How to Manage the Temperature of a Lithium Battery Bank: Heated Lithiums, Relays & Custom Controls

Compared with lead-acid, lithium iron phosphate batteries are a breeze when it comes to maintenance. The biggest issue, however, is that lithiums are sensitive to frigid temperatures. 

Both cold temperatures and temperature cycling negatively affect the life expectancy of lithium battery banks. They also affect its usable capacity. This is not always well communicated by battery manufacturers, but at the lowest operating temperatures, your capacity might be 2/3 of the nameplate battery capacity. Traditional lead acid batteries also experience a significant reduction in capacity at lower temperatures. 

Off-grid homeowners looking to upgrade their power systems will often make statements along the lines of: 

"Lithium batteries are great, but when it gets cold, they stop charging. Therefore, lithium is not a great energy storage solution for cold climates. I wish the manufacturer would just add a built-in heater to solve the issue." 

This article will address the practicality of heated lithium batteries and share our perspective on advanced battery management solutions for lithium banks in cold weather. As we've found, managing the temperature limitations of lithium technology with thoughtful solutions enables system owners to utilize them at their full potential and in all sorts of applications. 

Are heated lithiums a fix-all?

No. Heated lithiums incorporate a heating pad into the battery enclosure itself. The heating pad is a resistance-based electric heat source where electricity is turned into heat (think toaster). 

You can control the heating pad so that temperatures are not excessive, and depending on the battery, you can control it with a thermostat. When the temperature drops below a certain threshold, the thermostat activates the heating element and starts warming up the battery. We can add some logic to this to make it 'smarter,' but more on that later.  

The only neat thing about the built-in heating pad is that it's 100% efficient. There's no exhaust, and all of the electricity that goes in and doesn't come out the other end is turned into heat. Just because it's 100% efficient, however, doesn't mean it's free. Electric baseboards in your home are 100% efficient as well but are also one of the most expensive ways to heat your house. 

At face value, it makes things easier, and there are cold-weather scenarios where it's wonderful to have the ability to heat a battery up quickly. Our friends at Polar Bears International agree as they drive their tundra buggy along the shoreline to livestream polar bears hunting seals on the ice. Each of their 12V Pylontech batteries has a built-in heater that can be switched on manually or through a relay as the Pylontech battery management system communicates with a Victron Cerbo GX device. 

Related Reading >>> As Polar Bears Test the Ice, This Mobile Research Station is Camera-ready

Once the battery is warmed up and you're in a temperature-stable environment, the value of that built-in heater goes away, and a fundamental challenge remains: the batteries are discharging themselves to heat themselves, meaning if left on, your batteries will run themselves down to zero. 

At the end of the day, there are two options for heating batteries: you can either heat the space, or you can heat the batteries. 

Where will the batteries live?

Asking 'Where the batteries will live' is a more important question than 'Do I need a heated battery?' 

Part of the challenge is that we aren't used to being able to bring our batteries into interior living spaces that are temperature controlled. 

Instead, we expect to swap out older lead acids with brand-new lithium batteries in similar locations. And remember, lead acids were traditionally kept in pretty crummy spaces - the crawl space, the tongue of the trailer, outbuildings, sea containers, lean-to enclosures out in a field - whatever you could do to get that stinky battery bank away from humans. We had to ventilate the space, run fans and cut vent holes in the shed wall for the hydrogen gas to get out and to keep corrosion down while the batteries were bubbling away. Lead acid batteries also could self-regulate their temperature to some degree because they were relatively inefficient, and remember that inefficiency = HEAT (think incandescent vs LED light bulb).

Home lithium battery backup systems are much smaller in footprint - and safer than ever before. Putting a lithium bank in a heated garage or mechanical room is ideal, and people do it all the time. Pylontech's 48V rack-mount batteries are designed to be stacked vertically or put into a standing cabinet without taking up loads of indoor real estate. Remember, it's the iron in LiFePO4 batteries that keep them from spontaneously combusting (like a Tesla). 

