Lithium ion batteries have gained popularity in recent years, and rightly so. With applications ranging from tablets and smart phones to off-grid solar systems and electric cars, Lithium batteries have gained quite the reputation. Still, with all it’s popularity, I’ve been surprised lately by the lack of education when it comes to lithium ion maintenance, and the one critical aspect no one is talking about.
For off-grid solar applications Lithium ion batteries have many advantages:
- Does not degrade when stored at partial charge, as with lead-based batteries.
- Low self-discharge. Your battery bank can sit for long periods of time without needing to be charged.
- Low maintenance
- Long lasting with thousands of life cycles, as opposed to lead-based batteries with hundreds of life cycles.
- Lighter with higher energy density (more watt-hours per kilogram)
For off-grid solar, Lithium batteries are the most advantageous battery we have on the market right now. However, their biggest disadvantage is that they can be extremely dangerous if not managed properly. Which is kind of important small print that no one tells you about- which is why I’m writing this article.
Have you ever wondered why the top electric car manufacturers use an extensive system for maintaining battery temperature with their Lithium battery systems? Tesla and Chevy both use a liquid heating/cooling system integrated into their batteries to keep the temperature above 32 deg, and below 120 deg.
I’m here to tell you that they don’t do this just for fun. There is a good reason why Lithium batteries need to be maintained and charged within a specific temperature range, and I find it very disturbing when I see people claiming you can use lithiums at an extreme temperature range of -40 to 140 deg F. Lithium battery usage at temperatures up to 140 deg will significantly shorten their life span, and charging below freezing is extremely dangerous.
In other words, never charge lithium ion batteries in below freezing temperatures.
Doing so even once will result in severe and permanent capacity loss, and even worse, it causes damage to the battery that makes it unsafe to use and should be, safely recycled or otherwise discarded. When I say not safe, I mean it will work fine until it randomly explodes due to mechanical vibration, mechanical shock, or just reaching a high enough state of charge.
Lithium ion batteries are generally under a lot of internal pressure, unlike other battery chemistries. Lithium batteries store energy like two springs. During charging or discharging, lithium ions move through a separator, from one side to the other. As these ions move back and forth it causes stretching and contracting of the internal components which causes mechanical stress on the battery. It’s important to note that this is a completely normal process for Lithium batteries.
Another difference is their electrolyte is a volatile and extremely flammable solvent that will burn quite easily. There are a variety of different chemistries of lithium batteries, all with different levels of reactivity and inherent danger, but the ones with high energy density can undergo thermal runaway. Meaning, if they get too hot, the lithium ions will begin to release more heat, getting hotter until it catches fire or explodes, and there’s nothing you can do to stop it.
Now before you get too worried, the temperature for thermal runaway to start is around 300 to 500 deg F (depending on they type of battery). So this is unlikely to happen, unless there is damage to the internal components of the battery. Which leads us to the charging your batteries below freezing.
When you charge a lithium ion cell in below freezing temperatures, the lithium ions fail to bond in the anode correctly. Instead, they plate the anode with metallic lithium. This lithium platting of the anode isn’t smooth and even – it forms in dendrites, which are sharp tendrils of lithium metal growing on the anode.
The lithium coating damages the battery and reduces it’s capacity, but more importantly, the dendrites can put unexpected pressure on the separating membrane as the anode expands and forces them into it, and this will cause the membrane to one day fail unexpectedly. This of course makes the cell vent, and ignite its flammable electrolyte.
Note: I should add that discharging a lithium ion battery in below freezing temperatures is perfectly safe. Most cells have discharge temperature ratings of -20°C or even colder. Only charging a ‘frozen’ cell needs to be avoided. Also, if your battery has active heating then you can charge the battery in below freezing temperatures because the battery itselfwill not be below freezing.
So how do you avoid lighting up your battery bank like a bonfire? If you know you’ll be charging your batteries during freezing temperatures, make sure your battery bank has a temperature control feature that regulates the battery temperature, or keep it in a temperature controlled, insulated room like a heated garage or shop (like this photo of our HomeGrid solar generator hooked up to a 40Kwh lithium battery bank). If this isn’t possible, you’ll need to avoid using them during freezing temperatures, and save the bonfires for campouts.