Using Voltmeter Readings as indication of Battery Capacity left.......

Hi Kev

The best time is when the battery has been at rest for a while after either charging or discharge. (Like 1 hour)

However you can get a good indication within 5 minutes of charging if you put you headlights on for a minute then rest for 5

Your battery type will affect the actual readings at full/ half etc and so best do with help of manufacturers data.

For My cal/cal exide I use 12.7 as full and never discharge below 12

What sort of voltmeter ? - if its the plug in cig lighter Jaycar type then they are remarkably accurate even with temp change - they draw under 10ma and can be left in permanently (I have 2 and do this).

I use a clamp type current meter for measuring which means no circuit break needs to be made but also carry a 0.47ohm 10 watt resistor to put in series to measure smaller things.

Kevmac
I don't think it is an accurate way of telling the capacity level because batteries are all different in their charge capacity, ability deliver and type of battery. You will know hte voltage level but may not know the SOC.

Unless you test ie 2 amp draw for 50hrs on a 100ah battery you won't get much idea of the particular batteries you use.
Also a small load for many hours is quite different to a far heavier load for less but equivalent hours. Both may equate to be the same by maths but the batteries ability and characteristics will dictate what is real life and the resultant state of charge level.

Modern battery level indicators, microprocessor controlled, which read the charge amount input and then the output so it can be compared is Highly "testical" stuff, but even they aren't a real accurate/good indicator.

Once you get to know the readings for your particular circumstances from a known full charge down to 12.1v or thereabouts and the usage which caused it to drop you will be as accurate as the smart metering stuff. You will record typical use won't you?

Why don't you test the volt meter to know how much it draws, then you will know what to expect it to use and if to leave it constantly on or not.

Cheers
Ross M

Elite monitors

I hear that the Elite monitors have a great reputation and work well to show state of battery. I don't have one but others who swear by them.

Expensive but they work

Alan

This is going to be a BIG thread!

Hope you have your calculator ready...LOL

Check this site I think this is what you are looking for also google "battery state of charge"
heaps of info there
Kev

www.mmbalmainauto.com.au/PDF/State_of_charge_12_volt_batteries.pdf?

A member has previously posted a "Temperature Compensated Battery State of Charge (SOC) Table" which I found very useful in determining my battery charge and was easy to use. All readings were taken after four hours without charge/discharge. You may find it with the search function. If I remember it was Sidewinder who posted it but I cannot be sure. Hope this helps.

Just remember that using battery voltage to determine state of charge is like trying to work out someone's weight by using a tape measure ;)
You're using the wrong measuring instrument and measuring the wrong value. That said, you can use it, under certain conditions, to get an approximation of the state of charge. But that's all it will ever be.

Keep that in mind with the 'advise given' and you'll be ok.

Would like to thank all the respondents for their input and will soak them all up. At least I now have a better understanding