Not so for the series condition Notso.
Pos-Pos and Neg-Neg does connect the batteries in parallel, where the terminal voltage is the same as the individual battery terminal voltages and the output current is the sum of the currents in each battery. Thus two 12 volt 15 ampre-hour batteries connected in parallel produce a 12 volt 30 ampere hour power pack - delivering 30 amperes for one hour at a terminal voltage of 12 volts.
Connect the same two batteries in series by connecting one battery's positive terminal to the other's negative terminal and connecting the remaining positive and negative terminals to the load, produces a 24 volt power pack. The same current flows in both batteries, hence the capacity remains at 15 ampere hours - delivering 15 amperes for one hour at a terminal voltage of 24 volts.
Note that output power is doubled in both cases as 2V x A or V x 2A both equal 2(V x A)
A battery generally consists of a number of cells connected in series thus there are 6 cells in a 12 volt lead-acid car battery and 12 cells in a 24 volt aircraft battery; so a 24 volt battery is effectively the same as two 12 volt batteries connected in series within the same case.
Finally- and please, don't try this at home. Its very, very dangerous!
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Batt 1 pos --> Batt 2 neg
Batt 2 pos --> Batt 1 neg
= 24 volt how many amps?"
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The answer is lots and lots - until we reach the final melt down!!!