# Stumped by A/H calculation

Discussion in 'The Projects Forum' started by Crispin, Sep 24, 2011.

1. ### Crispin Thread Starter Member

Jul 4, 2011
88
2
Hi folks,

I have a 540AH battery powering an inverter. I want to work out how many AHs have been used since the inverter was switched on (only comes on in the evening when the solar panels have stopped working)

In a simple case, you could say it is C=A*T : 1A for 12 hours is 12AH.
As my load varies widely, this is not true any more.
I am measuring the current, via a shunt, every 500ms. Lets say it is 15A for a period of 500ms, that is 15A/ms. If the next measurement is 20A for 500ms, how do I add all this up to show me the current, cumulative AH used.

Its twisted my head...

I am trying to predict how much longer my 540AH battery will last with what has already been used.

note - Once I understand where I am going wrong, I will use the additional bits from http://www.powerstream.com/BB.htm based on load.

Thanks
Crispin

Last edited: Sep 25, 2011
2. ### Adjuster Well-Known Member

Dec 26, 2010
2,147
300
If you are measuring every 500ms, take each current measurement and multiply it by 0.5, and add it into a total of amp-seconds. Divide by 3600 to get amp-hours.

Edit: 15A for 500ms is 7.5As or if you prefer 7500mAs or even 7500Ams. It is NOT 15A/ms !

3. ### DerStrom8 Well-Known Member

Feb 20, 2011
2,428
1,328
Amp-Hours (Ah) is a value that remains constant throughout the life of your battery. You do not "use up" amp-hours. All it means is that if you draw one amp from a 12Ah battery, the battery will last roughly 12 hours. The Ah rating stays the same, though--it does not change.

4. ### Crispin Thread Starter Member

Jul 4, 2011
88
2
Thanks Adjuster - simples.

DerStrom8 -
I'm aware of that but looking to see how much I still have left to go before they run out.

As I think about it though, it's a daft way as the batteries age, their capacity will reduce.
A better way to guess SoC for a Pb battery? When fully loaded the inverter has a draw of 190A so the voltage drops a bit. Using voltage while under load to determine SoC seems unfair?

5. ### DerStrom8 Well-Known Member

Feb 20, 2011
2,428
1,328
So you're trying to figure out how much time you have left before the battery is completely dead?

6. ### Crispin Thread Starter Member

Jul 4, 2011
88
2
Yup, well, for when a lead acid battery is considered discharged. Think it is 80%?

7. ### Smoke_Maker Active Member

Sep 24, 2007
126
15
The easy way is to count AH in and AH out. Fill the battery and start with that number, don't forget to put a alarm on it to tell you when the battery is at 80% DOD.

8. ### Smoke_Maker Active Member

Sep 24, 2007
126
15
100% DOD is at 1.75 volts for a lead acid battery so make sure you stop the discharge before it gets below 1.82 volts.

9. ### Crispin Thread Starter Member

Jul 4, 2011
88
2
Thanks folks.