Question:  My CEL-BP came with 2100 mAh NiMH batteries. Can I use higher 2600 mAh batteries or will they cause a problem?
Answer:  Yes you can use an mAh rating higher or lower since the mAh rating refers to the batteries capacity. All AA size NiMH batteries are rated at 1.2V regardless of the mAh capacity. Higher mAh rated batteries will last longer between charges and allows for more use before they need to be charged.
Clear-Com recommend using Panasonic or Energize 2600mAh batteries 
mAh stands for milli Ampere hour or milli Amp hour.  It is a measure of a battery's energy storage capacity.  If you think of a battery as a small fuel storage tank, which in a sense it is, mAh a measure of how much "fuel" the battery holds.  (This is roughly comparable to using gallons to measure how much fuel a gas tank can hold.  The more gallons of capacity,  the more fuel the tank can hold.) With a battery the higher the mAh rating the more electrical energy it can store. While it is useful to think of mAh as being the rough equivalent to gallons, the analogy is not a perfect one.