ELECTRICS UPGRADE, PART 1
Battery isolation
In a multiple battery installation you must isolate
the service and engine batteries so that they
can be linked for charging, but kept apart
when not. A simple 3-way switch is the easiest
method and the most common, but care must
be taken not to switch it to ‘off’ with the engine
running or you risk seriously damaging the
alternator unless the switch is the protected type.
Although a 3-way switch enables you to
join batteries together in an emergency, it also
means you can flatten the engine battery if left on
‘both’ without the engine running. A diode splitter, Voltage Sensitive Relay (VSR)
however, will enable isolator sends all the charge to
both to be charged the engine battery first, until
simultaneously, it reaches 13.8V, after which
whilst it connects the service
automatically batteries into the charging
isolating them circuit – guaranteeing
under normal use. the start battery always
Better still, a remains fully charged.
Above left: A simple diode isolator. Above and right: The VSR
replaces the diode isolator and eliminates any voltage drop.
remembering when you’re trying to decide handling. If you were to buy one huge
which battery to buy: 400Ah lead-acid battery as a service bank
Above: a typical battery bank for a cruising yacht
it would work fine, but would put a great
made up from four deep-cycle service batteries
1. The bigger the battery, the more of a deal of weight (approx 150kg or 23½ connected in parallel and a single start battery.
hammering it will survive. stone) in one place on the boat and require
2. The larger the capacity of the bank, some sort of crane or tackle to manoeuvre
the less it will be discharged in a cycle, it into place. Fine if you can get it directly – i.e. connect them together electrically
consequently the longer it will last. over or inside the keel maybe, but for in parallel (+ to +, - to -) to form a larger
most of us a compromise has to be made. 12V or 24V battery bank. These will give
Using large batteries, however, creates A better way is to buy several smaller you a large overall capacity, but are far
another problem – that of space and batteries and ‘parallel’ them together easier to manhandle and can be placed
some distance apart – spreading the load
around the hull a little. Larger yachts often
go one step further, creating house banks
Fuse protection
out of 6V or even 2V cells, small enough
to fit all around the engine/machinery
When upgrading your battery system, do room without creating a vast power store
take the opportunity to install a high current in one place. At this point, larger yachts
battery fuse to protect against a possible that will be cruised extensively should
dead short between the batteries and the consider converting at least some of their
panel fuses or breakers. high consumption devices to 24V, which
Depending on your boat’s battery capacity will halve their current draw.
and maximum current draw, these fuses
– which usually range from 100A-500A – are Next month...
essential to provide protection against a Having calculated your total energy
possible dead short that is frequently the requirements and chosen the batteries
cause of onboard fi res. to suit, you will need to work out the
They can be placed in either the positive or best method of charging them. In Part
negative sides of the circuit – it makes no difference which is an ideal scenario. 2, next month, we will explain the
which – and should be as close to the battery Fast-blow fuses are a much safer protection features of modern AC battery chargers
terminal as possible, keeping the unprotected device than circuit breakers in this application, being and explain such terms as ‘smart’
section to a minimum. Some heavy duty fuse holders unlikely to go wrong and cannot simply be reset multi-stage charging, transformer
are designed to fi t directly to the battery terminal, without fi rst discovering the cause of the problem. isolation, equalisation and more. ■
July 09 Sailing Today 113
ST147
Practical_Electrics_RTG.in117 117 14/5/09 15:10:13
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