of lithium batteries is now really good.

There are different types of lithium batteries.
Lithium-polymer batteries are the ones that are in cell phones. They are expensive, have high capacity and are suitable for small compact batteries.

Lithium-ion can be found in hand tools, for example. They require constant monitoring as overheating can cause explosive collapses.

The third type is lithium iron phosphate batteries. They have slightly lower energy density than the other two, but on the other hand they are more stable, chemically. These are the best suited for use in boat, motorhome, caravan etc.

A lithium battery weighs less than half as much as a corresponding gel battery and takes about half the size. But besides the weight, the big advantage is the charging receptivity.