A GUIDE TO BATTERY CHARGING
1. CHARGING A BATTERY 7. CE
MARKING
2. 3-STAGE
CHARGING 8. VOLTS, AMPS and WATTS
3. TYPES OF
CHARGER 9. CHARGER SELECTION
4. PROTECTION 10. DC-DC CONVERTERS
5. REMOTE
SENSING 11. INSTALLATION
6. POWER FACTOR
CORRECTION
1. Charging a Battery
There are many types of battery but batteries on boats are
nearly always LEAD-ACID types -
similar to car batteries but heavier duty.
A BATTERY is made up of a number of CELLS.
A LEAD-ACID CELL generates around 2 volts. Small batteries
contain 6 cells in a container
which add together to give 12 volts at the terminals. Larger
cells are quite heavy and
individual CELLS are connected together 'in series' to make
batteries of either 12 volt (6 cells)
or 24 volt (12 cells).
Although a battery is called a '12 volt' battery, its
voltage varies from about 12.6 volts down to
10 volts when it is discharging and can rise to 15 or 16
volts during charging. It is very
important, however, to limit the maximum battery voltage
during charging otherwise the
battery will be damaged. The battery voltage should not
exceed 13.8 volts for long periods
and 14.4 volts for short periods (8 hours maximum).
2. 3-Stage Charging
There are two basic types of lead-acid battery charger:
FLOAT chargers and 3-STAGE
CHARGERS. Float chargers also come in two versions - good
ones and bad ones!
A good FLOAT charger charges the battery at a constant
current until the 'FLOAT' voltage
(13.8V or 27.6V) is reached, then it progressively reduces
the current to maintain that voltage.
This gets about 75% capacity back into the battery quickly
but then takes a long time to
restore the other 25%. A bad FLOAT charger commences
charging at the rated current but as the battery takes the
charge, its voltage rises and the current drops off long
before the float voltage is reached.
Thus, even 75% capacity takes a long time to restore and
full charge takes forever.
By far the best type of charger is the 3-STAGE CHARGER. This
starts charging like a good
float charger but continues charging at constant current
until the 'BOOST VOLTAGE' (14.4V
or 28.8V) is reached. Then, the current is progressively
reduced until it drops to one quarter of
its maximum. This corresponds to 90% capacity in the
battery. The charger voltage now
changes automatically to the float voltage where it then
remains, slowly restoring the last 10%
capacity. Thus almost full charge is restored quickly and
safely.
3. Types of Charger
There are several types of electronic circuitry used within
battery chargers for the marine
market
3.1 FERRO-RESONANT
(or CVT)
These use a low-frequency MAGNETIC control system, which
makes them very HEAVY, very
BULKY and are also only available with a poor FLOAT charge
characteristic, therefore very
SLOW recharging. They can also generate a large magnetic
field which can upset other
equipment on board. On the plus side, they are CHEAP and
RELIABLE due to the low
number of components used and they tend to appeal to
boat-builders who put price at the top
of their list of priorities.
3.2 LINEAR CHARGERS
These also use a low-frequency transformer to reduce the
input voltage to a lower level, but
they then use transistors to control the current and voltage
fed to the battery. This technique
can be used for either FLOAT or 3-STAGE chargers but is very
IN-EFFICIENT and therefore
HOT, HEAVY and BULKY. The biggest drawback is a LIMITED
INPUT VOLTAGE range - not
ideal for running from a generator or some marina supplies.
3.3 SWITCHED MODE
CHARGERS
These are more complicated than the previous two types and
use the techniques perfected
for and now universally used in computers and televisions.
The AC input is first turned into
high-voltage DC. It is then turned into high-frequency AC
using special types of transistor and
a high-frequency transformer (one thirtieth the weight of a
low-frequency transformer!)
reduces the voltage to the exact level needed to charge the
battery. A sophisticated control
circuit produces an overall design with HIGH-EFFICIENCY,
SMALL SIZE and LIGHT
WEIGHT. The extra complexity adds to the initial cost but
results in lower running costs and
the ability to run from a SMALLER (and cheaper) GENERATOR if
required. Switched mode
chargers can be either FLOAT or 3-STAGE types.
