Heat-shrink tubing needs heat — not fire. Use a heat gun (a hair dryer on high works in a pinch), keep it moving back and forth, and let the tube shrink down evenly over the joint.
Never take a lighter or torch to it, especially anywhere near a battery — scorched tubing cracks, and a cracked seal is no seal at all.
Got a wiring question? Drop us a message — happy to help.