Battery efficiency is a very big deal that very few really understand. The key to getting the best marine battery, is understanding that the battery efficiency plays a very large role in the rest of the system. A large inefficient Marine Battery will lose to a smaller, more efficient marine battery, every time. On the water, in my boat anyway, size and weight count. For instance, a battery that is 1/3 bigger but 50% efficient, defeats the purpose of purchasing the larger size battery. You would be better off buying a better battery, one the same size, and 87% efficient for instance. Let's say the marine battery in question is a 100AH deep cycle battery. The mathematical results would look like this.
100ah x .50 = 50ah of useful power.
100ah x .87 = 87ah of useful power.
87ah / 50ah = 1.74 or 174% or just 74% more power, in the same space.
The efficiency of a battery is a product of it's design, and then the quality of the marine battery's manufacture. First off, chemistry and design. The laws of chemistry and physics dictate much of the efficiency based on design. AGM batteries will always be more efficient than a flooded battery. Both flooded batteries and AGM batteries can sustain higher amp charge and discharge regimen due to manufacture. Therefore, the efficiency of the battery in your marine application is dictated by your choice of TYPE of Marine Battery FIRST! Then the choice of manufacturer comes in second. Basically, the better the battery is built, the better the efficiency, measured in resistance. The less resistance, the faster you can charge or discharge the battery, without it getting hot.
AGM Batteries, due to their construction will always have the lowest internal resistance, and inevitably, are part of the best marine battery installation. Lifeline Marine Batteries can be charged at 5C, or 5 times their rated amperage. Imagine charging your 1000 amp hour battery bank with a 5000 amp charger, and being done in 15 minutes; AGM Batteries can do that. Another interesting and useful feature of AGM batteries is their ability to start motors. Due to low internal resistance, even AGM deep cycle batteries have the cranking amps, because of their efficiency and lack of resistance, to routinely start your marine motor, and run the electronics.