4. Protection
Battery chargers should be designed to survive
a number of fault conditions that can occur
during or after installation
4.1 SHORT-CIRCUIT OUTPUT can cause instant destruction or
the charger may survive for
a few minutes or it may survive forever - it depends on how
it was designed.
4.2 REVERSE BATTERY CONNECTION can be disastrous if suitable
protection is not fitted.
A fuse is the simplest method. A relay is good at low output
currents but is unreliable at higher
currents. Electronic methods are uneconomic at high
currents.
4.3 OVER-TEMPERATURE PROTECTION is needed in case either the
engine room
temperature exceeds the rated level or the ventilation of
the charger is obstructed (air-flow
through a charger is essential to keep the electronic
components cool).
4.4 OUTPUT OVER-VOLTAGE protection should be present in case
of a fault in the charger
control circuit. Well-designed electronic circuits are very
reliable but components can still
occasionally fail on a random basis. If this happens, a
second circuit should prevent the
battery from receiving continuous over-charging.
5. Remote Sensing
5.1 TEMPERATURE SENSING
The FLOAT voltage of 13.8V/27.6V mentioned earlier is the
ideal voltage at a battery
temperature of 25C. At higher temperatures this voltage
should be progressively reduced by a
small amount. This is not much of a problem in the U.K. but
in the Mediterranean, for
example, engine rooms can reach 40-45C and a charger for use
at high temperatures should
have a remote temperature sensing facility. This consists of
a small sensing block which is
affixed to the battery and a cable connecting it to the
battery charger.
5.2 VOLTAGE SENSING
The thickness of cables joining a charger to a battery
should be chosen to keep the voltage
drop on the cable very low. However, any drop in the cable
will increase the re-charging time
to some extent. For the ultimate performance, the charger
should control the voltage at the
battery not at the charger. This is achieved by taking a
thin cable from the battery positive
terminal to a remote voltage sense terminal on the charger.
Some chargers have this feature,
others do not. It becomes increasing important at higher
currents.
6. Power Factor
Correction (PFC)
Although the AC mains voltage is a smoothly-changing
waveform called a SINE-WAVE, the
current taken from the mains by most battery chargers, T.V.s
and computers consists of
narrow peaks of current. This is inefficient for the power
generating companies and, with the
proliferation of T.V.s and computers in particular, the EC
have considered enforcing the
addition of an extra circuit called a POWER FACTOR
CORRECTION circuit to smooth out the
peaks. The date for introduction of this has already been
postponed twice but it will eventually
become compulsory. In the meantime, PFC is beneficial on a
boat because it uses the mains
power very efficiently and will therefore run from a smaller
generator or poor marina supplies
which non-PFC chargers might struggle with.
7. CE Marking
In January 1996, the EC introduced new legislation which
required battery chargers to meet
certain standards before being sold within the EC. The
regulations cover the amount of radio
frequency interference produced, the immunity of the charger
to RFI from other sources, dips
and surges in the mains and immunity to electrostatic
discharge. Conformity to these
specifications is indicated by attaching a CE label to the
charger
.
8. Volts, Amps and Watts
WATTS are the units of POWER. A hairdryer full-on might be
500 WATTS; on the low-power
setting it might be 200 WATTS.
The higher the POWER the bigger the charger.
VOLTAGE must be matched to the equipment in use and will be
either 12 VOLTS or 24
VOLTS in a boat.
CURRENT indicates the flow of energy from the battery and is
measured in AMPERES (or
AMPS). Zero current and the battery is not discharging. The
higher the current the faster the
battery will discharge.
A battery is rated in AMPERE-HOURS (abbreviated Ah) and this
is called the BATTERY
CAPACITY. For example, a small boat might have a 12 volt
100Ah battery. This battery will
provide 100 AMPERE-HOURS before needing to be re-charged.
