The battery as power source - Y
TECHNICAL BACKGROUND
The battery as power source
There are different kinds of
rechargeable batteries. The most
common type is the lead acid
battery. A less familiar one is the
nickel-cadmium (NiCad) battery,
which can still often be found in old
emergency power systems. Due to
the high charge voltage required
by a NiCad battery, and the fact
that they are very environmentally
unfriendly, these batteries are not
suitable for use onboard a vessel or
car/truck.
Principle of the lead acid battery
A battery is a device that stores electric
The upper limit on charge voltage for a
power in the form of chemical energy.
12 V battery is 14.4 V, and the
When necessary, the energy is again
corresponding value for a 24 V battery is
released as electric power for DC
28.8 V at 20 °C. The relationship between
consumers such as lighting and starter
how full a battery is and the specific
motors. A battery consists of several
gravity of the water/sulphuric acid
galvanic cells with a voltage of 2 Volt
mixture is as follows:
each. For a 12 Volt battery, six cells are
linked in series and fitted inside a single
percentage
charged
battery
voltage
specific
gravity
percentage
discharged
batteries are linked in series. Each cell
0%
11.64 V
1.100
± 100%
has positive oxidised lead plates and
20%
11.88 V
1.140
± 80%
40%
12.09 V
1.175
± 60%
60%
12.30 V
1.210
± 40%
80%
12.51 V
1.245
± 20%
100%
12.72 V
1.280
0%
casing. To achieve 24 Volt, two 12 Volt
negative lead metal plates, and has
an electrolyte consisting of water and
sulphuric acid. During discharging, the
lead oxide on the lead plates is converted
into lead. The acid content decreases
because sulphuric acid is required for this
Different types of battery - in terms of
process.
the thickness and number of plates
per cell - correspond to different
To recharge the battery, an external
applications. The maximum current
power source - such as a battery
that can be delivered is determined by
The starter battery
charger, alternator or solar panel - with a
the total plate surface. The number of
A starter battery has many thin plates per
voltage of around 2.4 V per cell must be
times that a battery can be discharged
cell, leading to a large total plate surface.
connected. The lead sulphate will then be
and recharged - the number of cycles -
This type of battery is, therefore, suitable
converted back into lead and lead oxide,
depends on the thickness of the plates.
for delivering a high level of current over
and the sulphuric acid content will rise.
A battery can feature either many thin
a short period of time.
There are limits set for the charge voltage
plates or a few thick ones.
The number of times that a starter battery
to prevent the release of an excessive
can be heavily discharged is limited to
amount of hydrogen. A charge voltage
around 50-80. But as starting the engine
of more than 2.4 V per cell, for instance,
uses only a small part of the energy stored
releases a lot of hydrogen gas, which can
(around 0.01%), the battery lasts for many
form a highly explosive mixture with the
years. This type of battery is generally
oxygen in the air.
unsuitable for cyclic use.
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BATTERIES
An added benefit is that Lithium Ion
And because lithium is the lightest metal,
batteries can supply a constant capacity,
Lithium Ion batteries are also more
regardless of the connected load.
lightweight. They can also be charged
The available capacity of a lead acid
at any time, while nickel-cadmium
battery is reduced in case of higher
batteries require complete discharge for
discharge currents. Lithium Ion batteries
an optimal performance and to prevent
The Lithium Ion battery
can be discharged to 80% without
memory effect. Furthermore, Lithium
Until recently Lithium Ion batteries were
affecting their lifespan, whereas lead
Ion batteries can be charged with a very
mainly available as chargeable batteries
acid batteries are more affected by deep
high current, up to 100% of the capacity,
with a small capacity, which made
discharge.
resulting in a very short charging time
them popular for use in mobile phones
and no memory effect.
and laptops. Mastervolt offers Lithium
Lasts longer
Ion batteries with large capacities.
Lithium Ion batteries also offer major
Battery Management System
Our Lithium Ion batteries have a high
benefits compared to nickel-cadmium
Mastervolt Lithium Ion batteries are
energy density and are perfect for cyclic
batteries, such as a much larger power
equipped with a Battery Management
applications. Compared to traditional
density and a longer lifespan.