Keeping any battery outside or in an uninsulated space has always been a bad design choice. The capacity is dramatically reduced. The ability to charge is reduced, and long-term performance takes a significant hit. So while lithium's lack of low-temperature performance is sometimes annoying, we see it shining a light on some bad practices and saying, "Stop it."  

Easy-peasy or added complexity? 

Again, the perspective we often hear from customers across markets is, 'If only there were a heater in my battery, I would no longer need to worry about my battery temperature.' 

Unfortunately, it's not that simple, as you now have to worry about when to run the heater and at what times it is advantageous for your system. A heater that is not controlled is a liability. 

Another problem is that you will likely need to insulate your heated battery from the elements regardless. If you already have it in a nice heated space, your internal heater will probably run less often anyways, so what have you gained with your purchase? 

The issue with lithium batteries is that they are already complicated. Heaters, on the other hand, are not complicated. You don't want a lithium battery to stay toasty warm all the time, so you need a way to prevent the heating pad from running more than necessary. If you have more than one battery and the heaters run independently without synchronized control, the batteries will unbalance themselves and cause additional headaches.  

How warm is warm enough? 

Heat loss is a function of the difference between the interior and exterior space, but this heat loss is not linear; it's exponential. When people think 'heated space,' they think 75 degrees. But lithiums don't need more than 50°F to stay happy. To plan for real cold snaps, we might design around 55°F. Keeping a small, well-insulated space at 55°F takes very little energy. 

Looking at a Victron Smart Lithium datasheet, you will notice two temperatures. One is the low-temperature discharge limit. The other is the low-temperature charge limit. If you are not heating the space and just discharging the battery, you can keep running a Victron Smart Lithium to -4°F. That's pretty cold. That also means that if you want to discharge your battery to heat the space heater, you can keep doing that all the way down to -4°F. If you're going to charge the batteries, then you need to keep them around 41°F. 

This means that if you go out to your motorhome and it's less than -4°F, you cannot use your power system. If warmer than this, you can use the system until the batteries die. If, instead, you start using the motorhome, it will heat up the interior space where your batteries live to more than 41 degrees, and you will be ready to hit the road.

How 'smart' do I need a system to be? 

With many things related to technology, there is a magic tipping point at which technology goes from being a great pain to a great benefit.  

Here's a scenario that shows how many 'dumb' or non-communicating lithium battery systems leave a lot of value on the table: 

Scenario 1:

I have a brand-new camper van, and I'm one day into a big hunting trip with the boys. In my camper, I have a lithium battery with an internal BMS. It's a frosty October morning, and the battery is below the charging temperature, 32 degrees. We partied a little last night after Jimmy got his first deer, so we're not feeling super sharp. The battery is discharging, we have all the lights on, and we're running the blender for Bloody Marys. We notice, however, that the batteries are getting low, so Jimmy decides to use the truck to charge things back up. The batteries cannot control the DC to DC charger from the alternator, so the second they start to do their thing, all the power goes out. Jimmy starts to swear. It's a top-of-the-line camper, and this is not the weekend adventure we had in mind. 

What happened? The battery can technically discharge completely, but it will not let itself be charged, so it had no choice but to isolate. No one knows what's happening or why. Panic descends as we realize we are out of cell range, and we start to think, "Who will go first? Probably the skinny one...."

Scenario 2: 

The truck starts, and the battery notices that power is available but does not allow the DC-DC charger to charge the batteries. The battery does, however, start to heat up, and when it's warm enough, it will begin charging from the vehicle. The blender keeps blending. The guys still don't know that the system did a number of things to protect their pride, and it certainly made for a better hunting trip.    

As most people have integrated BMS-type systems that don't communicate to the outside world, Scenario 1 is widespread and explains why lithium is still not used in many OEMs or RV super stores.  

Relays and Custom Controls

That said, how can Victron assist you in controlling the temperature of your battery or the space containing it? A handful of solutions come to mind and may be helpful as you think through your own system design: 

Solution 1: Use the temperature control relay on the Cerbo 

If you have a Victron Cerbo GX device, you can use one of its relays to control a heating pad based on temperature using either a wireless Ruuvi sensor or a wired GX temperature sensor. One of the advantages of this solution is that it offers lower temperature ranges than you can typically find with a standard thermostat.

