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Salvaged battery lead should be avoided at all costs. Since the advent of the maintenance free battery the lead content has been reduced and elements such as strontium, calcium and others have been added. Most of these elements cast very poorly, ruin a pot of good alloy they are blended with and are extremely toxic. The quantity and quality of lead from batteries is not worth the risk or the effort.
From "Linstrum" on the Castboolits forum - Maintenance free/low maintenance batteries use calcium metal-doped lead to catalyze the hydrogen gas. The lead alloy used in batteries also contains a bit of antimony and arsenic to help harden and strengthen the lead. When hydrogen comes in contact with arsenic and antimony, the hydrogen reacts to form ammonia analogues called arsine and stibine, AsH3 and SbH3. In World War One the Germans experimented with these as war gases. As such they were highly effective since they are deadly in amounts too small to easily detect.