System that automatically compensates
lead acid batteries, Lithium Ion
for the imbalance between the cells and
batteries offer savings of up to
increases the lifespan.
70% in volume and weight, while
the number of charging cycles is
three times higher, compared to
semi-traction lead acid batteries.
The traction battery
The semi-traction battery
(Mastervolt does not have this type of batteries in its portfolio)
This type of battery has even fewer, but
The battery is discharged unevenly,
very thick, flat or cylindrical plates. It can
significantly reducing its lifespan.
therefore be discharged many times and
In order to spread out the acid
fairly completely (1000-1500 full cycles).
evenly again, the battery has to
This is why wet traction batteries are
be purposefully overloaded using
often used in forklifts and small electrical
excessive voltage. This generates
equipment such as industrial-grade
a large amount of hydrogen gas,
cleaning machines.
which will form a dangerous
mixture with oxygen in the air.
But wet traction batteries require
The voltage required to recharge
a special charge method. Because
these batteries is around 2.7 Volt
these batteries are mostly tall, they
per cell, or 16.2 Volt for a 12 V
A semi-traction battery has fewer but
are sensitive to the accumulation of
system and 32.4 Volt for a 24 V
thicker plates in each cell. These batteries
sulphuric acid at the bottom of the
system. These high levels of voltage
supply a relatively lower starter current,
battery container. This phenomenon is
are extremely dangerous for the
but can be discharged more often and to
called stratification and occurs because
connected equipment and the large
a greater extent (200 to 600 full cycles).
sulphuric acid is denser than water.
amount of gas generated makes
This kind of battery is highly appropriate
Acid content increases in the lower part
these batteries unsuitable for use
for the combined function of starter and
of the battery, locally intensifying plate
in vessels and vehicles, except for
service battery.
corrosion, and decreases in the upper
propulsion.
part, reducing capacity.
175
TECHNICAL BACKGROUND
Frequently asked questions
about batteries
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How long will it take before my
battery is discharged?
What is series connection and parallel connection?
This depends on its capacity and the
Two serially connected 12 V/120 Ah batteries make a combined battery set of 24 V/120
amount of power consumed by the
Ah. In a series connection, the positive pole of one battery is connected to the negative
connected equipment. As a rule, the
pole of the other, with the poles that remain at the ends being connected to the system.
faster a battery is discharged, the less
Batteries with different capacities should never be linked in series.
A series connection is used to increase voltage, while keeping capacity at the same level.
power it supplies. This also works the
other way around: The longer it takes
before a battery discharges, the more
energy you can get from it.
A 100 Ah lead-acid battery supplies a
Examples
current of 5 amps for 20 hours, during
The examples below apply to the use of normal 12 V batteries. Mastervolt also
which time the voltage does not drop
supplies 2 V, 6 V and 24 V batteries; the principle of series and parallel connecting
below 10.5 Volt. This amounts to 100 Ah.
remains the same.
If a load of 100 amps is connected to the
same battery, the battery will be able
to power it for only 45 minutes. After
Series connection
this time the battery voltage will fall to
10.5 Volt and the battery will be empty,
having supplied no more than 75 Ah. In
contradiction to the lead-acid batteries,
the capacity of Lithium Ion batteries will
not be effected by the load connected.
A Li-ion battery will always supply 100%
Series connection
24 V/200 Ah.
capacity, independent of the connected
load.
Series connection 48 V/200 Ah.
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Parallel connection
How long will my battery last?
Parallel connection is used when you need to
The lifespan of a battery is related to how
increase your capacity. The positive leads are
often and to what extent it is discharged.
connected together, as are the negative leads.