Ruuvi sensors, which come in a few different models, all measure temperature and send real-time data back to a Gateway (in this setup, the Cerbo acts as the Gateway). Further, you can control the onboard relays on the Cerbo with a Ruuvi sensor. Remember, in 12V & 24V systems, the Cerbo relay is rated up to 6A DC, so depending on your heater, you may need a power relay. 

Available in fully waterproof versions with hard-shell cases, these sensors are easy to zip-tie or screw-mount anywhere, work very reliably, and are not expensive. For instructions on setting up temperature-based parameters from a Ruuvi sensor using Relay 2, read Monitor Temperature on Victron's Cerbo GX Using Ruuvi Sensors.

Place a Ruuvi in the garage or in the equipment bay of an RV next to the battery bank. Set the Relay to turn ON an electric heater or heating pad when the temperature reaches below, say, 41°F, and turn OFF when it reaches 55°F. If you wanted to, you could also use the second Relay to turn on a fan to improve airflow in a space that's getting too warm. 

Solution 2: Allow to discharge and preheat before solar charge

Say you have an internally heated battery or an external heating pad around your battery bank. Allow the temperature to fall during the night while you're discharging the batteries. Then, activate the heaters before the solar array starts in the morning. If you're using a Victron MPPT solar charge controller, you already have this solution available in your back pocket via the load output and the “streetlight” function. 

The diagram below shows a Pylontech RT12100G3 internally heated battery with external contactor control. A 12V heating pad could just as easily be wired directly to the controller. The SmartSolar MPPT current rating is between 15A to 20A, depending on the model. And remember, most tank heater pads and heated batteries have built-in override thermostats.

Open the VictronConnect App and open the Settings icon > go to Streetlight > toggle Streetlight function ON > At Sunrise > Select 'Switch on before sunrise.'

Solution 3: Utilize Node-RED

Attention nerds: did you know that your Cerbo GX can run Node-RED? Node-RED is a flow-based programming platform designed for easy programming. You can input "nodes" and then make decisions based on the data. For example: what if you could input the 5-day weather forecast, evaluate if the temperature is likely to drop during the night, and start preheating the battery room while the solar is charging? Or, maybe you want to wait until the MPPT has moved into a float state before using wasted energy on heat. Alternatively, what if you've insulated your battery room, run your heat loss calculations for that space, and are monitoring its temperature? Maybe the temp will drop to 15°F, but since you know there will be no wind, you can calculate the heat mass that can sustain the evening load. Then it's as simple as programming the Relay 2 on the Cerbo GX to trigger based on your logic.

Solution 4: Use a switch with your finger

On the 12V Pylontech, you can enable or disable the heater with a contact closure while leaving the battery online. If you want to let your cell phone charge up while out mushroom hunting (enable discharge) but don't want to waste energy keeping your battery warm all day, you can flip OFF the switch. When you return victorious, your battery will not be drained, and if you want to charge the battery, you can turn the switch ON and have plenty of power left in the battery to bring it up to temperature so you can begin charging it. 

Conclusion 

Heated batteries and battery temperature management are a conversation starter for system design. We've found that adding unnecessary complexity to systems rarely brings better results and in fact, it can introduce complex failure points. We must consider how systems will be used, how components work together, and design systems to anticipate and minimize real-world problems. Read our perspective on how Victron power systems reach their full potential when paired with intelligent, communicating lithium battery banks. Spoiler alert: lithiums that allow for closed-loop communications with the inverter share their internal temperature with the rest of the system, allowing for better independent decision-making and long-term health and performance of the power system. 

If you have a pain point with a system you're designing for yourself or for a customer, we'd love to hear about it and offer suggestions for improvement. 

Related Reading: 

Bad, Better, Best: Battery-Inverter Communications & Compatibility

Active Management: Expect More From Your BMS

Adding Capacity to a Pylontech Lithium Battery Bank

Battery Communication: Closed vs. Open-Loop Communications

Victron’s DVCC Function Explained