Can a 1500 mAh battery replace a 1600 mAh battery?

Yup......But only if u connect them in PARALLEL! Connecting them in parallel (e.g. the positive cap from Battery 1 is connected with the positive cap from battery 2.....the negative cap from battery 1 goes with negative cap from battery 2) adds the amperage (voltage remains the same).......Connecting them in SERIES (as in any flashlight the positive cap of battery 1 goes with the negative cap of battery 2 making a "bigger battery" that has the positive cap from battery 2 and negative cap from battery 1...) adds the voltage (the amperage is the same). Note: Use same batteries either parallel or series connection.