Electronics, Lighting, & Gadgets - Sticking unmatched batteries in a charger

What happens if I stick a 2000mAh and a 2500mAh battery side by side in a charger? Will the higher capacity battery charge the lower capacity battery so I get two batteries at 2000mAh or will I get what they're rated, or close to?

Depends on the charger. And the batteries

The best chargers will always charge each battery individually. This is OK

The cheapest chargers (overnight ones) just trickle charge the batteries for hours and hours and hours and they'll be as OK as any other battery setup.

The medium grade chargers charge in pairs and they'll likely screw things up.

Also, the "rated" capacity and actual capacity are often very different. So you might be OK because your 2000mAh and 2500mAh batteries are actually fairly similar batteries that just have different ideas of what the "mAh" part really means. And/or have been worn down differently.

No easy answer. Batteries are trouble.

I have a Maha charger which specifically advertises its 4 independent charging units. http://www.thomas-distributing.com/mhc401fs.htm

+1 on depends on charger and that best chargers treat each cell individually. But it likely won't really hurt anything. Just may not supply optimum charge to either cell. Try it and see how they work.