Battery charge percent

Hi all, I have a 12v 7Ah battery which I can't find the discharge profile for - if that's the name - from the manufacturer. Its an eBay battery, so I'd assume is pretty standard.

Anyway, I have a project which monitors its voltage and the instantaneous current draw.

Is there a way I can have a percentage charged figure from these two values (and potentially a discharge profile)? I've read that a 7Ah battery doesn't necessarily last for 7 hours at 1 amp load, and not 3.5 @ 2A; the more amps drawn means a lower capacity?

Basically these things are not very good, you can plot your discharge profile but this will change over the life of the battery.

Just like the gas miles left in your car, it makes the assumption that you are either going to do what you are doing now until the end of the discharge or else gathers a history of usage and tries to guess how much you are going to use.

the more amps drawn means a lower capacity?

To some extent, some batteries don't do so well on very light loads either.

I believe it is possible to guestimate the state-of-charge of a lead-acid battery if you know the battery's temperature, capacity, current drain and voltage. There was a recent thread that touched on this here: http://arduino.cc/forum/index.php/topic,70927.0.html One thing worth looking at this this: http://www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf

The graphs in this writeup are for large flooded batteries. I suspect the curves for VRLA gelled electrolyte or AGM batteries would be different.