The simple rule: parallel increases capacity and current. Series means wiring the positive terminal of one battery to the negative terminal of the next.
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For lithium-ion packs, a series circuit connects cells end-to-end so voltages add while current rating stays equal to one cell, whereas a parallel circuit ties all positives together and all negatives together so capacity and current rating add while voltage stays at one.
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The right battery configuration for you depends on the energy needs of your device and your own. Here's a quick look at how each type of connection works: Series Connection = Voltage Adds Up, Capacity Stays the Same.
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The parallel combination A and B is in series with the parallel combination C and D. Again, the total battery pack voltage is 24 volts and that the total battery pack capacity is 40 amp-hours.
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