Technical Voltmeter readings Electrical experts

The open circuit (no load) voltage does not give any real indication of the battery's condition. If the battery has a 40% charge and shows 12.4 v (no load), it will drop down to probably around 5 volts if you connect up a decent load, like a headlight.

 

Acceptable on a modern car, with electronic alarms, computers that never fully turn off, and so on. In an old car i wouldn't be happy at that level, zero mA should be within reach.

The chemical reaction in the battery when the sulfuric acid reacts with the lead plates create a voltage of about 2V per cell. There's a small difference between the start of the process with strong acid and pure lead, and the end of the process with weaker acid and a bunch of lead sulfate, so the voltage remains relatively stable all the way between empty and full battery.
It isn't at all like the air tank on a compressor where the pressure rises proportionally to the amount of air we push in there.

 

It's because of the way wet lead acid batteries work. The "charge percentage" reading given on one of those meters isn't actually to a full zero volts, it's to about 11.6 Volts. Below that voltage the battery has very little to give, coupled with the battery chemistry being a problem.
Below that voltage the plates don't have enough ions on them so will start to attract the sulphur from the acid and that begins to coat the plates (you might have heard the term "sulphated battery") which cause the plates to be able to hold less of a charge than they did when they were new.
That's why it's bad to fully discharge a lead acid battery, too.

Edit: Missed the post by G-son right there...

Phil

 

@hudson48 , have you located the electrical 'leak'?

 

Phil, that is my next job. Set up a trickle feed to keep it charged. It is charging well on the alternator and reads up around 14V on the voltmeter on the battery. Fixed a few other things like dash lights on the speedo that wern't fitting right and the left turn indicator on the dash. I also need to remove the under dash panel and tighten up the fitting that prevents the car starting in a drive gear. I am sure that is loose
as I need to juggle the column gear shit lever to get the sweet spot to start. Could be a source of power drain? Do that first and then do the drain check as below.
I will get my mate to help and use the old fashioned way of checking the drain. Remove the negative cable and run the voltmeter between the cable and post. Currently 60 milliamps. Then with someone holding that I will pull fuses and watch for a drop and hopefully we find the culprit.
In the meantime it starts and everything works fine,just battery goes goes down over a few days but now with the trickle feed on it is fine.