Why Do Extra Boat Accessories Require A Second Battery?

If you mostly own or run small craft, you might be used to having a single battery on your boat. A single starting battery is usually sufficient if your primary concern is simply starting a small outboard motor. In many cases, a simple flooded lead-acid battery will usually provide enough power for this application while also being highly cost-effective.

However, the power situation can quickly become more complicated as you add more power-hungry accessories to your boat. In many cases, you'll need to add at least one additional deep-cycle battery to the mix, but why is this the case? Why can't you run your accessories off the starting battery, just as you run your car's accessories from a single battery?

Understanding Marine Charging Systems

Small outboard engines will either use a marine alternator or stator. Marine alternators work similarly to the ones found in cars, and they'll charge your starting battery as the engine runs. Stators provide a similar function but are smaller, simpler devices integral to the engine unit. A stator will still charge your battery, but typically at a lower rate.

However, the key difference comes from the power of the engine. Small boats typically use outboard engines with much less power than most cars, limiting the ability of the engine to keep the battery charged under heavy load. Numerous power-hungry accessories will stress the engine and may discharge the starting battery deeply enough to damage it.

Upgrading to Dual Battery Systems

Dual-battery systems solve these issues by adding a second battery to the mix. Unlike your main battery, this second battery should be a deep-cycle type. Deep-cycle batteries don't need to remain charged like starting batteries, so it's less important for your boat's charging system to keep them topped up. You can discharge these batteries more deeply and recharge them ashore. 

Knowing when to upgrade to a dual battery system can be somewhat difficult, although it's often a good idea if you're adding anything beyond the essentials to your boat. A second battery will protect your starting battery and charging system, providing you with a simple "house bank" that can run your boat's accessories without stressing your starting system.

Choosing a Second Battery

Once you've decided to add a second battery, the next step is to select one. While simple flooded lead-acid batteries can be cost-effective as starting batteries, more advanced options are often better for deep-cycle batteries. AGM and lithium-ion (LiFePO4) are maintenance-free and lighter, making adding them to almost any boat much easier.

Of course, you can use any deep-cycle battery for a dual-battery system, and there's nothing wrong with going with a cheaper option to fit within budget constraints. What's most important is that you recognize the need for adding a second battery as you upgrade your boat, allowing you to protect your starting battery while ensuring you have enough power on the water.

Contact a supplier for more information about marine batteries