13.2 v may be the charge voltage, 12 volts delivered. If the voltage is different, however, and you connect in parallel, then the voltage will tend to equalise, so that the higher voltage mis-matched batteries will constantly be at part load and so deteriorate rapidly (assuming lead acid). If you connect in series, using a step up transformer, you overcome this problem, but may run into others, as if any cell in series breaks down, none of the cells will charge