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At a recent event, out of the seven Sevens that graced my back lawn, there were no less than three that had charging complaints of one form
or another. The charging circuitry and all the associated gubbins is
well known as being the "Achilles Heel" of many an Austin, which
prompted me to put together a few notes.
For those who wish to go the 12V route, the excellent article by Seamus
on how he converted JJ (see Focus, Jan '05) shows exactly what is
necessary. It also shows what is involved either with a 12V or a 6V
version. The intent here is to look at some of the major bits and how
they work. I’ll cover these a chunk at a time over the next few issues
of Focus.
First then, the battery. In its simplest form, it is a bucket of
sulphuric acid with a couple of lead plates dipped in. (This is one
cell, there are three cells to a six volt battery; six cells for 12V).
Not much there to go wrong then. But actually, killing a battery is
easier than it looks. For starters, they don't like getting dry. Too
little distilled water and the plates will go dry at the top. This is
very bad news, as the capacity of a battery is set by the area of the
plate. Once part of the plate is allowed to dry out, that proportion is
never as good again. Secondly, and hugely important, lead acid batteries
do not like "deep discharge". In other words, don't let them go
completely flat. What happens here is that some of the sulphate from the
acid coats the plates and again you get a loss in capacity. Deep
discharging a battery for more than a couple of hours will do it
irreparable harm. I was told of the exploits on the Irish trip, where
due to a charging problem, it was necessary for two cars to swap
batteries at the end of each day. This was just fine, because the
battery wasn't allowed to be too flat for too long.
The state of charge is important. What do I mean by the state of charge?
For this, we need a voltmeter, and I strongly urge every body to have
one at their disposal. Not that old 12V panel meter that you picked up
at an auto jumble, (probably ripped out of the dash of a Dolomite), but
a proper instrument capable of reading voltages as small as a hundredth
of a volt.
Now some numbers. Take your voltmeter, set it to the 20V DC range and
connect it across the battery posts, firstly with everything turned off.
Better still, isolate the battery by disconnecting it.
| Battery State | For 6 Volts | For the 12 volt |
| 100% charged | 6.32v | 12.65v |
| 75% charged | 6.22v | 12.45v |
| 50% charged | 6.12v | 12.24v |
| 25%charged | 6.02v | 12.06v |
| Flat | Less than 5.95v | 11.89v |
Having established what the battery is like disconnected, let's look
at what happens when we try to charge it. Start your engine, and use a
fast tickover (or enough revs to replicate sensible driving around
speed). Now we can look at the state of our charging circuit.
Let the car run fast for a few minutes, (to replenish the cranking
charge) and measure the voltage again. (By the way, keep the meter away
from the coil or the HT leads. Just like a radio, they go doo-lally due
to the ignition interference).
For the six volters, the voltage on the battery should have increased to
between 6.75V and just over 7V. The minimum voltage necessary to
maintain the battery in it's present state of charge, (known as the
float charge voltage) is 6.75 V. This is what those expensive little
boxes that "condition" a battery when not in use are supposed to give
out. If your charging circuit is absolutely spot on, then a full
charging voltage is 7.2V. Don't worry if this increases to about 7.4 V,
but much more will cook the battery.
I've just tried this on my 'Seven': Engine off, it was 6.30 V as I know
it's pretty well charged. The ignition warning light goes out at 6.40 V;
at fast idle I get 6.65V which means that the battery is neither
charging nor discharging, at 30 mph I get 6.80V and at as high revs as I
dare go, I get 7.10V.
By the way, if your battery is quite a bit flat, then you wont see these
voltages until it has recharged. The clue that the charging circuit is
working is that the voltage will slowly increase as the battery becomes
less flat. To make this all less confusing, if you suspect that your
battery is flat, charge it on the bench before trying to diagnose the
car’s own charging circuit.
Switch on the lights: you'll see the voltage drop a bit. If it is just
on the verge of 6.7V at speed, you know that the system is holding it's
own and the battery won't let you down if driving in the dark.
Now you twelve volters. For you, the float charge voltage is 13.5 V,
the ideal charging voltage, (lights off) is between 13.5 and about 14.2
V and at really high revs you should see up to about 14.8V.
If this is what you're getting, every thing is fine. If not, we’ll need
look at the rest of the system. Next month we will look at how to test a
dynamo.
This article, written by Geoff Hardman, originally appeared in CA7C Seven Focus in Aug 2005 pp20-21.