This may be taken from the
battery as
1 AMP for 100 hours
2 AMPS for 50 hours
10 AMPS for 10 hours etc.
WATTS are VOLTAGE multiplied by CURRENT, so taking the above
example with the 12 volt
battery
1 AMP x 12 VOLTS = 12 WATTS for 100 hours
2 AMPS x 12 VOLTS = 24 WATTS for 50 hours 10 AMPS x 12 VOLTS
= 120 WATTS for 10 hours
Re-charging a battery follows the same principle. The
requirement is usually to re-charge the
battery over-night - say in 10 hours.
Because a battery is not totally efficient at converting
electrical energy into chemical energy
and vice-versa, re-charging a 100Ah battery requires about
120Ah to be put back into it, and
this can be achieved by either
120 Amp-hours / 10 hours = 12 Amps for 10 hours
120 Amp-hours / 15 hours = 8 Amps for 15 hours
120 Amp-hours / 24 hours = 5 Amps for 24 hours etc.
CURRENT = WATTS / VOLTS
therefore if, say, the lights add up to 36 WATTS and the
battery VOLTAGE is 12 VOLTS then
the CURRENT taken from the battery will be
36 WATTS / 12 VOLTS = 3 AMPS
If these lights are on whilst the battery is being charged,
then the battery charger must also
provide an extra 3 AMPS to power them.
Below is a sample calculation to determine which of our
products would best suit your
requirements.
1. Determine the battery voltage - 12 volt or 24 volt
2. Determine how many batteries are to be charged (1,2,3 or
4)
3. What are the battery capacities (Ah, Amp-hours)?
Add them together
e.g. 2 x 60Ah, 1 x 100Ah = 220 Ah total
4. What re-charge time is required?
(6 hours minimum, usually 24 hours maximum - if not known,
assume 10 hours)
5. Divide TOTAL Ah by the re-charge time to get CHARGING
CURRENT
e.g. 220Ah / 10 hours = 22 AMPS.
6. ADD 20% to allow for battery in-efficiency
i.e. 22 AMPS + 20% = 26.4 AMPS
7. Is there any additional load current on the battery
during re-charging? If so, add this to the
CHARGING CURRENT.
e.g. 24 WATTS / 12 VOLTS = 2 AMPS
CHARGER CURRENT REQUIRED = 26.4 AMPS + 2 AMPS = 28.4 AMPS.
8. Choose the next biggest charger in the range
e.g. 12 VOLT, 30 AMP, 3 BATTERY BANKS = BCM 12/30-3
Note:
Much too big a charger could damage the battery and too
small will take longer to re-charge.
10. DC-DC Converters
-Some equipment is only available for 12 volt operation. If
the boat's main battery is 24 volt,
there are three options to run 12 volt equipment on board
1.Install a separate 12 volt battery with its own 12 volt
mains battery charger.
2.Use a DC-DC converter to generate 12 volts using energy
from the main 24 volt battery.
3.Install a separate 12 volt battery and charge it using a
DC-DC converter from the main 24
volt battery. This requires a 13.8 volt output converter
designed for the purpose.
For options 2 and 3 it is useful to have a remote on/off
switch otherwise the DC-DC converter
could drain the 24 volt battery whenever the 24 volt battery
is not being charged.
11. Battery Charger Installation
1. The INPUT to the charger is AC mains voltage and is
capable of causing death. A circuit
breaker of suitable rating MUST be fitted between the
charger and the shore power in
accordance with good marine installation practice. If the
charger is replacing an existing
charger, then this should already be in place.
2. The BATTERY itself stores large amounts of energy and is
quite capable of causing a fire
or explosion if the output terminals are shorted together.
Extreme care must be taken when
wiring the charger to the battery. Cables should be of
suitable thickness for the current rating
for the charger, cable runs should be kept as short as
possible and all cables must be
securely held in place with cable fixings to prevent wear to
the cable insulation.
3. The charger should be securely fixed to a vertical
surface using ALL mounting hole
positions ( 3 for BCM, 4 for BCH and BCO).
4. At least 75mm clearance must be left all around the
charger to allow the free flow of air over the
charger. The ventilation holes or slots must not be
obstructed in any way.
5. The chargers should be protected from water and spray.
What is a Rotary phase converter?
