I know there's a depth of battery knowledge on the forum, so this should be an easy question:

I have a set of 4 AA NiMH batteries (IKEA's 'Ladda 2450') and both my caving light and the charging unit take all 4, so that's easy, I always charge the full set together.

Today, some Christmas lights were short of a set of 3 batteries, so I 'borrowed' the caving batteries (with little prospect of them being used before Christmas ).

When I'm finished with them, what's the best option for recharging - should I charge the 3 partly-used batteries separately from the remaining 1 (barely-used), or will the charging unit be smart enough to do the right thing if I charge them together as normal?

The charger was also from IKEA (it's called Kvarts, which is nice).  Actually, I now see they only cost ?6, so I may have answered my own question - you don't get a lot of smarts for that price, do you?

If you're worried charge the 3 then the one

From the Kvarts manual (emphasis my own):

• Charge 1 to 4 pieces of HR6/AA and/or 1 to 4 pieces of HR03/AAA rechargeable batteries.
• 4 charging channels.
• Charge is terminated by individual minus delta or zero delta voltage sensors (-dV/dT or 0dV/dT).
• Charging status with LED indicator for each channel

So you needn't worry about overcharging I don't think?

Edit:
Well, you needn't worry about overcharging any more than usual. There is some research suggesting that using the minus/zero delta voltage point for NiMH might be overcharging the batteries and reducing their lifespan! 

Thankyou Ari 
- and thanks for not saying RTFM , which would have been an appropriate response