Charging a Deep Cycle Battery with an Alternator  30 July 2005

A logical choice for the charging source for the deep cycle house battery in a vessel is an alternator driven by the propulsion engine. As simple as this sounds, if not done right, this alternator charging system can be a source of a lot of headache.

It all starts with the selection of the alternator: why would anyone pay three or four times more for a marine type alternator as compared to a large automotive type alternator? As we will see there appear to be some very good reasons, not the least being cost.

 Charging the engine start battery  

To charge a standby battery (a battery that is not being cycled) such as an engine crank battery, a two-stage charge characteristic is a good charging regime. After cranking the engine, a short bulk phase of several minutes during which maximum current is supplied to the battery will quickly bring the battery voltage up again.

When the battery voltage is up, the voltage is kept at a fixed level; commonly 14.1V for a 12V engine start battery. Most automotive type alternators will have a built in regulator that regulates the alternator output to this fixed voltage.  

Charging the house battery  

To charge a deep cycle battery such as the house battery, which will go through deep discharge and subsequent re-charge cycles, a different charging regime is required. First of all there will be a long bulk period of several hours during which maximum current is supplied to the battery. When the battery is about 80% full, the voltage will start to rise. The last 20% of charge will be absorbed by the battery if it is kept at a higher voltage

(about 14.4V-14.8V for a 12V battery, depending on the type of battery) for one- to six hours. After this absorption stage, the battery should be kept at a lower voltage, the float voltage, to prevent damage to the battery.

The device that gives the alternator the charge characteristics as described above is the multi stage regulator, also called a “smart regulator” or “external regulator”.

 Using the automotive type alternator

 A cost effective solution appears to be using an external multi stage regulator controlling an automotive type alternator. Unfortunately, this often leads to premature failure of the alternator.

 So why does the automotive type alternator fail when used on a deep cycle battery and controlled by a multi stage regulator?

The answer is quite simple: it has not been designed for prolonged maximum output. It has been designed to deliver a high current for several minutes and then go into float.  

As we have seen above, the recharge cycle of the house battery can take several hours. The automotive alternator will not be able to dissipate the heat that is generated internally while delivering full output for extended periods of time and the windings will eventually overheat.

When using the automotive type alternator this way, the system heavily relies on the user to make sure that it is not overloaded and that the alternator receives sufficient air cooling.  

Using the marine type alternator  

Marine type alternators are designed to provide charging current at or near their rated outputs for the extended periods needed to fully charge large deep-cycle house banks. This means that they are thermally stable when delivering full output. They are specifically designed for the long recharge periods as experienced with deep

cycle house batteries. In addition to being thermally stable, a good marine alternator will further feature heavy - duty bearings, extra large diodes and have a corrosive resistant coating.  

Output rating

 The output of an alternator is proportional to the resistance of the internal windings. When the alternator is cold, this resistance will be low and the alternator will deliver a high current. The temperature of the windings quickly rises when the unit is delivering current. Normal operating temperature can be as high as 90 to 100 degrees Celsius.

 It is common to rate automotive alternators by their maximum current when cold

It is common to rate marine alternators by their continuous current when hot (hot-rated alternator)

The difference between the two ratings can easily be as high as 25%, this means that a 140A marine alternator may actually put more current into the battery than a 175A automotive type alternator.

 Frame size and alternator rpm

 In addition to the generic type of alternator (ie automotive or marine), another important consideration is the frame size of the alternator. Alternators come in small, large and extra large frames.

Apart from mechanical differences such as size of bearings and diameter of rotor shaft, an important difference between the differently sized frames is the way in which the alternator dissipates its generated heat. The small frame alternator primarily dissipates its heat through forced-air cooling. Special design such as dual internal fans in small frame units will increase the airflow but to ensure sufficient cooling, the revolutions of the small frame alternator should not be too low.

The larger frame designs have the advantage of increased metal mass that works as a heat sink. This makes these designs the most reliable solution for heavy duty charging in combination with low rpm engines such as marine diesels.

 A note on corrosion

 Regardless of the type of battery that is being charged (deep cycle or engine start), in a marine application the alternator should have an isolated ground output. This means that the negative output is isolated from the alternator frame. The isolated ground prevents earth leakage currents from flowing through the alternator mount, engine and hull, which can cause galvanic corrosion. Saving a few dollars on the alternator could ultimately cost the boat!

 The right alternator system for the right job

In summary:  

• When charging a standby battery such as an engine crank battery, the automotive alternator with (internal) fixed voltage regulator is a good choice. It offers short duration high current at low cost. 
• When charging a deep cycle battery, use a marine type “hot rated” alternator with multi stage regulator. The larger the frame size, the more rugged the system will be.
• Do not be tempted by the low cost and apparent high rating of large frame automotive alternators designed for truck engine cranking systems. When these units are operated at prolonged output, the actual current output will be disappointing and worse, the alternator may fail.
• In a marine environment, select an alternator with isolated ground.
• Balmar 6-series small frame marine type alternators

• Balmar 98 series 5kW extra large frame alternator
• Balmar Max Charge multi stage regulator          

 Source: Alphatron Pacific                                            

The Galvanic Isolator for the Marine Vessel Application

Whenever a marine vessel is connected to AC shore power, a galvanic circuit, which can cause corrosion of the vessel and its drive system components, is established. Any time there is a direct bond between dissimilar metals in an electrolyte, a galvanic corrosion cell is established. In this application, the direct bond is the grounding conductor in the vessel-to-shore power cable; the electrolyte is earth and water; and the metals in the marina or port electrical grounding system are normally different than the metals in the vessel. If adjacent vessels are also connected to the shore power from the same source, then a vessel-to-vessel corrosion cell can also be established since they will be interconnected through a common grounding conductor.

The rate of metal erosion is directly proportional to the magnitude of galvanic current. Typical erosion rates for metals are in the 3.6 to 9 kg of metal per ampere per year. If the galvanic current is confined to a small area or a critical drive component, even a small amount of metal erosion can result in damage costly to repair.

A simple way to eliminate this corrosion causing circuit, while retaining the safety against shock hazards provided by the grounding conductor, is to install a galvanic isolator in series with the grounding conductor of the vessel-to-shore power cable. This interrupts the corrosion causing circuit (by preventing the flow of DC galvanic current) while retaining all of the safety features of the grounding conductor, provided an appropriately rated and certified device is used.

A galvanic Isolator provides isolation of the grounding conductor, similar to the function of an isolation transformer, but in a smaller, lighter package, and without the inherent losses. For certain transformer arrangements already installed on a vessel, a galvanic isolator may still be needed for isolation of the grounding conductor from shore.

These notes are taken from DEI Galvanic Isolation information

Testing for Electrolysis activity

Our Electrolysis Analyser is designed to detect the potential difference between dissimilar metals and stray current when submerged resulting in galvanic action and corrosion.

We take measurements between underwater protrusions and the ‘silver chloride half cell’ (the water) to test the integrity of the green wire bonding and any DC leakage.

Take the mystery out of corrosion protection and the need or not, for a Galvanic Isolator with a report in real time mille-volt readings.

It is to your benefit to have this report before a vessel is slipped.

Here's how: call 09 428 0863 to make an appointment to check out your vessel.