April 2

Understanding Marine Electrics: Best Practices for Paralleling Batteries on Your Boat

Best Practices for Paralleling Batteries on Your Boat

Battery failures are the most common electrical system failure on boats. Many of these failures are avoidable. This is not just a matter of adequate charging and maintenance. Frequently there are paralleled batteries which are improperly wired together at the time of installation. In this blogpost we’ll discuss best practices for installing paralleled batteries on your boat.

The Importance of Properly Paralleling Batteries

Paralleling batteries involves connecting two or more batteries together to increase the available energy. When paralleled, the positive terminals on the batteries are wired together, and the negatives wired together, with a positive feed to the boat from the common positive terminals and a negative feed from the common negative terminals.

Frequently, the positive and negative feeds to the boat are taken from one of the batteries (we will call this the feed battery). But now, because of small voltage drops in conductors and connections, the batteries that are further away from the feed battery see a different duty cycle to the feed battery - they see lower current flows on discharge and recharge. In a multi-battery bank, the further away, the lower the current flows. Lower current flows result in less internal heat generation, which exacerbates the differences between the batteries in the bank. The warmer batteries work harder than the cooler. The net result is the life expectancy of all the batteries is reduced, sometimes significantly.

Optimized Paralleling

What we want to see is equal length conductors and equal resistances between every battery terminal and the connection point of the positive and negative feeds to the boat. The ideal way to do this (which is almost never done!) is to establish positive and negative busbars or connection posts close to the batteries. Then measure the longest distance to a battery terminal and make all conductors from the battery terminals to the busbars or connection posts the same length. By connecting each battery separately with an equal resistance connection to the central connection points, you'll ensure that all batteries are doing the same amount of work, thus increasing their lifespan. The greater the number of paralleled batteries, the more the benefit. The cost of this approach is small when compared to the cost of quality batteries. It obviously must be implemented at the time of battery installation.

Investing in Proper Battery Management

The installation is just the first step in ensuring batteries live up to their potential. Next is ensuring proper management of both the discharge and recharge cycles, especially the recharge cycle. This is significantly dependent on battery type, chemistry and use. We cover all these things in detail our courses on Marine Electrical Systems. If you are not getting at least 5 years out of quality batteries, and preferably 10, you are doing something wrong. Check out our courses to see how to fix things!

If you want to delve more into the topic, you could also take a look at Nigel's Talk on Advances in Batteries & Solar!

About the author 

Nigel Calder

Nigel is often referred to as THE guru when it comes to technical systems on boats.

He is a long-time member of the American Boat and Yacht Council (ABYC) electrical Project Technical Committee (PTC) which writes the standards for recreational boat systems in the USA, and has also been involved in European standards development.

Nigel is best known for his Boatowner’s Mechanical and Electrical Manual (now in its 4th edition), and his Marine Diesel Engines (in its 3rd edition), both considered the definitive English-language works in their field.

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