Most acids for recharging lead-acid batteries are higher concentration than 10%. Available at just about any hardware store and not particularly expensive either. What concentration do you need?
As a side note, I personally wouldn't heat glassware ever with a Bunsen burner. Extremely uneven, a good way to shatter a glass container or have a runaway or uncontrolled reaction. Plus open flames are just, IMO, a good thing to avoid generally in a chemistry lab unless absolutely necessary.