Proper charging with the right charger is
The cabling from the battery to the system should
also crucial. At a normal use for holidays
be: Positive from battery 1 and negative from
and weekends, a lifespan of between
battery 2 (or the last in the parallel connection).
five and seven years is quite common for
Parallel connection
12 V/400 Ah.
gel and AGM batteries. When batteries
are frequently discharged you will need
Series/parallel connection
to adjust the capacity. There is also an
If you need a 24 V battery set with a higher capacity,
option to use 2 Volt cells. A lifespan
you can combine series and parallel connections.
of 15 years is not exceptional for this
The cables from the battery to the system must be
type of battery as long as they have the
crossed: Positive from battery 1 and negative from
right capacity and are properly charged.
battery 2 (or the last in the parallel connection).
Lithium Ion batteries are top of the bill.
You can discharge and recharge them
super fast and they last up to three
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times as many cycles than other types of
Make sure there is sufficient space between the
batteries.
batteries when installing multiple batteries:
There should be a ‘finger’ of space between them
to allow the heat to be diverted.
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Series/parallel connection
24 V/400 Ah.
BATTERIES
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What NOT to do with batteries, especially gel and AGM
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Incorrect charge voltage. Too low a voltage means that the battery cannot
charge to 100% - the sulphate then hardens on the plates and the battery
loses its capacity. Excessive voltage causes the batteries to generate gas,
leading to water loss and drying out.
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Excessive discharging. Discharging a battery further than its capacity
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Too large ripple on the charge voltage. Cheap and old-fashioned chargers
shortens its lifespan.
often have a significant voltage ripple (voltage variation) in the output
voltage.
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The use of an alternator without 3-step regulator, a high ambient
temperature or charging without temperature compensation.
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How should I maintain gel, AGM
and Lithium Ion batteries?
Can I leave the batteries onboard during winter?
Gel, AGM and Lithium Ion batteries do
This is fine for all batteries as the lower temperature will actually prolong their
not need maintenance, which means they
lifespan. Remember to charge the batteries completely and to ensure that no
can be installed anywhere. However, we
consumers are left on. Voltmeters, timers and car radio memory are some of
recommend checking all the connections
the stealthy consumers to look out for in this respect. Wet batteries have to be
once a year to make sure that they are
regularly topped up and charged to avoid freezing. It is advisable to connect the
properly attached, and to clean the top
power once every two to three weeks so the batteries can be fully recharged. If you
surfaces with a slightly moist cloth. The
do not have access to power during the winter we advise you to fully charge the
batteries also need to be completely
batteries before the winter and then disconnect the battery poles so small users
charged every time for a maximum
cannot discharge your battery. We also advise charging your battery every two
lifespan.
months.
What are maintenance-free batteries?
Various types of batteries are used, each with its own specific characteristics.
Here is a summary:
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The second design
The gel battery
This process results in tiny hairline cracks
is a 2 Volt traction
With wet lead acid batteries, that use a
in the gel between the plates.
battery, available
liquid electrolyte of water and sulphuric
During the charging process, oxygen O2
in capacities up to
acid, the water is seperated into
is generated on the positive plate and
2700 Ah and highly
hydrogen and oxygen during charging,
hydrogen H2 on the negative plate. The
suitable for systems
mostly at the end of the charging cycle.
cracks in the gel let the gases combine to
with frequent and
These gases subsequently escape
create water. The gel then absorbs the
significant dischar-
through the filler cap. This means water
water so that no water disappears from
ging where a long
is used and distilled (battery) water needs
the system and no gases are produced.
lifespan is needed.
to be added.The electrolyte in a gel
For a battery of 12
battery is a gel that binds the water with
Gel batteries are not a new technology
the acid. While the batteries are being
and have been in use since the late
twelve gel batteries need to be connected
or 24 Volt, six or
filled, the gel is heated and liquefies.
1950s. The most important applications
in series to provide the required voltage.
After the battery has been filled with the
are in emergency power systems,
liquefied gel, the gel cools and solidifies.
telecommunications systems, power
Major benefits of gel batteries include very
supply and, for the last 20-25 years, as
limited self-discharging, the possibility of
service batteries in various systems. Gel
a short charging time, and the lack of gas
batteries come in two different versions.
production under normal circumstances.
