Hi Peter,
The charger is MH-18a, which can charge EN-EL3 series batteries
http://www.bhphotovideo.com/c/produc...k_Charger.html
The D90 uses EN-EL3e, so it should work. I believe later D70s models came with EN-EL3e batteries (earlier models had the older EL-EN3 and EL-EN3a batteries). Nikon issued the same charger for all their D70s cameras, so I am pretty sure the MH-18a charger would charge EN-EL3e batteries (ie. D90).
Cheers,
Bo