Panasonic FX-55 Series Operating Instructions Manual page 133

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The battery indicator lights when the battery pack is properly inserted with the AC adaptor connected.
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Four battery packs can be inserted simultaneously. Each of them can be inserted in any slot.
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When removing the battery pack, pull it out in the opposite direction of attaching the battery pack.
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Charging begins automatically.
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The battery pack or battery charger will become somewhat warm during normal use. This is not a malfunction.
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