Boat Battery Charging

About boat battery charging systems and systems redundancy. The key to minimising failure or mitigating the effects of failure is the provision of redundancy.  Redundancy in simple terms is having backup systems as part of the design, so that on one system failing the other will provide the charging or starting. 

In the average single engine yacht, normal systems design and installation rarely incorporates any redundancy on boat battery charging, boat battery power or boat engine starting systems.  In most commercial shipping, this is a basic premise in all systems design.  There are several methods for improving boat battery charging system redundancy and the following are the easiest and most economical to carry out.  While not easy on some yachts there are some measures that can be implemented. 

Boat Battery Charging

Spare Parts. This includes carrying appropriate boat battery charging spare parts. It is quite astounding how few people carry either a spare alternator or a spare boat engine starter motor, a very cheap investment or insurance I would have thought.

Alternative Charging SourcesAn AC generator with mains powered boat battery charger. The on board generator with a boat battery charger offers the best battery charging source. It is relatively expensive and has space and weight constraints limiting it to larger yachts.        

A combination of wind, solar and water powered boat battery charging systems. They are supplementary only as they depend on environmental factors, if the sun doesn't shine and the wind doesn't blow then no charging is possible.

Boat Battery Charging

There are several relatively low cost modifications that can be carried out on the boat battery charging system. This can provide some measure of redundancy and increased reliability.

Install a second alternator on the engine. This will generally require the addition of a second pulley.  Some yachts have very tight engine spaces and this may be difficult.  The second alternator is for the house battery charging circuit, with the existing boat battery charging alternator being used just for charging the start battery. A high output alternator with smart regulator makes this a viable modification.

Alternator Wiring Modifications. Each alternator will have a separate positive circuit without any switches or other devices in it.  This will eliminate changeover switch problems on alternators that commonly destroy the alternator rectifier diodes, and just as importantly reduces electrical connections to just 2.  It also eliminates the accidental (human error) switch operation under load, or switch contact failures, which are both very common. 

Each alternator will have a separate negative circuit cable running back to the respective battery from the alternator.  This provides separation from the starter motor to battery negative, with the main starter negative serving as a backup. This also reduces circuit connections to just 2.  It also takes the engine block out of the circuit, and generally reduces voltage drop in the circuit.  There is anecdotal evidence that current flow though a bearing also results in reduced engine bearing life.

Boat Battery Charging

Separate the charging system. Separate the charging system from the starting circuit, in the long term this will considerably reduce problems and increase reliability.  This process entails the deletion of battery selection changeover switches, and the installation of a separate charging circuit, which may include charge splitting diodes or relays.  An emergency crossover switch between battery banks can be installed, however this does not affect the circuit during operations.   

Install Separate Negative Cables.  Install a separate negative conductor of at least 15mm² (6 AWG) from each alternator case or negative terminal directly back to the corresponding battery negative.  This bypasses the engine block and all the cumulative resistances of mountings and brackets. 

This offers a good low resistance path and reduces stray currents through the block, which can cause pitting of bearings.  It also eliminates a single point failure of the main negative connection to the engine block. If it comes loose, comes off or fails the spike generated also blows the alternator diodes as well as causing serious charging efficiency losses.  In some cases heat at the connection can start a fire.

Boat Battery Charging

Upgrade Alternator Positive Cable.  Most installed positive cables are too small, especially if a fast charge regulator is installed.  The cable size should generally be doubled over existing sizes.  Ideally install a minimum of 15mm² (6 AWG) cable size.  I usually take the alternator maximum rating and add 25% as a rule of thumb. 

A common problem is that besides having a maximum current going through it with fast charge devices connected or when heavily discharged batteries are recharged, the heat of the engine compartment also de-rates the current capacity of the cable. 

In most cases a significant voltage drop develops across the cable under full output conditions.  This also should ensure that cable connections are also properly rated for the maximum current, a common failure point is undersized terminals causing high resistance and hot spots.  This drops charging efficiency and can lead to major failure.  When a main positive cable connection fails the spike usually blows the diodes in the alternator.

Boat Battery Charging

After carrying out our FMEA and boat battery charging systems assessment, and implementing some modifications, just how much further ahead are we?  The most salient point is that there is now a significant reduction in single point failures.  With two separate charging circuits there is full charging system redundancy. There are now significantly reduced electrical connections in both the positive and negative circuits, the negative having a backup with the starter motor negative. 

In a typical boat battery charging system using a changeover switch arrangement there is a reduction of up to 75% in possible failure points.  In addition, the person has been designed out of the system as the possibility for accidental switching off the changeover switch is removed. This is very common.

There is a significant improvement in boat battery charging efficiency. There is a gain of up to or exceeding 0.5 VDC due to lower circuit resistance in both the positive and negative circuits.  This reduces alternator loads, and can shorten battery charging time. It can reduce charge current and extend alternator life by imposing less stress.  Boat battery charging problem resolution and more about the boat battery.