Don't. Do. It.
Battery lead has Bad Stuff in it.
I had the same idea since I've been casting my own since my Dad thought I was old enough to handle hot lead. A little research killed that idea fast. Too toxic. From a random forum post out in the ether:
"You don't want to use car battery lead. The chemicals added to car batteries, to make them "maintenance free" become very bad when melted. You melt the lead, the non-lead stuff floats to the surface, and you skim it off as "dross". Now, this dross, with the calcium and other stuff from the maintenance free batteries, when it gets wet releases nerve gas."
I buy lead from ebay and any company that will ship good stuff via USPS flat rate. The poor girl at the post office hates me when a heavy package comes in, but I can feed my 44 and 45 a steady diet of home brewed bullets at prolly around 1/4 the cost of factory ammo. No lead wheel weights here in Hawaii since Kalifornia switched to all steel weights... so no cheap / free lead there. But you also don't have to pay full price from the reloading shops like MidwayUSA.com.
Just not with battery lead. Ever. Not worth the risks.