Aha !
There is a known problem with some older Li-ion batteries.
It is a non-problem really.
Each battery has an internal chip that monitors battery performance.
Amongst other things it reports over-charge.
This triggers the red light if a battery is over charged.
There is a fault in that chip, it falsely reports an overcharged battery, at completion of the charging process.
It's annoying but not a real problem.
There was no fix available, to the best of my knowledge.
This only affected an early batch of batteries.
About 3-4 years ago, from memory.
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