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Cranking Amps Vs. Battery Capacity — What's The Difference?

by Konsta Latt

Buying a new battery for your boat can be a little more complicated than buying a new battery for your car. While vehicles typically only use a single starting battery, boats may use banks of multiple batteries for power. Depending on your boat's configuration and needs, you may require a starting battery and a separate house battery bank.

Since you'll need to make a few extra decisions when purchasing a marine battery, you'll probably want to understand as much of the terminology as possible. This guide will review two of the most essential (and basic) aspects of any marine battery purchase: cranking amps and battery capacity.

Start Your Engines: CA, CCA, and MCA

Starting batteries are the closest marine analog to a car battery but are not quite the same. A typical marine battery will have numerous features you won't find in most car batteries, including spill-proof designs that can tilt well beyond what's possible (or safe) with an automotive battery. These features make them more suitable for the harsher environment found on boats.

However, their fundamental role is the same as an automotive battery: to provide a quick jolt of power to get your boat's motor turning. Manufacturers use the cranking amp (CA) metric to help you understand how much power the battery can deliver. This value tells you the sustained amps delivered by the battery over a relatively short period while maintaining the minimum required voltage.

Cold-cranking amps (CCA) are similar, but measure sustained amperage at freezing temperatures. This value matters for cars that often operate in winter, but it's less important for seasonal boats. Instead, you'll want to match the battery's marine cranking amps (MCA) to your motor's requirements. MCA is essentially the same value as plain cranking amps since it's measured at the same (higher) temperature.

Sustained Power: Battery Capacity

Capacity is an entirely separate concept from cranking amps. If you need to power accessories on your boat, capacity is the name of the game. For capacity, you need to look at the battery's amp-hour (Ah) rating. The more accessories you run on your boat, the more capacity you'll need. For larger boats, you'll typically use one starting battery and a separate (deep cycle) house battery for powering accessories.

Determining your capacity needs can require a bit of work. A marine power usage chart is a good place to start, especially if your boat has many electrical systems requiring power. You can estimate your total power usage and use that number to determine the appropriate capacity for your house battery bank in amp-hours.

While most boats require at least one starting battery and one or more house batteries, dual-purpose batteries are an option for smaller craft. These are effective deep-cycle batteries that can also provide starting amperage for your motor. Although generally unsuitable for larger boats, these batteries work well on boats where space and weight are at a premium.