They are not inexpensive AA's in a plastic casing. They are 18650 Li-ion rechargeable. In the picture these appear to be rather old 2.2Ah versions. you can buy these in pairs for around $5 and there is nothing to stop you replacing each one.
You need to test the voltage of each cell when the pack is fully charged. any cell showing significantly less than 4.2 volts is probably worth changing.
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