When it's real cold the battery is just like the ones on the news in the electric cars. They will not charge until they're pre-conditioned, which means they're heating it up first. This is the same with a trickle charger. Most trickle charger instructions say you shouldn't charge a battery below a set temperture. So to get the max cranking amps from you battery when cold you need to pre-condition it, heat it up. These pads are not cheap and I would not want the cheap ones. Anything with electric and heat is a fire hazard. Stick with Canadain or USA made. I can't remember which one I bought but the wattage is really low and you can stick your hand under the wheel well and feel the heat where the battery sets. Cranking when it's real cold it much faster than when I forget to plug it in. There not something you leave plugged in all the time. It only takes a few hours and it's heated.
