Essentially, you know the nominal capacity Cn [Ah] of the lead-acid battery you are studying. The Coulomb-counting method is a technique that exploits the integration of the active flowing current [A] over time [h] in order to compute the total sum of energy leaving (or entering) the battery. This computation produces a "discharged" (or "charged", in the case of entering current) capacity deltaC that is measured in [Ah]. The actual capacity C is computed as C = Cn + deltaC. Please keep in mind that this method suffers problems of the integration error over long time and there is the need of anexact knowledge about the initial battery capacity (or the SOC, similarly).

Go to Hand Book of batteries byDavid Linden.(electronic form is available in net)

You can refer " Maintenance - Free batteries " Third Edition by Berndt