When two 12-volt batteries are connected in series, the voltage doubles to 24 volts, while the amp-hours remain the same. So, if each battery has a capacity of 100 amp-hours, the total amp-hours of the series connection will be 200 amp-hours.
To summarize, two 12-volt batteries connected in series result in a 24-volt battery with the same amp-hours as each individual battery.
