Marine-Battery Charging


This article is about boat-electrics and marine-battery charging and this is based on my presentation to the ARC Rally Seminar for cruising yachts doing the transatlantic crossing rally. This information is also based on the contents contained within the Motorboat Electrical and Electronics Manual.

Over the last 10 years or so I have been actively involved in many new large commercial ship and oil rig systems, and as I write am the Commissioning Superintendent for large FPSO project. Before we commission a new offshore installation we carry out what is called a Failure Mode and Effects Analysis (FMEA), and then perform extensive testing and trials to prove and verify the conclusions.


After a few lectures on boat-electrics and many emails from people and continually seeing the same mistakes I decided that the same principles could easily be adapted to boat-electrics on smaller vessels, in particular cruising yachts and power boats. This exercise isn't as difficult as the name suggests, and it essentially entails the analysis of the marine-battery charging and boat-electrics starting and battery charging systems, as both are critical to propulsion or electrical power generation.


The primary function of the exercise is the analysis of the systems and identification of any single point failures. In simple terms this means looking at what can happen in a worst-case situation for all main boat-electrics systems. It is a fact that in most cruising yachts, trawler yachts and motorboats there is virtually no redundancy on any critical marine-battery charging or boat-electrics systems. It is necessary to first identify all of the points that upon failure will also fail the entire system, these are called single point failures for obvious reasons, and then devise suitable methods and strategies to improve boat-electrics reliability and provide some redundancy.


It is important when assessing the failure risks in any marine-battery charging and boat-electrics system to consider the following factors and statistics:

1. A failure in the marine-battery charging system means no charging of batteries and therefore an eventual loss of all electrical power, and possibly propulsion engine starting, loss of communications, navigation instruments etc.

2. A failure in the engine starting system means no propulsion and no charging of batteries, and eventual loss of all electrical power.

3. Approximately 80% of all boat-electrics system circuit failures are due to faulty or failed electrical connections.

4. Approximately 70% of boat-electrics equipment and machinery failures are attributable to poor or improper maintenance or installation. In general most people look at the charging system in terms of a series of discrete components. A charging system must not be viewed as a collection of components, but as an entire electrical power supply system, the boat power station or onboard electricity utility. The typical charging system comprises a considerable number of elements and these are summarized.


Make a simple drawing of your boat-electrics systems. It is a good idea to trace out each circuit on your boat, and draw in each component and mark each connection on it. As a minimum the average boat-electrics system will have 4 main positive circuit connections, 4 main negative circuit connections, 4 control circuit connections, 2 changeover switch contacts, a meter shunt, the alternator, the regulator and the batteries.

1. The alternator (which includes several integral components such as the brushes, brush holders, sliprings, bearings, diodes and stator windings)

2. The regulator, (which may be integral or separate)

3. The DC positive circuit, (which includes the connections at the alternator and battery, and the changeover switch or isolation switch)

4. The DC negative circuit, (which includes the connections at the alternator and battery, the cable back to the battery, and the meter shunt if fitted). In addition the engine block also becomes part of the negative circuit, along with alternator bracket, holding bolts etc.).

5. The batteries. The math of this simple analysis is that on an average boat-electrics system there are an average total of 14 connection points plus the alternator, regulator and battery that can directly impact on the starting system, plus the human element, the “boat owner”.

Each point represents a single point failure with subsequent total system failure, with no apparent redundancy. For this exercise wind, water and solar panels are considered extra or supplementary boat-electrics battery charging sources, as are diesel generators with AC battery chargers. These however can be factored into the redundancy provisions.

The operational factors also must be considered, the human factor in particular. If a battery changeover switch is opened or fails during operation, the alternator can be destroyed, and it is more common than one might think on marine battery charging systems. Read PART 2 of this 4 Part Series on the marine battery charging system risk assessment and improve your power systems reliability. Click on this link to read PART 2 and all you need to know. Marine-Battery Charging - Assessing the Risks