Is it ok to charge two AA batteries together, one nearly full, one nearly empty?

I have a battery powered shaver that takes 3x AA batteries. The charger I have will only charge batteries in pairs. So in the charger I’m going to have to put in an empty battery with a full battery (the spare one that wasn’t used in the shaver). Is this safe? Or am I better draining the full battery first somehow (maybe in a torch)?

An additional question: most chargers seem to charge AA’s in pairs. Is there any particular reason for this?

I’m in the UK with 240V mains if that makes any difference.

Generally, that’s not a good practice.
If the charger is charging them in series, it will tend to over-charge the nearly full cell.
If it’s a trickle charger, and stays below the “float” charging current of the battery, it’s ok.

I think most chargers do pairs because most items take two cells…

It’s common to charge NiCads, Lithium, Alakaline etc. in parallel. Check your charger. If it is able to charge a single cell, do that.

(Lead Acid cells are charged in series.)

Some chargers discharge the battery before it charges it.

Some batteries do not have to be dead before it charges.

My laptop can show 70% of battery life and I can put it on the charger.

Most Ni-Cad chargers charge batteries in pairs because they can reduce the cost of the charger as it requires less components and more common and thus cheaper components.

It may reduce the life of the charged battery to do as you asked but assuming you are using ni-cad batteries and the charger is not a high current charger which is unlikely seeing as you can only charge in pairs it should be safe.

As low self discharge nicad batteries are fairly inexpensive it would be better to buy more and keep them on hand in a charged state or buy a charger that will deal with individual cells.

Yes, its to help extend the life of the AA Cells by ensuring one does not drive another down to 0Volts , for the case of where the battery (a collection of cells) contains a series connection…
Your charger might require two cells, or it might require one cell and a piece of wire, paper clip, aluminium foil, or other suitable conductor.
If you do use a conductor as a stand in cell, and the two cells are (would be ) in series, the charger won’t detect fully charged state, so it returns to being a manual charger.
(unless its a timing based charger, rather than detecting voltage.)

Aren’t NiMH cells more common than NiCads nowadays?

Yes I should have said, non lithium based. I am old and NiCad is still a generic term for rechargeable battery in my mind.

Thank you for pointing out my error.