Partially charged (or discharged) batteries

If you have a lithium ion battery that has been part used (but you don't know HOW part used) what happens when you put it in the charger?
The 'old fashioned' way was for the charger to fully discharge it and then start from scratch -my old Fujifilm camera even has the facility to fully discharge batteries in the camera and this is recommended prior to re charging its AA batteries. If I'm intending to take a spare battery out with me I always stick it on charge as a matter of principle (having discovered once that I had one that had gone flat since charging it some time prior) but I've no idea if this is likely to harm it.

Lithium batteries don't work in the same way as other types, so they need to have some charge left in them at all times. Just charge them up as needed. I rotate mine but there's never any attempt to exhaust a battery completely.

There's no need to discharge a lithium ion battery before charging.
Not needed.

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Thank you both - that has been a minor worry for a little while.