# Stumped by A/H calculation

#### Crispin

Joined Jul 4, 2011
91
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.

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:

Joined Dec 26, 2010
2,148
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 !

#### DerStrom8

Joined Feb 20, 2011
2,390
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.

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
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.

#### Crispin

Joined Jul 4, 2011
91

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?

#### DerStrom8

Joined Feb 20, 2011
2,390
DerStrom8 -
I'm aware of that but looking to see how much I still have left to go before they run out.
So you're trying to figure out how much time you have left before the battery is completely dead?

#### Crispin

Joined Jul 4, 2011
91
Yup, well, for when a lead acid battery is considered discharged. Think it is 80%?

#### Smoke_Maker

Joined Sep 24, 2007
126
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.

#### Smoke_Maker

Joined Sep 24, 2007
126
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.