Batteries have three basic ratings: the voltage output, the cold cranking amps (how many amps it can supply for starting when cold) and the amp-hours or reserve charge. Battery size doesn't affect voltage -- that's a function of the number of cells in the battery -- but it does affect the CCA and RC. The RC is the factor to be concerned with, since that's how much power the battery can hold. While a 1 amp charger will eventually get the job done, odds are that it won't do it any time soon.
Disconnect the battery terminals, remove them and allow the battery to sit for at least an hour. To get an accurate voltage reading, your battery must be completely "at rest." This means that it's not supplying power to anything and that the voltage has had a chance to settle for an accurate reading.
Test the voltage with a digital multimeter, and connect your battery charger while you're doing the math. A fully charged batter will read right at 12.10 to 12.20 volts, while a fully discharged battery will read about 11.4 volts. The percentage of battery charge is equal to the measured battery voltage minus 11.4, divided by 1.5 and multiplied by 100. For example, a battery's measured voltage is 11.8; so (11.8 - 11.4) = 0.4, then (0.4 / 1.5) = 0.26, and (0.26 x 100) gives you a total battery charge of 26 percent.
Multiply the battery's total charge in amp-hours (its reserve capacity or RC when noted) by the percentage of charge remaining, then subtract that figure from the total amp-hours to determine how much charge it needs. The example battery above has an amp-hour rating of 600. So (600 x 0.26) = 160, and (600 - 160) = 440. This is the number of hours you'll hypothetically need to leave a 1 amp charger connected to fully charge the battery.
Account for the battery's self-discharge rate, which, over the course of 24 hours, is usually equal to about 1 percent of the battery's amp-hour capacity. One percent of 600 equal 6, which is how many amp-hours the battery will lose for each 24 hours it's on the charger. Divide your theoretical figure (440, or about 18 days) by 24 (the number of hours in a day), multiply by the self-discharge rate (6) and then add that figure to the theoretical charge. For the example battery, the self-discharge rate will add approximately 110 hours to the charge time.
Add the additional charge time (110) to the theoretical charge time (440) to equal your new hypothetical charge time (550 hours). Now, divide the additional charge time (110) by 24, multiply by the self-discharge rate (6) and add that figure to your new hypothetical charge time to account for the discharge rate of the discharge-rate-adjusted figure (27.5 hours here). The final figure is just about 577.5 hours, or 24 days even.
