No Charge - Batteries

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.