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I have some battery chargers for AA rechargeable batteries. They are differnt make and models by different vendors, but have the following in common: Both have four charging slots, and both can only charge in pairs, i.e., they charge well if I put batteries in slots 1&2, or 3&4, or 1&2&3&4.

What is the technical reason behind this? Why can't these devices charge only one slot or other combinations? (Also: Could these cause problems when I add a full or semi-full batterie just in order to turn a singleton into a pair and charge a single empty battery?)

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    \$\begingroup\$ Mine does on any combination of batteries, yours is probably just too cheap. \$\endgroup\$ – PlasmaHH Apr 16 '18 at 13:33
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    \$\begingroup\$ Price. Two subcircuits is cheaper than four. \$\endgroup\$ – winny Apr 16 '18 at 13:48
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In a word... Simplicity.

The chargers you refer to will charge pairs of batteries in series, rather then all batteries using individual charge circuits. This simplifies construction, component count, size and cost.

They cannot usually employ parallel as otherwise if batteries of different charge levels were inserted at the same time, you would have the least charged cell take the majority of current until they became balanced, and this could be unsafe or damage cells.

Its also easier to produce circuit that provides 2.4V charge voltage than 1.2V, since the losses in the switching for the circuit would be less of a concern, and the components are likely to cost less in this case.

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