Boat Starting Battery 

Boat Starting Battery technology is essentially the same as their automotive relatives. The open flooded cell has gone the way of the dodo on motor vehicles and boat starter batteries are the same. Vehicle technologies have advanced and so has the battery. Manufacturers have come along way with the improvement in performance and efficiency.  This includes better performance when partially discharged, a better level of deeper cycling, a multiple and frequent engine start capability and much improved charge acceptance rates. Similar to boat requirements they also have improved the thermal stability of batteries when used in hot climates and enclosed and poorly ventilated engine spaces.

Boat Starting Battery 

The boat starting battery must be capable of delivering the engine starter motor with sufficient current to turn and start the engine. This starting load can be affected by engine compression, oil viscosity, and engine driven loads.  The key specifications are the Cold cranking amps (CCA), which is amp delivery capability the battery can deliver for a time of 30 seconds at a temperature of -18 degrees (0°F) until cell voltage reaches 1.2 volts per cell. The Reserve capacity (RC) is how much power the battery can deliver when the charging ceases and you can run all the systems such as when an alternator fails. This is also defined as the time in minutes until the battery voltage falls below the nominal value.  About the rating known as Marine Cranking Amps (MCA), this is similar to the CCA test however as a boat is highly unlikely to go to -18 degrees the test is at 0 degrees (32°F). But many leave their trailerboats parked in those conditions and wonder what happened in the spring. 

Boat Starting Battery Facts

The following characteristics are required for a boat starter battery.

Construction. The starting battery is characterized by thin, closely spaced porous plates, which give maximum exposure of active plate material to the electrolyte and offer minimal internal resistance. This enables maximum chemical reaction rates, and therefore maximum current availability. The physical construction is very similar to deep cycle batteries.

Deep Cycling. The boat starter battery is not designed to and cannot withstand deep cycling, and if deep cycled or flattened then they will have an extremely short service life. Ideally the boat starter battery should be maintained within 95% of full charge, preferably fully charged.

Sulfation. In practice, sulfation is not normally a problem for a starter battery, as batteries are generally fully charged if used for starting applications only. If improperly used for deep cycle applications and under charged, they will sulfate and the life of the battery drastically shortened.

Self Discharge. Starting batteries have relatively low self-discharge rates and this is generally not a problem in normal engine installations.

Boat Starting Battery Efficiency

Battery Efficiency. Cold temperatures dramatically affect boat starter battery performance. Diesel engine lubricating oil viscosities are also affected by low temperatures, and this further increase the starting loads on the battery. The reduction in battery capacity in low temperatures, combined with the increased starting current requirements, amplifies the importance of having fully charged batteries.

The typical cranking power loss when the temperature decreases from 27°C to -18°C using a typical 10W-30 multi-viscosity lubricating oil and the increased percentage of power required to turn over and start an engine is dramatic. At 27°C with 100% battery power availability you get 100%.  When the temperature drops to 0°C the battery capacity availability falls to 65% and the power required to turn the engine increases due to effects of cold on the engine to 155 %. 

When the temperature falls to -18°C (scary that it could be that in your boat in the depths of an Arctic winter) the battery power availability falls to just 40% and the power to turn over the diesel engine jumps to 210% of normal. There is a reason many commercial vessels have crankcase oil heating, a it improves the start capability.

Boat Starting Battery Charging

Battery Charging. Recharging of starting batteries is identical to deep cycle batteries. Additional factors to consider are:

1. Discharged current must be restored quickly to avoid damage. Similarly temperature compensation must be made.

 2. Normally after a high current discharge of relatively short duration, there is no appreciable decrease in electrolyte density. The battery is quickly recharged, as the counter voltage phenomenon does not have time to build up and has a negligible affect on the charging.

Boat Starting Battery Ratings

Battery Ratings. Starting batteries are normally specified on the basis of engine manufacturer’s recommendations, although I have found these to be often imprecise and not allowing for worst case start scenarios. The following is given as a guide only.  Recommended battery ratings as well as typical starter motor currents.

1. Start Capability. Calculate a good safety margin allowing for a multi-start capability. Some classification societies specify a minimum of 6 consecutive starts, and that should be the absolute minimum value.

2. Temperature Allowance. Additional allowances should be made for the decreased efficiency in cold climates as a greater capacity and greater load current is required.

10 hp 7.5 kW Engine Draws 60 amps with a Battery CCA of 375 amps

15 hp 11 kW Engine Draws 67 amps with a Battery CCA of 420 amps

20 hp 15 kW Engine Draws 67 amps with a Battery CCA of 420 amps

30 hp 22 kW Engine Draws 75 amps with a Battery CCA of 450 amps

40 hp 30 kW Engine Draws 85 amps with a Battery CCA of 500 amps

50 hp 37 kW Engine Draws 115 amps with a Battery CCA of 550 amps

Outboard Boat Starting Battery Ratings

Typical battery ratings are as follows. Check with your outboard engine supplier for the recommended battery rating, and then add a margin for safety. In most cases with smaller boats the start battery is being used for some instruments when fishing so don’t end up with deeply cycled battery or run it down so you can’t start the engine. If you use a lot of extra gear then look at installing different batteries or dual batteries.

70 hp Motor Battery CCA of 580 amps, MCA of 750 amps

150 hp Motor Battery CCA of 680 amps, MCA of 880 amps

350 hp Motor Battery CCA of 780 amps, MCA of 1000 amps

350 hp plus Motor Battery CCA of 1000 amps, MCA of 1200 amps

Boat Starting Battery Loadings

Additional Starting Battery Loadings. The starting battery is often used to supply short duration, high current loads. I prefer to do this as I all cases the engine should be running during operation of a windlass and a thruster. The boat alternator then takes a lot of the load. 

Anchor Windlass. The very heavy current loadings that an electric anchor winch can demand requires a much higher rating. The battery can be doubled up or a larger unit installed, so that two identical batteries are then parallel connected. Deck winches get used a lot without an engine running and so they are usually connected to the house battery bank.

Thrusters. DC powered thrusters are generally powered directly from engine starting batteries. As the engine should be running much of the load is supplied directly from the alternator. It must be noted that as the engine is in slow or idle, the full output is not available, and considerable load will be taken from the battery. Boat starting battery and boat battery information,