Choosing an on board charger is important and, it requires careful consideration. You can't select a boat charger as you would a car charger because of the harsher environmental conditions facing marine chargers.
For example - moisture. An on board marine charger has to endure higher humidity and moisture conditions .Most boat battery chargers are moisture resistant to some degree to prevent premature failure from wet conditions. Usually, on board boat battery chargers aren't directly exposed to water beacuse they are permanently mounted in the engine compartment or in another nearby location. A completely waterproof marine battery charger is needed when direct contact with water ocurrrs such as in smaller fishing boats.
Another important consideration is the type of batteries in your boat and their amp-hour rating, as well as their voltage. .
What voltage charger do you need?
Well, it all depends on your configuration.
Here are some examples.
Single 12 Volt battery = 12 Volt battery charger
Two 12 Volt batteries setup in parallel = 12 Volt battery charger
Two 12 Volt batteries setup in series = 24 Volt battery charger
Do not use a 24 Volt battery charger to charge a 12 Volt battery. Doing so will damage or destroy your battery battery and charger and may also have dangerous consequences such as a fire or explosion.
Marine Battery Amperage:
How many amp marine battery charger do you need? A simple calculation (that works in most cases) is to examine the amp-hour output rating of your battery and take 10% of that value as the proper amperage needed to recharge it.
If you have multiple batteries you will have to add up the amp-hours of all the batteries and multiply that figure by 10%. So in the case of three 105 amp-hour batteries, you would need approximately 10% of 315 amp-hours or 31.5 amps..
The best boat chargers are the new smart chargers that have onboard computer processors capable of detecting the state of your batteries and adjusting their output automatically. They can also be used as battery tenders providing a minimal charge to your batteries to keep them fully charged even during longer periods of downtime without overcharging.