How Do Self-Heating Lithium Batteries Work?

When teaching RV Solar 101 seminars at RV Shows around the U.S., we encourage folks to switch their RV batteries to lithium when building a solar powered system -- simply because they are more efficient, lightweight, and long lasting.  But, we’ve come to realize that there is some confusion about the self-heating device that is included in the latest generation of Lithium Iron Phosphate (LFP) batteries.

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Let’s start with how temperature affects deep cycle batteries.  

Standard Lithium Iron Phosphate batteries can charge normally between 32°-131°F.  Outside of this range, their ability to transfer lithium ions efficiently is decreased and can damage the battery.  To keep this from happening, most LFP’s are equipped with a Battery Management System (BMS), a computer chip inside the battery, to prevent it from charging when too cold or too hot.  LFP batteries will continue to discharge (power your DC and AC appliances) between -4° and 140°F.  This discharge rate (current flow) may decrease depending on how close the core temperature of the battery gets to either end of this temperature continuum, which could affect the ability to run high amperage appliances.

Keep in mind, it’s not just LiFePO4 batteries that experience this problem with cold climate charging. Flooded lead acid batteries will struggle to charge below 40°F, and gel batteries have the same cold charging limitations as LFP at 32°F.  Only AGM outperforms its rivals as it is capable of charging well into the single digits (down to 5°F). But, none of the deep cycle lead acid batteries offer the self-heating option.  Self-heating is unique to Lithium Iron Phosphate deep cycle batteries.

Now, let’s dive a bit deeper into LiFePO4 battery chemistry to find out why cold causes problems.

It’s all about some high school chemistry. When lithium batteries are exposed to low temperatures, the rate of lithium-ion transfer in and out of the anode decreases. This slow down in the rate of lithium-ion transfer is caused by the lithium-ion alloy that plates onto the surface of the anode preventing entry of the ions into the carbon site of the anode. Depending on how cold it gets, the electrolyte could become stiff and circulate less smoothly in colder temperatures, thereby decreasing the rate of lithium-ion transfer and consequently the battery voltage and output power.  Want to nerd out even more? Check out this blog.

While most RVers may never experience temperatures on the upper end of this range, many enjoy RVing during the winter, or at the very least, traveling through winter on the way to a warmer place.  When you think about how an automatic self-heating device may work, it’s easy to assume that the internal thermostat simply turns on the heater whenever the core temperature drops below 41°F.  But, that’s not entirely the case.  The self-heating device within the battery will not heat unless BOTH of these conditions are met:  

• The core battery temperature, which is monitored by the BMS, is less than 41°F.

• There is a charging current of at least 4A coming from the solar panel array, DC to DC charger, AC to DC charger, OR gas generator.  

Why is it designed this way?  Let’s play out a scenario.

You run your RV on solar and enjoy camping off-grid in the winter. Your lithium RV batteries are stored in the battery box on the tongue of your RV.  It’s nighttime and the outside temperatures drop below freezing, causing the core temperature of your batteries to slowly drop below 41°F.   If the self-heater used a small amount of battery power to run itself, it would unnecessarily deplete your battery bank – especially if it needed to run all night.  So, unless you are trying to run high amperage appliances at night and the core temperature of the battery gets below -4°F (low end of the discharging temperature range), there’s no need to heat the battery overnight.  

Then, when your solar array is illuminated in the morning and generates at least 4A (most 100W panels produce around 5A when fully illuminated), the BMS will automatically turn on the self-heating device within the battery, utilizing that small flow of current. The self-heating device automatically turns off when the core temperature of the battery reaches 50°F.  If the outside temperature remains below freezing while your solar array is illuminated, the self-heater will regulate the battery temperature all day long, keeping it between 41° - 50°F so that it can charge efficiently.  If it’s a cloudy day, and your system is not getting enough solar gain, you could use a battery heater or possibly your tow vehicle’s engine to create a trickle charge – either by itself or with the increased power of a DC to DC charger.  

How long this heating up process takes depends on how cold the lithium RV batteries get overnight.  From our personal experience with Renogy’s Self-Heating 100Ah Smart Lithium Iron Phosphate batteries installed on the floor inside of our vintage camper, we’ve noticed a significant delay between the outside temperatures dropping and the battery core temperature dropping.  It needs to get down into the low teens outside at night in order for the battery core temperature to get anywhere near freezing inside.  

Recently, even when the nighttime low dipped into single digits (9°F), the core battery temperature only showed 30°F at dawn the next morning. Once our panels were illuminated by 9am, the batteries started charging back up normally within 20 minutes of the self-heater kicking on.  Bottomline, these self-heating batteries are a total game changer for us!  Check out this short video below to see what our Renogy ONE Core monitor displayed during this test.

Want to speed up that self-heating time?  Here are a few key strategies to keep your batteries warmer overnight:

1. Install lithium RV batteries inside.  Lithium batteries are completely sealed and do not off-gas, making them safe to install inside your rig. If batteries are installed on the floor of your rig, try AirSkirts to keep the floor warmer, save on propane usage, and prevent pipes from freezing too.
   
   

2. Add insulation inside the battery box. If you store your batteries on the tongue of your RV, lining the inside of the battery box with a layer of foam board insulation will help keep the battery warmer in the winter.
   
   

3. Use a battery blanket. Battery blankets are designed to fit snugly over a battery, providing good insulation by trapping the heat generated by the lithium battery bank.
   
   

4. Use a battery heater. Battery heaters with adjustable temperatures and settings are a perfect way to keep lithium RV batteries warmer in extremely cold conditions. 

Which Renogy LiFePO4 Batteries are Self-Heating?

In addition to the 100Ah Smart LFP Self-Heating batteries that we have installed in our own rig, Renogy offers self-heating lithium batteries in voltages ranging from 12V to 48V and storage capacities ranging from 100Ah to 400Ah.  Take a look at their latest line up…

12V Options: 12V-100Ah PRO, 12V-300Ah CORE , REGO 12V-400Ah

24V Options:  24V-100Ah CORE, 24V-200Ah CORE 

48V Options:  48V-50Ah SMART

Taking advantage of all 3 ways to charge your lithium RV batteries (via solar panels, AC to DC charger, and DC to DC)  gives you multiple options no matter what the weather is doing outside.  With self-heating batteries in place, you’ll be able to recharge your batteries to meet your energy demands more efficiently and effectively in cold climates.  Don’t let the night time lows keep you from enjoying your favorite winter location, or discovering a new one!

In 2012, Shari Galiardi & David Hutchison left behind careers and a comfortable home in North Carolina to travel with the vintage camper trailer they lovingly restored, outfitted with solar, and named "Hamlet."

What began as a short break from careers and responsibility quickly turned into a love affair with roadlife. They have parlayed their higher education backgrounds, desire for life-long learning, and thirst for adventure travel into writing, photography, video production, and public speaking gigs from coast to coast.

Known to their friends as simply Shari & Hutch, you can learn more about their full-time, solar-powered adventures on their website at freedominacan.com. Or, follow them on Facebook, Instagram, and YouTube as “Freedom in a Can, LLC.”