A marine battery combiner allows two or more batteries to be automatically connected while charging from a single source. Some people want to know if a battery combiner is necessary. If multiple battery banks on board require redundancy, a battery isolator ensures no one battery drains other batteries.
A battery combiner is better if the battery bank consists of a single charging source that requires no excessive draws. A combiner works like a battery but is automatic. There is no drop or loss in voltage. An isolator has diodes that separate batteries. There is a voltage drop of one-half.
When wiring boat batteries, a common question is, 'Should the connection be in series or parallel?' It depends on the needs and the boat's electrical setup. Batteries connected in series combine the voltage while keeping the capacity the same. 
Series
Choosing to wire boat batteries in series means connecting a battery's positive terminal to another's negative terminal. The configuration adds the voltage of each battery while keeping the capacity the same. Increased voltage and balanced charging are the advantages of series wiring.
It allows an increase in voltage output, which is beneficial for high-powered electrical devices on a boat. Each battery receives an equal charge, which ensures balanced performance. The risk of system failure and no increase in capacity are disadvantages of a series circuit. If one battery gets disconnected or fails, a complete loss of power occurs. Runtime is a concern for a battery that is not increasing in capacity.
Parallel
Connect all positive terminals and all negative terminals together in parallel wiring. The configuration increases the overall capacity while keeping the voltage the same. An increase in capacity and redundancy are the advantages of parallel wiring. If one battery fails, the others continue to provide power. It also provides more runtime for the electrical system.