What Voltage Should You Store Your Li-Po Batteries At?
LiPo batteries, or lithium polymer batteries, have the unique characteristic of being more compact and lightweight than other portable battery alternatives. It is because of this trait that it has become a mainstay in many hobby industries, including in vapes, RC cars, and drones. However, LiPo batteries are a bit more fragile and finicky, demanding more deliberate maintenance practices.
One such good practice is the storage of LiPo batteries at the right voltage level. Most of us know not to store batteries completely full or completely empty, but how can we know if it’s just at the right capacity for long-term storage?
What is the safe voltage for the long-term storage of LiPo batteries?
Assuming that you are using typical LiPo batteries that hold a full charge of 4.2 V per cell, then you should be targeting a storage voltage between 3.6 V to 3.8 V, corresponding to roughly 85% to 90% of the battery’s full capacity. This is necessary if you will be storing your LiPo batteries for longer than 3 days without using them.
Just to be clear, storing your batteries either above or below the prescribed voltage is similarly harmful. Overcharging your battery and letting it run empty are also poor practices that will lead to the premature deterioration of your LiPo battery. Never leave it charging once it has reached the 4.2 V full capacity, and don’t let it drain to below 3.0 V.
What happens when LiPo batteries are undercharged or overcharged in storage?
As we’ve mentioned, storing a LiPo battery outside the prescribed voltage range is harmful, whether the actual voltage is higher or lower. This is best illustrated by the following table, lifted from BatteryUniversity.com.
|Permanent Capacity Loss versus Storage Conditions|
|Storage Temperature||40% Charge||100% Charge|
|0 °C||2% loss after a year||6% loss after a year|
|25 °C||4% loss after a year||20% loss after a year|
|40 °C||15% loss after a year||35% loss after a year|
|60 °C||25% loss after a year||40% loss after a year|
As we can see from the table, storing a LiPo battery that is either above or below the prescribed storage voltage results in deterioration of the battery. More specifically, it results in a decrease in the battery’s capacity over time.
The same table also shows us that it’s much more ideal to store LiPo batteries in as low a temperature as possible. Simply storing at 0 °C instead of room temperature massively reduces the rate of deterioration of a LiPo battery, even if it’s not stored at the ideal voltage.
Why do LiPo batteries get damaged when stored over or under the prescribed voltage?
All this talk about storing LiPo batteries at the proper voltage begs the questions: why is it necessary? What happens inside the LiPo battery while it is stored? What causes a reduction in the battery’s full capacity? To answer those questions, we’re going to have to go back to the basics and understand what’s inside a LiPo battery and the chemistry behind a battery.
All batteries have three basic components: a cathode (the positive terminal), an anode (the negative terminal), and an electrolyte. The energy is stored in a battery through the chemical reactions that happen between the electrolyte and both the cathode and anode. When charging the battery, ions are forced into the electrolyte solution from the cathode to the anode. This process is simply reversed when the battery is put into use.
While older batteries, like nickel-cadmium (Ni-Cd) and lithium-ion (Li-ion), use a liquid chemical solution as an electrolyte, a LiPo battery instead has a lithium polymer electrolyte. This gel-like substance can be configured into a very thin and long semi-porous layer which helps keep LiPo batteries compact and lightweight.
Much like any other battery, a LiPo battery has a limited lifespan. This happens because of a process called electrolytic decomposition – the natural degradation of a battery’s electrolyte over time. The dissolution or the formation of deposits in either the cathode or anode terminals of a battery also results in a permanent reduction in the capacity of a battery.
When it comes to battery storage, two culprits lead to permanent damage: storing them undercharged or overcharged. Different mechanisms occur in these two cases:
- Storing a LiPo battery near 100% capacity results in the formation of a restrictive layer of deposits in the battery’s anode
- Allowing a battery to go into deep discharge – or the state where it goes below 3.0 V – can happen whether during use or long-term storage. When this happens, there is a huge risk of the anode material starting to dissolve into the electrolyte solution
Whether the anode material starts to dissolve or suffers deposit formation, the process is largely irreversible. This means that lost capacity probably cannot be gained back, even if you immediately switch to more sound battery management practices.