The 12 Volt design is appropriate for
All of this makes gel batteries very suitable
regular use and available in capacities up
for heavy cyclical applications.
to 200 Ah.
177
TECHNICAL BACKGROUND
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The AGM battery
discharge current is no problem.
(cheap) unregulated battery charger, a
A different type of lead
The charge current could be
small amount of hydrogen gas is formed.
acid battery is the AGM
a little lower than with gel
This gas escapes through a special vent
(Absorbed Glass Mat)
batteries (approx. 30%) because
in the battery casing that is designed
battery. In this
the glass fibre mat is also an
to prevent oxygen from entering the
model, the electrolyte
efficient heat insulator, and
battery. Incorrect charging will reduce the
(water and sulphuric acid) is absorbed
heat generated by charging is gradually
into an extremely delicate glass fibre
conducted to the outside of the case.
mat. Just like with any other battery,
This requires the charge current
charging generates hydrogen gas and
to be somewhat restricted and
oxygen, which are transported through
results in a slightly longer charging
the capillary tubes of the glass fibre mat.
time. AGM batteries are highly
Once the two gases are recombined,
suitable for applications requiring
water is once again obtained and
a high discharge current, such as
subsequently reabsorbed into the glass
a bowthruster or winches and for
fibre mat. The recombination process
medium cycle use.
is then complete. The glass fibre mat
battery’s lifespan.
Conclusions and
recommendations
also serves as insulation between the
The AGM battery is entirely closed
plates, allowing the plates to be close
and therefore maintenance free.
batteries makes them highly suitable
together and leading to very low internal
If the AGM battery is overcharged,
for powering winches, windlasses and
resistance. This means that a high
for instance due to the use of a
bowthrusters, for starting engines, and
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The low internal resistance of AGM
for limited cyclic use.
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Gel batteries are very suitable as
service batteries due to the fact that
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they can be quickly charged and have a
The Lithium Ion battery
long lifespan, even with many charge/
Mastervolt’s Lithium Ion batteries are
discharge cycles
based on Lithium Ion iron phosphate,
which has an energy density of three
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For a service battery you can choose
times higher than that of lead acid
for either a 6 Volt, 12 Volt or 24 Volt
batteries. Although there are materials
version or the 2 Volt model.
with an even higher energy density,
these are generally considered less safe.
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Mastervolt batteries are completely
Mastervolt’s Lithium Ion batteries are the
maintenance free and in normal
safest batteries of their kind. A unique
circumstances do not release acid or
feature is their built-in Battery
generate dangerous gas. They are
Management System (BMS). The system
This means that you can only
easy to install anywhere onboard,
controls cell voltage and temperature,
use up to 200 Ah from a 400 Ah
such as next to the bilge or in the
and guarantees optimal safety. Lithium
battery. A Mastervolt Lithium Ion
engine room (reduced lifespan due to
Ion batteries are MasterBus compatible
has a DOD of 80%, almost 60%
higher temperatures). Special battery
and up to 15% more efficient than lead
more usable battery capacity.
cases or external ventilation is usually
acid batteries.
With this percentage, a battery of
unnecessary as natural ventilation will
400 Ah supplies 320 Ah, or 120
suffice.
This gives you:
Ah more.
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Shorter charging times.
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Less generator time required for
ideal for electric and hybrid
in space and weight, last three times
charging.
propulsion. Mastervolt Lithium
longer and can be recharged and
More power than from a traditional
Ion batteries can be paralleled
discharged very quickly, 2000 charge
battery of the same dimensions.
up to ten units.
cycles is no exception.
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A normal open lead acid battery,
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Lithium Ion batteries are also
Another benefit is that Lithium
for example, has a DOD (depth of
Ion batteries weigh less and
discharge) of 50%.
require less space.
178
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Lithium Ion batteries save up to 70%
BATTERIES
Determining lifespan
The average lifespan of a 12 Volt gel
It is therefore not necessary to replace
or AGM battery is up to six years if the
the battery after six years or 500 full
battery remains unused and is kept in
cycles. An average use of seven years is
a charged state. After five or six years
perfectly normal for 12 Volt gel batteries.
