Yes I had the identical experience. Went to Home Depot and figured it out. The 1830 model 3 amp battery will fit all models. The 1850 model battery is tool specific. For example a two tool kit, (hammer drill and 3 select speed drill set). The hammer drill uses the 3 and the 5 amp battery but the 3 speed select drill only uses the 3 amp battery. Bad engineering from Makita in my opinion.
THE MODELl BJR182 SAWZALL, MODEL BHP451 THREE SELECT SPEED DRILL, BSS610 SKILLSAW, AND BGA452 GRINDER WILL ONLY ACCEPT 3 AMP BATTERYS (MODEL 1830)
Larry