There are several types of the electronic circuitry used within battery chargers for the marine market.
FERRO-RESONANT (or CVT)
These use a low-frequency MAGNETIC control system, which makes them very HEAVY, very BULKY and is also only available with a poor FLOAT charge characteristic, therefore very SLOW recharging. They can also generate a large magnetic field which can upset other equipment on board. On the plus side, they are CHEAP and RELIABLE due to the low number of components used and they tend to appeal to boat-builders who put the price at the top of their list of priorities.
LINEAR CHARGERS
These also use a low-frequency transformer to reduce the input voltage to a lower level but then use transistors to control the current and voltage fed to the battery. This technique can be used for either FLOAT or 3-STAGE chargers but is very IN-EFFICIENT and therefore HOT, HEAVY and BULKY. The biggest drawback is a LIMITED INPUT VOLTAGE range - not ideal for running from a generator or some marina supplies.
SWITCHED MODE CHARGERS
These are more complicated than the previous two types and use the techniques perfected for and now universally used in computers and televisions. The AC input is first turned into high-voltage DC. It is then turned into high-frequency AC using special types of transistor and a high-frequency transformer (one-thirtieth the weight of a low-frequency transformer!) reduces the voltage to the exact level needed to charge the battery. A sophisticated control circuit produces an overall design with HIGH-EFFICIENCY, SMALL SIZE and LIGHT WEIGHT. The extra complexity adds to the initial cost but results in lower running costs and the ability to run from a SMALLER (and cheaper) GENERATOR if required. Switched mode chargers can be either FLOAT or 3-STAGE types.