Although frequently not recognized as such, premature battery failures are the most common electrical system failure on boats. If a quality battery does not last five to ten years, or even longer, something is wrong with either the installation or the way in which the battery is being managed. Typically, it is both! On the installation side, a mistake I see over and over again is the way paralleled batteries are wired into an electrical system.
Incorrect Paralleling
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 ‘feeder’ conductor to the boat’s electrical system from the common positive terminal and a negative ‘feeder’ conductor from the common negative terminal.
Frequently, the positive and negative feeds to the boat are taken from the positive and negative terminal on one of the batteries (I 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.
When discharged and recharged, lead-acid batteries have significant internal inefficiencies which generate heat. The lower current flows in the batteries further away from the feed battery result in less internal heat generation. Less heat generation increases the disparity in discharge and recharge rates between the batteries in the bank; the warmer batteries work harder than the cooler. We get into a vicious circle. 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 battery’s positive and negative feeds to the boat’s electrical system. In the past it has been commonly recommended (including by myself) to connect the positive feeder conductor to the battery at one end of the bank and the negative feeder conductor to the battery at the other end of the bank. This improves the situation but still does not fully balance the loads on the batteries, especially when you get to three or more batteries.
The ideal way to balance out the loads on the batteries (which is almost never done!) is to establish positive and negative busbars or terminal 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 terminal posts the same length using the same size conductor.
Given the way heat impacts battery performance, the batteries must all be in the same ambient temperature. If installed close to one another, there should be some spacing to allow convective cooling.
By keeping batteries at the same temperature and connecting each battery separately with an equal resistance connection to the central connection points, you 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.
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. With lead-acid batteries, the two critical voltages are the absorption voltage and the float voltage.
With proper installation and management, you will get five to ten years life out of quality lead-acid batteries, and sometimes more. From time to time, I come across batteries that are 15 years old and still performing well. 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!
