Colin Haines explains how to install a simple voltmeter to check the
condition of your engine start battery to make sure it is fit for purpose
A
boat’s domestic batteries
live in a sort of battery hell,
while the engine’s starter
battery, on the other hand,
resides in a battery heaven. Unlike
the domestic batteries, it won’t be
steadily drained until it is giving up its
life-limiting last 10 per cent by driving
a fridge, nav system, lights and all the
other kit carried on board. The lucky
starter battery gives up a fraction of its
stored energy as a massive discharge
in the few seconds needed to start
the engine, after which the engine’s
alternator immediately feeds the
power back in again – gently.
Devices exist that will monitor both the
current taken out of an engine’s starter
battery when it is used to start the engine,
and the current delivered back into it when
the engine is running. However, the
amount generated by an alternator is
much less than that drawn by a starter
motor. In fact, before the starter motor
starts to turn, it is almost like a dead short
between the battery terminals. Measuring
these figures is not difficult, but because
the battery charge power is much lower,
displaying it accurately can be.
The germane question – is there enough
energy in the battery to start the engine –
can be answered with a much simpler bit
of kit: a voltmeter. It can also reveal if the
engine’s alternator is charging the battery
properly, because the voltage should rise
to >13.2V. A rested and fully charged
battery will show 12.8V and anything
below 12.5V may see the battery having
difficulty starting the engine.
A constant display of this simple bit of
valuable information can be acquired
without needing a degree in electronics or
breaking the bank. The device in the picture
is an EMV1200 and is made by Lascar
Electronics (www.lascarelectronics.com).
It has just two wires (red/positive and
black/negative) to connect to the battery.
Fixing it in place requires a 5.5 or 6mm
diameter mounting hole to be drilled. The
supplied wires are not really long enough
to run between a starter battery and a
switch panel beside a navigation station,
so they will most likely need extending.
The tiny amount of current needed to
drive the display means that only small
size wires are needed, such as those used
for burglar alarms or telephones. Better
still, these wires are usually tinned and
carried in an outer sleeve, thus meeting
the requirement to have two layers of
insulation enclosing every core. Being
small they also lend themselves to being
wriggled through tight spaces and can be
secured simply with cable ties.
Before you install this handy bit of kit you
need to consider the consequences of
any damage being done to the wires and
the risk of a short-circuit starting a fire. The
answer is to install 100mA fuses in both
the positive and negative wires (caution
tells you that there is no way of knowing
which wire may get shorted) as near as
possible to the battery, an obvious point to
connect the extended supply wires.
All devices consume electricity, but this
meter needs just 0.003A to work, meaningit would take well over two years to drain a
typical 60 amp-hour battery if left on.
Nevertheless, I installed an 8A rocker
switch into the circuit to disconnect it
when not needed.
After installing, ensure the voltage
displayed by the meter is the true voltage
of the battery by taking a measurement
taken directly across the battery’s terminal
posts with another meter accurately
calibrated. The display’s read-out can
then be adjusted to match using the tiny
screw at the back of the meter.All info is on hand with regard to engine battery voltage on this auxiliary switch panelGet the measure of your battery’s power
with this two-wire, three-digit voltmeter
Monitoring an engine
start battery
WHAT IS COSTS1 x EMV1200 voltmeter*
(Lascar) £28.65
1 x rocker switch (Maplin) £3.00
2 x fuse holders (Maplin) £2.50
2 x 20mm x 100mA
Quickblow fuses £0.44
10m x 4-core alarm cable
(Maplin) £8.50
Scrap aluminium and spray paint
for switch panel £0
TOTAL £43.09*www.lascarelectronics.com/
lascar-panel-instrument-emv-1200/MONITORING AN ENGINE START BATTERY