The role that temperature plays
Although we’re focusing on the proper voltage for LiPo battery storage, it would be a disservice not to mention the importance of temperature. As we’ve seen in the table above, storing a LiPo battery at low temperature has a highly positive effect on slowing down the rate of its degradation.
This apparent “preservation” happens because the process of electrolytic decomposition proceeds much slower at low temperatures. If you can store your LiPo batteries at 0 °C, then that would be ideal. If not, then the second-best option would be to store your LiPo batteries in a spot that is as cold as possible, preferably below 25 °C.
Other tips for LiPo battery use and storage
What we’ve written here about how LiPo batteries work and the chemistry behind them isn’t a comprehensive explanation of battery technology. However, it should be enough for us to make more informed decisions about how we use, charge, and store our LiPo batteries. Aside from keeping them at low temperatures and between 3.6 to 3.8 V, here are some more good practices to make sure that you can use your LiPo batteries for as long as safely possible.
1. Throw away a swollen LiPo battery
The most obvious sign of a battery that has undergone significant electrolytic deterioration is swelling or puffing up. This isn’t a phenomenon that’s unique to LiPo batteries. If you have an old phone with a Li-ion battery, you might have observed the same phenomenon after a few years.
The swelling of a battery happens because electrolytic deterioration results in the formation of a suite of gases, including carbon dioxide, carbon monoxide, and oxygen. Again, this is a process that can be arrested but not completely avoided and is irreversible. In some cases, the battery can burst, releasing toxic and highly corrosive electrolyte solution. This can be problematic inside your RC car or drone, as it can heavily damage any metallic or electrical components.
The biggest risk of continuing to use a swollen LiPo battery comes from the highly flammable nature of oxygen. Thus, there are a lot of documented cases of batteries getting punctured and immediately catching on fire and exploding.
Our recommendation is to immediately take steps to dispose of a battery once the swelling is evident. It should no longer be used and under no circumstances should it be charged.
2. Don’t leave a LiPo battery charging unattended
There are two good reasons why you should make sure to unplug a LiPo battery as soon as it’s fully charged. The first reason goes back to our recommendation to never overcharge a LiPo battery, as it results in the formation of deposits on the battery’s anode terminal. For this reason, you can expect a LiPo battery that is constantly being overcharged to have a rapid drop in capacity.
The second reason has to do with safety. Overcharging any battery results in heat buildup. Given what we know about LiPo batteries, there could be pockets of oxygen in the battery that could easily start a fire. The terminals of a LiPo battery are also very thinly separated by a polymer electrolyte layer, thus accelerating the formation of heat.
If you have to walk away for a substantial length of time from a LiPo battery that’s being charged, then it would be best to unplug it momentarily.
3. Store, charge, and transport your LiPo battery in a LiPo-safe bag
Many things make LiPo batteries much more hazardous compared to other types of batteries – the thin separation between the terminal, the pliable encasement, and the lack of mechanical durability among others. For this reason, it’s often recommended to practice much stricter safety standards when handling LiPo batteries.
The bare minimum requirement is to keep your LiPo batteries inside a LiPo-safe bag while they are stored, charging, or being transported. LiPo-safe bags are typically made with fiberglass treated with some fire-retardant material. Some are also lined with an aluminum coating outside to avoid any heat buildup because of external sources. LiPo-safe bags are invaluable and there are probably hundreds of testimonies online on how LiPo bags probably saved their houses from burning down.
While LiPo-safe bags don’t provide much mechanical protection either, they can at least contain a fire and avoid collateral damage. If you’re traveling with a LiPo battery by air, then a LiPo-safe bag may be required under your airline’s carry-on policy. You can get LiPo bags designed for storing individual batteries, but you can also get larger bags that can fit up to 20 batteries. The latter is safe enough for storage, but we suggest separating multiple batteries into individual bags during charging.
Despite the deficiencies of LiPo batteries, there’s no doubt that it has characteristics that make it useful not just to RC cars and drones, but to other forms of technology moving forward. The fields of electric cars and renewable energy, in particular, have been at the forefront of research on battery technology. As you would expect, lithium batteries remain the most significant development in this field, although not in the same form as LiPo batteries.
Until such time when LiPo batteries become more robust, the best we can do is to do all the best practices in the interest of safety. Fortunately, safe battery practices also coincide with those that are designed to maximize their lifespans.