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Came across this and I thought it might help those who are considering buying a new battery for the boat. Anatomy of a Typical Battery In recent times there have been advances in battery technology and VRLA batteries have become very popular. VRLA stands for valve regulated lead-acid and is the designation for low maintenance lead acid batteries, also called recombinant batteries. VRLA batteries are commonly further classified as: Absorbent glass mat battery (AGM) Gel battery These batteries are often called sealed lead-acid batteries, but this term is misleading. A sealed battery would be a safety hazard due to overpressure risks when overcharged and there is always a safety valve present, hence the name valve-regulated. Sealed is opposed to vented (also called flooded). Because VRLA batteries use much less electrolyte (battery acid) than traditional lead-acid batteries, they are also occasionally referred to as an “acid-starved” design. The name “valve regulated” does not wholly describe the technology; these are really “recombinant” batteries, which means that the oxygen evolved at the positive plates will largely recombine with the hydrogen ready to evolve on the negative plates, creating water–thus preventing water loss. The valve is strictly a safety feature in case the rate of hydrogen evolution becomes dangerously high. One result of this design is a much higher ratio of power to weight than large, flooded type battery systems; another is a high-rate power capacity, though of relatively short duration. As a result, VRLA batteries are frequently employed in UPS or other high-rate applications. Batteries can be subdivided further by application. They are many jobs batteries are asked to perform but the everyday jobs can be listed as follows: Engine Starting Lead acid batteries designed for starting engines are not designed for deep discharge. They have a large number of thin plates designed for maximum surface area, and therefore maximum current output, but which can easily be damaged by deep discharge. Repeated deep discharges will result in capacity loss and ultimately in premature failure, as the plates disintegrate due to mechanical stresses that arise from cycling. A common misconception is that starting batteries should always be kept on float charge. In reality, this practice will encourage corrosion in the electrodes and result in premature failure. Starting batteries should be kept open circuit but charged regularly to prevent sulfation. Deep Cycle Specially designed deep-cycle cells are much less susceptible to degradation due to cycling, and are required for applications where the batteries are regularly discharged, such as Photovoltaic systems, electric vehicles and UPS systems. These batteries have thicker plates that can deliver less peak current, but can withstand frequent discharging Marine/Motorhome batteries, sometimes called “leisure batteries”, are something of a compromise between the two, being able to be discharged to a greater degree than automotive batteries, but less so than deep cycle batteries. Sizing Your Deep Cycle Battery The chart below shows some typical examples of calculations necessary when trying to size your deep cycle batteries. Description Watt @12V Amps X hrs/day = Ah Autopilot 20 1.67 5.0 8.33 Echo Sounder 4 0.33 7.0 2.33 Instrument Lamp 10 0.83 5.0 4.17 Log 2 0.17 7.0 1.17 Nav. Lamps 80 6.67 6.0 40.00 VHF Transmission 50 4.17 0.2 0.83 VHF Reception 5 0.42 5.0 2.08 Fridge 55 4.58 8.0 36.67 Bilge Pump 50 4.17 0.1 0.42 Sink Pump 50 4.17 0.3 1.25 FM Radio 40 3.33 2.0 6.67 TV 40 3.33 2.0 6.67 Reading Lamp 15 1.25 3.0 3.75 Sundry 60 5.00 3.0 15.00 TOTAL 129.34 The total load in this scenario is 129.34Ah. Unfortunately you can not use a 130Ah battery as the battery will not be able to provide a useful voltage at 100% discharge. The recommended maximum depth of discharge for optimum battery performance and life is 50%. Therefore in this example you should have a minimum battery capacity of 260Ah. This can be made up using one very large 12V battery or multiple smaller batteries. It is important to remember most manufacturers recommend a maximum of three batteries in parallel. This is to avoid battery imbalance which can occur during normal cycling.