pracTical
7.2V. MCA is the same figure, only at a
temperature of 0°C (32°F).
The amp-hour rating is a measure
of the amount of current a battery
can supply over a known period,
usually 20 hours (known as the C20
rate), before the voltage drops below
10.5V. For example, a 100Ah
battery might supply 5A
for 20 hours, after
which the battery
would be completely It pays to know what you’re
flat. In actual fact this is
doing before attempting to
not strictly true, because
upgrade your power system.
its discharge properties
change as the charge capacity, you must decide how you are
reduces, but it is close going to construct the battery banks and
enough for general purposes. what type of batteries you want to use.
Most deep-cycle batteries also have If you already have separate starter
a ‘reserve’ rating, which is the number and service banks, but want to increase
of minutes they will run a 25A load, the service capacity by adding further
before their voltage drops to 1.5V. batteries, they should be the same type and
Another figure you might notice either capacity as the existing ones.
capacity – that’s four years or more of printed on the battery or in the literature Better still, you should start with two
continuous daily cycling. is the number of charge/discharge cycles or more brand new, identical service
Table A shows the average life it can stand. Be wary of these numbers; batteries, so a good time to upgrade is
expectancy and approximate cost per they vary greatly depending on how the when your existing one is on its last legs.
Amp-hour of a variety of popular battery is treated. Ideally, they should all Having got this far, two points are worth
marine batteries. be quoted at a 50 per cent discharge rate,
which is the ideal maximum to which a
Battery capacities deep-cycle lead-acid battery should be
BATTERY 1 STATUS BATTERY 2 STATUS
Typical
RESET DATA LOCK FUNC RESET DATA LOCK FUNC
12.
The capacity of a battery is measured discharged, but the figure can double if
installation
V A Ah t
SEL SET BAT1 BAT2
in Cold Cranking Amps (CCA), Marine it is only ever discharged by 20 per cent
with monitor.
Ampere-hour
Fuse
From
Cranking Amps (MCA) or Amp-hours before fully recharging. With care, regular
meter
distribution
To distribution
circuits
(Ah). The CCA and MCA ratings are used maintenance, minimal discharging and
board
for starter batteries and are a measure of fully recharging, a regular deep-cycle wet
Shunt
how much current they can produce under battery should give you at least 1,000
certain conditions when fully charged. cycles before it packs up. AGM and gel
The CCA of a starter battery is actually cells even more.
MARINE
the current in Amps that a battery can
HEAVY DUTY
supply over a 30-second period at -17°C Battery system design
(0°F), without the voltage dropping below Once you have worked out your required
Calculating your requirements
One of the commonest mistakes boat
Example 24hr consumption list
be run over your chosen cycle period
owners make today is putting too (time between c75harges – usually 24 or
Battery
capacity
large a service bank on a boat without
ITEM CURRENT (A) TIME PER TOTAL
48 hours). Total the result (which will
providing the means to fully replenish
24HRS (HRS) CONSUMED (AH)
now be in
12.
Amp-hours) then double the
its charge.
Nav lights 4.5 10.0 45.0
figure (as the batteries should never be
Voltmeter
When calculating your required Interior lights 7.0 4.0 28.0 discharged deeper than 50 per cent)
capacity it is a good idea to first
Water pump 5.0 0.4 2.0
and add 25 per cent to ensur6.5
Ammeter
e you
reduce some of your unnecessary
Windlass 90.0 0.1 9.0
will have enough overall capacity with
power consumption by changing some to spar
filament bulbs to fluorescents or LEDs,
20
e. A
VHF & SSB (Rx) 0.5 14.0 7.0
Battery
It is worth bearing in mind at this
size
improving the insulation on the fridge,
VHF & SSB (Tx) 15.0 0.2 3.0
point that you will probably need to
installing manual water pumps at the Fridge 3.5 8.0 28.0 increase your vessel’10.
Time
remaining at
current
s charging power
sinks etc.
Radar 5.0 2.0 10.0
source in order to accommodate any
consumption
rate
Then, to accurately calculate your
Other instruments 2.0 14.0 28.0
increase in battery capacity – so don’t
total power requirements, multiply go too over the top. Chargers and
TOTAL 160.0
the maximum current draw of all the charging will be fully explained in Part
equipment in Amps by the time it will Total battery capacity required = (160 x 2) x 1.25 = 400Ah 2 of this series, next month.
100%
112 Sailing Today July 09
80-99%
60-79%
40-59%
ST147
Practical_Electrics_RTG.in116 116 20-39% 14/5/09 15:09:50
0-19%
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