A CITATION 650 EXPERIENCED A BATTERY OVERTEMP RESULTING IN AN EMER LNDG. WHILE ACFT WAS DSNDING; THE FLT CREW OVERSHOT AN ASSIGNED ALT.

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WHILE CLBING OUT OF APPROX FL380; OUR #2 HOT BATTERY ANNUNCIATOR LIGHT CAME ON. WE PULLED OUT THE CHKLIST AND STARTED TO FOLLOW IT AND ALSO TOLD CTR WE NEEDED A DSCNT -- KNOWING THAT WE WOULD HAVE TO MAKE A LNDG SOON; BUT HAD NOT DECLARED AN EMER YET. THE CTLR GAVE US A DSCNT TO FL350 AND A HDG OF 30 OR 40 DEGS TO OUR L. WITHIN A VERY SHORT PERIOD OF TIME; THE ANNUNCIATOR WENT FROM A STEADY ANNUNCIATOR LIGHT (60 DEGS) TO A FLASHING ANNUNCIATOR LIGHT (71 DEGS). WE DECLARED AN EMER AND INFORMED ATC THAT WE NEEDED TO LAND AS SOON AS PRACTICAL AND ASKED FOR ALL AVAILABLE ARPTS WITH EMER EQUIP. EVEN THOUGH WE FOLLOWED THE CHKLIST AND ISOLATED THE BATTERY FROM THE ELECTRICAL SYS; THE TEMP CONTINUED TO CLB AND THAT HAD OUR UNDIVIDED ATTN. UNFORTUNATELY; DURING THIS TIME; I DSNDED BELOW THE FL350 ASSIGNED ALT. I CANNOT REMEMBER IF WE HAD DECLARED AN EMER BEFORE OR AFTER THE ALTDEV; BUT THE 2 WERE ALMOST SIMULTANEOUS. FORTUNATELY; THE BATTERY TEMP STABILIZED AT 73 DEGS ON THE DSCNT AND WE DIVERTED TO NASHVILLE WITH AN UNEVENTFUL LNDG. SUPPLEMENTAL INFO FROM ACN 592293: UPON LNDG AT BNA WE CHKED OUT THE BATTERY AND TAXIED TO FBO WITH NO FURTHER PROBS.