Transport
temperature of 25 ºC, the battery still
2 Volt traction gel batteries
retains 80% of its original capacity.
The lifespan for 2 Volt traction gel cells is
Transportation of
Mastervolt gel and AGM
batteries
Higher average temperatures will
around 10 to 15 years and the maximum
Mastervolt gel and AGM batteries
shorten the lifespan of the battery.
number of full cycles is 1000-1500 when
are considered as non-spillable
The number of charge and discharge
discharging to 20% of capacity. These
batteries. This means that they can
cycles of a 12 Volt battery is strongly
batteries are therefore highly suitable for
be transported as non-dangerous
correlated to its structure and quality.
larger systems that require intensive use
goods as they are exempt from
Mastervolt’s 12 Volt gel batteries can
and a very long lifespan.
Dangerous Goods Regulations
of float voltage at an average ambient
take around 500 full cycles of being
which cover transport by road, rail,
discharged down to 20% and charged
Lithium Ion batteries
sea freight or air cargo. So they can
back to full capacity.
Mastervolt Lithium Ion batteries have a
be sent to any destination in the
lifespan of more than 2000 cycles, which
world quickly and relatively cheaply.
Most manufacturers consider batteries to
is three times longer than most standard
be spent at a remaining capacity of 80%.
lead acid batteries.
This does not, however, mean that the
This can be attributed to a wide range of
battery has to be replaced immediately.
features including cell management, the
Transportation of
Mastervolt Lithium Ion Ultra
batteries
negligible self discharge, the absence of
Extra care is to be taken for
‘memory effect’ and a discharge to 20%.
proper transport of Lithium Ion
For example, the battery can still be used
if only 50% of the battery capacity is
batteries. Mastervolt’s Lithium
actually required.
Ion batteries and their packaging
have undergone all the required
safety testings as prescribed by the
United Nations and the transport
authorities (both road, rail, sea and
air) to achieve this. Below you will
find the technical details of what
this means.
The Mastervolt Lithium Ion batteries
have been tested according to UN
Handbook of Tests and Criteria,
part III, sub section 38.3 (ST/SG/
AC.10/11/Rev.5). For transport the
batteries belong to the category
UN3480, Class 9, Packaging Group
II and have to be transported
according to this regulation. This
means that for land and sea
transport (ADR, RID & IMDG) they
have to be packed according to
packaging instruction P903 and
for air transport (IATA) according
to packaging instruction P965. The
original packaging of the Mastervolt
Lithium Ion batteries satisfies these
instructions.
179
TECHNICAL BACKGROUND
3-Step+ charging
This modern charging technology
throughout this stage, and the charging
switches back to the bulk phase.
allows a battery to be quickly and
current depends on the degree to which
If consumption is reduced, the charger
safely charged in three phases
the battery was initially discharged, the
will start charging the battery again via
(steps).
battery type, the ambient temperature,
3-step+ charging. A battery charger with
and so on. With a wet battery this phase
3-step+ charging can remain connected
The first step is the BULK PHASE, in which
lasts some four hours, with gel and AGM
to the battery, even in winter, and
the battery is charged quickly.
batteries around three.
ensures a long lifespan for your batteries
The output current of the battery charger
This does not apply to Lithium Ion
as well as being safe for the connected
is at maximum (100%) during this phase
batteries as these are charged to 100%
equipment.
and the battery voltage depends on
with full current.
Absorption time
the charging degree of the battery. The
duration of this phase depends on the
Once the battery is 100% charged,
The duration of the second phase in the
ratio of battery to charger capacity, and
the Mastervolt charger automatically
charging of a battery. The battery will,
on the degree to which the batteries were
switches to the FLOAT PHASE.
in general, be charged from 80 to 100%
discharged to begin with.
In this step, the batteries are kept in
during this phase, which lasts around
optimal condition and the connected
four hours with a wet lead battery, and
The bulk phase is followed by the
users are supplied with power. If power
three hours with gel and AGM batteries.
