A battery consists of one or more electrochemical cells. Each of these electrochemical cells produces a voltage difference, which can be harnessed to do work. Many rechargeable batteries can only be safely charged. Two or more six-volt batteries can be charged at the same time if both are connected in series, provided the proper measures are taken.
Battery construction
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While some batteries are composed of a single electrochemical cell, many batteries are composed of several electrochemical cells connected in series. This allows the battery to produce a higher voltage difference.
Primary Cell vs. Secondary Cell Batteries
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When the electrolyte in a battery reacts with the electrodes, a voltage differential is created across the electrodes. The electrolyte combines with the electrode material, and over time, the voltage differential decreases. In some types of batteries, such as alkaline batteries, the electrochemical reaction cannot be reversed. These are primary cell batteries, which can be used until the charge is too weak and they must be discarded. For other types of batteries, such as lead-acid batteries, the reaction can be reversed by applying voltage to the electrodes. This process is called recharging. Many batteries can be recharged hundreds, if not thousands, of times before they must be replaced.
Charge two six-volt batteries connected in series.
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If two batteries are connected in series with each other, the voltage difference will be equal to the sum of the voltage difference of each battery. Therefore, two six-volt batteries connected in series will produce 12 volts. Similarly, if you want to charge two batteries connected in series, the charging power source must have a voltage equal to or slightly greater than the total voltage of the batteries. If you want to charge two six-volt, lead-acid batteries, for example, you would need a charging voltage between 12.6 and 14.4 volts.
Precautions
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Do not attempt to charge the batteries if one battery is rechargeable and the other is not. If you try to do so, there may be a risk of fire or leakage.