Some Lithium Iron Phosphate (LiFePo4) batteries can be bought with internal heaters. However, most batteries don’t have them. I added my own heating pad in the bottom of my battery box. Still, I had to ask myself, “Is a heater really necessary in my application?” Is one necessary in yours? Sometimes I take a bit to get to the point. So, here’s my answer right from the start: If your battery is portable and will be brought indoors when not in use, then the easy answer is NO. I think most hams use or build portable batteries. But what about everything else?
If your battery is in a garaged vehicle or in a temperature-controlled space such as an RV or camper van, then you probably don’t need a heating pad because it’s already being warmed by either a furnace or residual heat that’s retained within the garage structure. However, a battery without those advantages could benefit from a heating pad, especially if the vehicle is not driven every day. What? You’re still here for more details? Let’s go! 😉
To understand the impact of adding heating pads, we first need to address the issue of battery consumption by the heating pad itself. After all, what good does keeping the battery warm do if it’s going to be dead when you need it? The combined load of my heating pad and refrigerator averaged 70Ah while parked for three-days during Winter Storm Fern. That’s low consumption for my 314Ah battery because I was still at 79% state of charge (SOC) after three days. However, a 70Ah drain would leave a 100Ah battery at only 30% SOC, a more significant deficit! My old 100Ah battery had degraded to 74Ah and would be nearly dead after Fern. Even at 30%, would you need the battery immediately or would you be able to drive and charge a bit before placing a demand on it?
Another consideration is when a car sits outside unattended on a cold day: there’s no heat being generated by the engine, equipment operation, or the battery charger. Regardless, a large LiFoPo4 battery has a lot of mass that holds heat far longer than air inside the car, perhaps enough to stay above 32°F (0°C) for the first 24 hours of inactivity, even when it’s below freezing outside. But there’s a fair chance that it will be at least a little below freezing after two nights in the cold. I’ve found that if I drive my car for just one hour each day, then the battery temperature seldom drops to below 32°F (0°C) before the following morning unless the cabin temperature falls to below 25°F (-4°C) early on an overcast day. That almost suggests that I don’t need a heating pad at all!
But what happens if the battery drops to 30°F (-1°C), just a small dip below freezing? The Battery Management System (BMS) will cut-off charger input until the battery warms to 41°F (5°C), the “Resume Charging” temperature shown in this image. In my experience, warming the battery with cabin heat alone can take over 90 minutes of driving! 😀 That’s time and fuel wasted when the battery could have been charging right away. If you live where the temps drop to much lower, then you might not drive long enough to warm the battery to above freezing at all! Charging can begin almost immediately if the battery is already warmed to above 32°F.
That alone is why I wanted a heating pad under my battery. If you saw my battery box project, then you know that I built it with a 23-watt RV pipe heating pad in the bottom. I tested it to see how hot it gets without regulation. If left unchecked, the pad reaches 185°F or 85°C over the course of just 30 minutes! I added a standalone thermostat and experimented with a variety of settings before settling on the pad being turned on at 68°F (20°C) and then off at 113°F (45°C). Note in this photo that the thermostat sensor is measuring the temperature of the heating pad and not the internal battery cells and BMS. Still, I’m sure that 68°F will be too high during spring and fall. I can mitigate that by pulling the thermostat’s fuse during warmer months.
I noted above that my heating pad and refrigerator consumed 70Ah during a three-day period of inactivity. That’s about 23Ah per day, a value that’s easily replenished within 30-40 minutes of driving and charging at 40+ amps! That’s hardly a penalty for having heat under my battery! Okay, there’s a one-time penalty of $30 to buy the heating pad and thermostat. However, the reward is gained by being able to charge the battery at any point after the engine starts. That’s worth the price of admission to me. What do you think?
Not Frozen,
Scott