ABSORPTION PHASE, which begins once
consumption is higher than can be
With Lithium Ion batteries the absorption
a battery has been charged to ± 80%
supplied by the battery charger, the
time is very short as they can be charged
(90% for gel and AGM batteries), and
remaining power is supplied by the
to 100% with full current. This phase is
ends when the battery is completely
battery. The battery is then (partly)
automatically set for Mastervolt battery
full. Battery voltage remains constant
discharged and the charger automatically
chargers.
3-Step+ charging characteristic (IUoUo)
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3 STEP+ CHARGING
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Charge factor
Cycle
The charge factor indicates the efficiency
A battery only lasts a certain number
If the battery is only discharged to 50%
of a battery. The efficiency of the average
of charge/discharge cycles, depending
of capacity, 600 cycles are available.
wet battery is approx. 80%, which means
on its type and quality. In theory one
Assuming 25 weekends of sailing (50
it must be recharged 1.2 times the
charge/discharge cycle is the process
days) plus 20 days of holiday and
eventual capacity in Ah to get the same
of discharging a battery to 0% of
discharging only to 50%, the battery will
capacity. This translates into a charge
capacity and recharging it back to 100%.
go through 70 half cycles or 35 full cycles.
factor of 1.2. The lower the charge factor
Twice recharging after discharging to
or the higher the battery efficiency, the
50% is also one cycle, as is four times
better the quality. Mastervolt’s gel and
discharging to 75% and recharging. A
AGM batteries have an efficiency of > 90%
starter battery, for instance, can take
and a low charge factor of 1.1 to 1.15 and
around 50 to 80 cycles, which may
offer the very best quality.
seem little but is in practice more than
sufficient: While the current used for
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starting an engine is high, it only lasts
a short time and represents 0.001 of a
Discharge factor
cycle. In other words, an engine can be
This is also known as Peukert’s Law,
started 80,000 times before a battery is
and allows you to determine how long
worn out. A high-quality semi-traction
a battery can be used at a given load
battery lasts for around 250 to 300 cycles.
before it needs recharging.
181
TECHNICAL BACKGROUND
Charging batteries
Charge voltage
The charge system
Gel (12 and 2 Volt) and AGM (6 and
Ensuring the longest possible lifespan
12 Volt) batteries need to be charged
for gel, AGM and Lithium Ion batteries
with a voltage of 2.4 Volt per cell at
requires a modern battery charger
a temperature of 25 ºC. For a 12 Volt
with 3-step+ charging and a sensor for
battery set, this corresponds to 14.4 Volt,
measuring battery temperature. These
and for a 24 Volt battery set to 28.8 V.
battery chargers will constantly regulate
The maximum time that a battery can be
charge voltage and charge current and
charged at this voltage is four hours, after
adapt the charge voltage to the battery
which the voltage has to be reduced to
temperature.
2.2 Volt per cell, or 13.25 and 26.5 Volt,
As there is always equipment onboard
respectively. Lithium Ion batteries need
such as refrigerators that draw power
to be charged with a voltage of 29.2 Volt
from a battery even when it is being
for a 24 Volt system and 14.6 Volt for a 12
charged, a maximum charge voltage
Volt system. The float voltage is 26.5 and
has been set to protect the connected
13.25 respectively.
appliances. This maximum is 14.55
Volt for a 12 Volt system and 29.1 Volt
With emergency power systems, where
for a 24 Volt system, which is also the
gel batteries can be in float condition for
charge voltage applicable at an ambient
long periods of times (years), the float
temperature of 12 ºC.
voltage needs to be slightly increased
Mastervolt’s modern battery chargers
to 13.8 and 27.6 respectively at a
come with a temperature sensor for
temperature of 25 ºC. Mastervolt supplies
attaching to the battery, which allows
DC-DC converters that regulate the
the charger to automatically regulate the
onboard voltage to a lower level (13.8 or
charge voltage in accordance with battery
27.6 Volt) thus ensuring that (halogen)
temperature. Adjusting voltage to high or
lamps do not fail during charging.
low temperatures is not necessary with
Mastervolt chargers are, of course,
Lithium Ion batteries.
all equipped with an excellent
The charge current
voltage regulation that keeps ripple
A rule of thumb for gel and AGM
In order to prevent premature failure
voltage below 100 mV. For GMDSS
batteries states that the minimum charge
of the battery, the ripple voltage of the
(Global Maritime Distress Safety
current should be 15 to 25% of the
battery charger has to stay below 5%.
