According to the Japanese Nikon website, all EN-EL18, 18a and 18b MB-D18 are compatible with MB-D18.
The capacity of 18 "without the letter" is rated at 2000mAh, and 18a/b, 2500mAh. The voltage is rated at 10.8V for all versions.
I don't know why Nikon changed the model from 18a to 18b. But so long as they are all compatible, keeping two 18's could be better than one 18b for the virtually the same price.