To tell the truth, I've never really found them to be particularly useful. If there is a small opening, cold is going to get in there and since everything moves from warm to cold, you are heating the outdoors a lot, which doesn't work well. A heating pad directly under the battery (don't bother getting one oversized, undersized is actually better) works better in my experience than trying to wrap the thing somehow. Lead-Acid batteries often need venting, which is counter-productive if you're trying to keep it warm.