Thanks for the clarification. Would it be worth the $10 more for a battery from B&H over amazon?I seriously doubt the EN-EL3a will work at all in a D90. The "a" was for the older D70, D100, D50, which did not have or use the center of the three contacts on the battery (which communicates battery status to the camera). Design changed, center pin added, and later model cameras unequivocally need the EN-EL3e. Will not recognize the "a" battery at all.
You can use EN-EL3e in older and newer cameras, no issue. Reputable places like B&H will not sell the "a" now, the replacement is the "e".
You cannot use EN-EL3a in newer cameras. Would be unwise to buy one even for an old camera, because the "e" surely must be newer stock. Lithium battery life goes downhill sitting unused on shelf, every year is important.
I was going to go with a Nikon battery either way but wanted to make sure that I was going to get the right one. Also thank you for the information on the battery, I never knew that they were that complex.If the $10 difference is "e" vs. "a", then absolutely yes, you need the "e". "a" won't work in the D90. See D90 manual page 22.
If both are genuine Nikon brand "e", then I would save the $10.
I doubt "Amazon" themselves ("Sold by Amazon") have the "a" for sale now either (Nikon no longer offers "a", its replacement is "e"), but Amazon has many small vendors, which are sometimes a mixed bag.
Opinion: If one is off-brand imitation, then the best insurance is to go with the genuine Nikon brand.
The cost is high because these are more than just a battery. They have a logic chip inside them to control and monitor charging (for Lithium fire hazard safety). Some of the off-brands may work (communicate with camera, so camera LCD can still report same "71% charge" when you plug the battery back in after a few months- "a" cannot do that). Possibly one of the off-brands may even be the same supplier that Nikon uses? But it is quite common that many of the cheapies simply don't work at all, and are false economy (wasted money).
It's an expensive camera, buy the proper battery. How often do we need to do that?