System) systems onboard larger
battery capacity. Connected equipment
If the battery also powers navigation
ocean-going vessels, the battery
usually also needs to be powered during
or communication equipment such as
charger also can be equipped with
charging, so include the power used for
GPS or VHF, the ripple voltage must be
an amps and voltmeter plus an
that purpose in the abovementioned
less than 100 mV (0.1 Volt) or problems
alarm contact. The alarm contact
figure. This means that, with a battery
may occur with the equipment. Another
is connected to the ship’s alarm
set of 400 Ah and a connected load of
advantage of a low ripple voltage is
system so that any interruptions to
10 amps, battery charger capacity has to
that onboard power systems will not be
the operation of the charger – due
be between 70 and 90 amps in order to
damaged if a battery pole is corroded or
to a cut in the 230 Volt supply, for
charge the battery in reasonable time.
incorrectly attached. A low ripple voltage
instance – are detected on time.
even allows the charger to power the
The optional Mass Charger Interface
The maximum charge current is 50%
system without being connected to a
makes Mass chargers very suitable
for a gel battery and 30% for an AGM
battery.
as GMDSS chargers.
battery. For a Lithium Ion battery the
charge current can be the same as the
capacity. A 180 Ah Lithium Ion battery,
for example, can be recharged with
180 amps.
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CHARGING BATTERIES
Calculating charging time
Various factors have to be taken into account when
calculating the charging time for a battery.
The first consideration is battery efficieny. With a
standard wet battery, efficieny is about 80%. This
means that 120 Ah has to be charged into the battery
in order to be able to draw 100 Ah later. With gel, AGM
and Lithium Ion batteries the efficieny is higher - 85 to
90% - so there is less losses and charging time is shorter
compared to wet batteries.
Another issue that has to be considered when calculating
charging time is the fact that the last 20% of the charging
process (80-100%) takes ± four hours (this does not
apply to Lithium Ion batteries). In the second stage, also
known as the absorption phase, the battery dedicates
how much current it needs to absorb independently of
the output of the battery charger.
The amount of power depends on the type of battery
(wet, AGM, gel or Lithium Ion) and other factors such
The following formula can be used to calculate
the charging time of a gel or AGM battery:
Lt =
Lt
Co x eff
Al - Ab
as the extent to which it was charged to begin with,
temperature, lifespan and the ambient temperature.
+ 4h
25 ⁰C reference
temperature
30
= charging time
Co = capacity drawn from the battery
29
eff = efficiency; 1.1 for a gel battery, 1.15 for a AGM battery
absorption
28
and 1.2 for a wet battery
Al
= battery charger current
27
Ab = consumption of the connected equipment during the
Taking a battery that is discharged to 50% and applying the
example of a 400 Ah gel battery and an 80 amps charger,
charging up to 100% will take:
charging voltage in V
charging process
float
26
25
24
-30
Lt =
200 x 1,1
80 - 10
+ 4h ≈ 7h
-20
-10
0
10
20
30
40
50
60
battery temperature in ⁰C
Temperature compensation curve
183
TECHNICAL BACKGROUND
n
Peukert’s Law
On the surface it seems easy to calculate how much longer a
battery will continue to supply sufficient power. One of the most
common methods is to divide battery capacity by discharge
current. In practice, however, such calculations often turn out to
be wrong. Most battery manufacturers specify battery capacity
assuming a discharge time of 20 hours.
A 100 Ah battery, for instance, is supposed to deliver 5 amps per
Checking the remaining capacity of a sealed
AGM or gel battery
hour for 20 hours, during which time voltage should not drop
The simplest way to check the remaining capacity or
a current level of 100 amps, a 100 Ah battery will deliver only 45
condition of a battery is with an Ah meter, such as
Ah, meaning that it can only be used for less than 30 minutes.
