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All you need to know about

alternators and charge regulators

These alternators also tend to be

temperature-sensitive: In a higher

ambient temperature, such as is common

in an engine room, their output quickly

falls by 50 % or more. This is not a

problem in cars as the small amount

of energy used during, for instance,

starting can be recharged in no time,

and windscreen wipers, ventilators, etc.

do not need a great deal of power. In

general, a car engine also runs at a far

higher rpm than a boat’s engine, and the

temperature under the bonnet is lower

due to the cooling effect of the head


Why a Mastervolt alternator?

Mastervolt Alpha alternators are specially

designed for ships and professional

mobile applications to provide sufficient

power even with a low rpm. A pulley ratio

of 1:3 and an engine idle speed of around

700-800 rpm will generate substantial

current for charging the battery sets and

powering the connected equipment.

Mastervolt alternators are also resistant

to the high temperature of the engine

room, allowing the engine to serve as the

energy source for onboard consumers

and as a quick charger for the service


Make sure you do not choose your

alternator too small.

A larger one will ensure that the battery is

charged faster and engine hours are kept

to a minimum – we suggest choosing an

amperage between 30 and 50 % of the

battery capacity.

The standard alternator designed for

road vehicles has a voltage regulator

mounted to the back of the alternator

and set to a single charge voltage of

14 or 28 volt. This is sufficient for a

car battery, which is rarely (if ever)

discharged. Furthermore, the voltage

regulator of an automotive alternator

is often temperature sensitive and in

high temperatures regulates the voltage

even further down, often to 13.5 or

26.5 volt. This is much too low for

sufficiently recharging a discharged

battery. The maximum achievable

battery capacity for these voltage levels

is around 60 to 70 %. The lifespan of a

battery is significantly lower if it is never

properly charged. In order to adequately

recharge a partially empty or completely

flat battery at 25 ºC, voltage need to be

14.25 volt for a 12-volt battery and

28.5 volt for a 24-volt one. Once the

battery is 100 % full, this voltage must

be reduced to 13.25 or 26.5 volt (float

More information on alternators and installation diagrams

are available at


To quickly charge the batteries while the engine is

running, we recommend an additional ‘high output’

Mastervolt alternator on the engine.

phase) to prevent the batteries from

becoming overcharged.

Ungrounded – also for

aluminium vessels

Mastervolt alternators are delivered

ungrounded, i.e. the negative pole of

the alternator is not connected to the

alternator casing but has a separate

connection. This means that they are also

suitable for aluminium boats, where the

negative needs to be separated from the


More output

Mastervolt alternators offer a much

higher output than the alternators

supplied with engines. As a result, the

standard single belt is insufficient to

transfer the power from the engine to the

alternator. Two belts are required and

the pulley of the engine will often need to

be changed as well. Your engine supplier

can help you choose an appropriate

double pulley and give advice on setting

up the alternator. In order to handle the

high output you will also have to adjust

the alternator support.

Standard alternators, originally designed for the

car industry, only deliver sufficient energy to both

charge the batteries and power the various onboard

consumers when they reach a very high rpm.