ACMone
brew
job
74Is your battery suffering a dishonourable discharge? Time to find out why...
Words & photography: Dave SmithCharge!
The two main components in
your charging system are your
battery and your alternator or
dynamo, and their fates are
intertwined. A dying alternator
will soon cause your battery to
go flat, and repeated flattenings
will dramatically shorten the
life of your battery. Likewise, a
battery that won’t take or hold
a charge will soon overwork
your alternator. Winter is on
the way, and cold weather will
soon highlight any faults in your
charging system, but finding the
culprit is a job you can do before
your first brew has finished
steaming.
You’ll need a decent
multimeter, like this Sealey piece
(1). With the meter set to read
voltage, and the engine running
at idle, place the probes across
the battery terminals. In this
instance, the battery is charging
at 13.3 volts (2), which isn’t much.
Get an assistant to rev the engine
(or, if you can reach the throttle
cable, do it yourself) and see if
the voltage rises as the revs pass
a certain point – some alternators
don’t charge too much at idle.
You should be looking for a
maximum of around 14.5 volts.
Then turn the engine off
and watch the voltmeter. The
reading should fall fairly rapidly
towards the mid-12s and then
hold steady (3). If it falls below
about 12.2v, you should put the
battery on charge and start
looking for a new one soon. As a
double-check, get your assistant
to switch the car’s lights on.
The voltage should drop, but
if it plummets, your battery’s
knackered.
In this case, the alternator
would not charge above 13.3 volts
at all. The first things to check
are the fan belt (4), which may be
slipping, although this is usually
accompanied by an excruciating
squealing noise, and the battery
terminals (5). These could be
loose or damaged, or there
could be a build-up of crud and
corrosion between the battery1 2
3 4
5 6
terminal and clamp. Now you can
see why you need a big tub of
Vaseline in the garage – it’s great
for coating battery terminals.
Don’t forget to check the other
end of the earth lead, and the
earth strap between engine
block and body, both of which
can cause confusion in voltage
regulators if they’re dodgy.
If the battery appears healthy
but goes flat when parked
overnight, there is a current
drain somewhere. With yourmultimeter set to read amps,
remove the earth lead from the
battery and connect the meter
between them (6). Modern cars
that have alarms, immobilisers,
radios with memory settings etc
will have a constant, tiny drain
on the battery. In this instance,
the drain is around a third of an
amp, or about four watts. That
could be due to the alarm and
radio. It could also be because
you’ve got the bonnet open and
there’s a four-watt lamp underthere somewhere, dunderhead...
If your car is older and quite
simple, try pulling fuses in turn
and seeing if one causes the
current drain to stop. Then you’ll
know which circuit the drain is
on. Check the plunger switches
on the doors, boot and glove
box to see if there’s a courtesy
light staying on. If the answer’s
not that simple, or your car is
electronically complex, then best
of luck. You might want to make
yourself another brew...one brew.indd 74 21/09/2015 14:14:28