Mastervolt’s MasterShunt or BTM-III battery monitor.
This phenomenon is described in a formula – Peukert’s Law -
In addition to charge and discharge current, the monitor
devised more than a century ago by the battery pioneers
also tracks battery voltage, the number of amp-hours
Peukert (1897) and Schroder (1894).
consumed and how much longer the battery has before
Peukert’s Law describes the effect of different discharge values
it needs recharging. The device also provides data on
on the capacity of a battery, i.e. that battery capacity is reduced
how often the battery has been discharged and to what
at higher discharge rates. All Mastervolt battery monitors take
extent, with both the average and highest discharge
this equation into account so you will always know the correct
level shown. The MasterShunt is easy to connect to
status of your batteries.
below 10.5 Volt (1.75 V/cell). Unfortunately, when discharged at
the MasterBus network and, with its integrated system
clock combined with command-based events, you can
Peukert’s Law does not apply for Lithium Ion batteries as the
program the system to your preference.
connected load will have no effect on the available capacity.
A different but very imprecise method of checking your
battery is to measure the voltage, which can only be
done when the battery has not been used (discharged)
The Peukert formula for battery capacity
at a given discharge current is:
or charged for at least 24 hours. While measuring
voltage provides a rough estimation of how discharged
a battery is, small variations in voltage make an accurate
Cp = Int
digital voltmeter essential.
Cp = battery capacity available with the given discharge current
remaining
battery capacity
battery voltage
25%
between 11.7 and 12.3 Volt
50%
between 12.0 and 12.6 Volt
75%
between 12.1 and 13.0 Volt
100%
between 12.6 and 13.35 Volt
I
= the discharge current level
n = the Peukert exponent =
T
log T2 - logT1
log I1 - log I2
= discharge time in hours
I1, I2 and T1, T2 can be found by carrying out two discharge
This method is only 15-20% accurate and gives a rough
tests. This involves draining the battery twice at two different
indication of the power remaining in the battery.
current levels.
One high (I1) - 50% of battery capacity, say - and one low (I2) around 5%. In each of the tests, the time T1 and T2 that passes
before battery voltage has dropped to 10.5 Volt is recorded.
Carrying out two discharge tests is not always simple. Often, no
large load will be available or there will be no time for a slow
discharge test.
184
CHARGING BATTERIES
n
Ventilation
Under normal conditions, gel, AGM and Lithium Ion batteries
This air flow is so small that normally natural ventilation will be
produce little or no dangerous hydrogen gas. The little gas that
sufficient. If the batteries are installed in a closed casing, two
escapes is negligible. However, just like with all other batteries,
openings will be needed: One on the top and one underneath.
heat is generated during charging. To ensure the longest possible
The dimensions of the ventilation opening can be calculated
lifespan, it is important for this heat to be removed from the
using the following formula:
battery as quickly as possible. The following formula can be
used to calculate the ventilation required for Mastervolt battery
A = 28 x Q
chargers.
A = opening in cm²
Q = ventilation in m³
Q =
0.05 x I x f1 x f2 x n
Q =
required ventilation in m³/h
In our case, this amounts to 28 x 6 = 168 cm²
I
maximum charge current of the battery charger
(around 10 x 17 cm) for each opening.
=
f1 =
0.5 reduction for gel batteries
f2 =
0.5 reduction for closed batteries
Lithium Ion batteries do not produce any hydrogen gas and are
n
number of cells used
therefore safe to use. When batteries are charged quickly there
(a 12 Volt battery has six cells of 2 Volt each)
is some degree of heat production, in which case the above
=
formula can be used to remove the heat.
Returning to the example of a 12V/400Ah battery set and an
80 amps charger, the minimum ventilation necessary will be:
Contact your installer for larger systems with multiple battery
Q = 0.05 x 80 x 0.5 x 0.5 x 6 = 6 m³/h
chargers.
185
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