The 12 volt battery for boat has a primary role of a power storage device. They have a secondary role serving as a buffer, absorbing power surges and disturbances that arise during charging and discharging. If there is one enduring and almost constant debate over or problems with, it is boat batteries and boat battery charging.
Rule. A separate battery shall be provided for starting each engine. The engine start battery capacity should be based on the provision of 10 consecutive start attempts of 5 seconds duration with a 30 second period between each attempt, at an ambient temperature of 5ºC. The engine manufacturer’s recommendations should be the minimum battery specification.
Rationale. Starting loads require large current levels for relatively short time periods. Boat batteries loads in this category include engine starter motors, engine pre-heating, anchor windlass, thrusters and electric toilets. The boat starting battery is normally used for these applications. The rating should allow for worst case starting scenarios. These can be issues with the engine fuel system and have to crank over and bleed the system several times. In cold temperatures battery efficiency is lowered and engine starting requires greater power due to increased oil viscosity. I have seen way too many people discharge the start battery while trying to resolve a problem and then not be able to start the engine.Marine Electrical Electronics Bible
Rule. The service or house battery capacity shall be based on calculation of the boat power consumption for a 12-hour period. Sailing vessels should base calculations on a 24-hour period. The power calculations should include all equipment and systems that will run continuously or intermittently during the calculated period.
Rationale. Service loads on boat batteries draw current over relatively long periods. Equipment in this category include lights, instruments, radios, radar, autopilots, inverters and entertainment systems. Deep cycle 12 volt battery for a boat are normally used for these applications.
Calculations are based on the maximum power consumption over the longest expected period between battery recharging. If you don’t charge boat batteries for 2 days then calculate for 48 hours. If you have an electric deck winch also factor this in. The takeaway is to match your battery consumption calculations on your actual reality, not a regulation, the regulation or rule is the minimum start point.
There are many boating standards and recommendations about boat batteries. Click on the link to reference the page on boating standards.
They "rules" are my own generic recommendations and incorporate best installation practice based on decades of professional experience. Unlike official rules and standards, which often don’t have context or a rationale behind the recommendation for the average boating person to understand.
Please check and refer to your local rules and standards or recommendations as required. It doesn't replace them but helps with those who don't have access to them or can't understand them.
Rule. Generator units should have a dedicated engine starting battery. The battery should be of the sealed type. The battery should comply with installation requirements for other batteries.
Rationale. Generators should have an independent battery, which also increases starting power redundancy. If you put in a battery that will also start your main propulsion engine, then you have some redundancy as the battery always gets charged when it runs.
If you need to know more the Marine Electrical and Electronics Bible has all the information you require,
Battery Ratings. Manufacturers of boat batteries use a range of ratings figures to indicate performance levels of boat batteries. When selecting a 12 volt battery for a boat it is essential to understand the ratings and how they apply to your requirements. The various ratings are defined as follows.
Amp-hour Rating. Amp-hour rating (Ah) is one of the primary boat batteries ratings. It refers to the available current over a nominal period until a specified final voltage is reached. Rates are normally specified at the 10 or 20-hour rate. This rating generally applies to deep cycle boat batteries. For example, a battery is rated at 84 Ah at 10 hour rate with a final voltage of 1.7 Volts per cell. This means the battery is capable of delivering 8.4 amps for 10 hours, when a cell voltage of 1.7 volts will be attained. (Battery Volts = 10.2 VDC). Where a battery is discharged faster than the nominal rating the available capacity also decreases. This is called the Peukert effect; the decline follows a logarithmic curve.
Reserve Capacity Rating. This rating specifies the number of minutes a battery can supply a nominal current at a nominal temperature without the voltage dropping below a certain level. This rating, normally applied to automotive applications, indicates the power available when an alternator fails and the power available to operate ignition and auxiliaries. Typically, the rating is specified for a 30-minute period at 25°C with a final voltage of 10.2 volts.
Cold Cranking Amps (CCA). This rating defines the current available at -18°C for a period of 30 seconds, while being able to maintain a cell voltage exceeding 1.2 volts per cell. This rating is only applicable for engine starting. The higher the rating, the more power available, especially in cold weather conditions.
Marine Cranking Amps (MCA). This newer rating defines the current available at 0°C for a period of 30 seconds, while being able to maintain a cell voltage exceeding 1.2 volts per cell. Again, this rating is only applicable for engine starting purposes. If you are in a cold climate area (UK/Europe and US) then CCA is more relevant.
Plate Numbers. Data sheets state the number of positive and negative within a cell. The more plates, the greater the plate material surface area. Greater plate surface area increases the current during high current rate discharges and subsequently improves cranking capacity and cold weather performance.
Casing Type. Battery casings are either a rubber compound or plastic. Where possible, always select the rubber types as they are more resilient to knocks and vibration.
Marine Battery. This often misused sales term with boat batteries applies to certain constructional features. Plates may be thicker than normal or there may be more of them. Internal plate supports are also used for vibration absorption. Cases may be manufactured with a resilient rubber compound and have carry handles fitted. Filling caps may be of an anti-spill design. These days, batteries are of a similar design, with very little to distinguish marine batteries from the automotive types except the label. In many cases you are paying a premium for a label. All you need to know about 12 volt battery for boat and the boat battery.