Do Power Tool Batteries Degrade When Not in Use?

Your power tool batteries are degrading even in storage. This silent loss costs you money and reliability. Let's explore the simple fixes to maximize their lifespan and value.

Yes, power tool batteries degrade when not in use due to a process called calendar aging¹. Factors like storage temperature² and charge level significantly impact this degradation rate. Storing them improperly can permanently reduce their capacity and lifespan, even if they are brand new.

This issue is something we see all the time in the battery industry. It’s not just about use; it’s about how you care for them when they’re idle. I've spoken with many clients, like Michael from a US-based medical device company, who manage large inventories of battery-powered equipment. For them, a battery failing prematurely isn't just an inconvenience; it's a significant cost and a potential operational risk. So, let's dive into the specifics of what you can do. The right knowledge can save you significant costs and headaches down the line.

What is the best charge level for storing lithium-ion power tool batteries?

Storing batteries at full or zero charge feels logical. But this common practice silently damages their internal chemistry. Discover the optimal charge level to prevent premature battery death.

The best charge level for storing lithium-ion batteries is between 40% and 50%. This state of charge (SoC) minimizes stress on the battery's internal components, slowing down chemical degradation and significantly extending its overall shelf life.

At our facility, we are very careful about the state of charge when we ship batteries to our customers. It’s a critical part of ensuring quality from day one. Storing a lithium-ion battery at 100% charge is one of the fastest ways to accelerate its aging. The high voltage places constant stress on the cathode materials, causing them to break down faster. On the other end, storing a battery at 0% is also risky. All batteries self-discharge over time. If a depleted battery's voltage drops below a certain critical level, it can trigger internal safety circuits that prevent it from ever being recharged again, effectively killing the battery. This is why we never ship fully depleted or fully charged batteries. We aim for that sweet spot, around 40-50%, to give the product the longest possible shelf life.

The Impact of State of Charge (SoC) on Battery Health

For businesses that stock large quantities of batteries, this is not a small detail. It's a core part of inventory management. The upcoming EU battery regulations³ will even require more transparency on battery health, and storage history will be a part of that. Starting with good practices now is a smart move.

How does storage temperature affect battery degradation?

You store your tools in the garage or a work van. But extreme temperatures are secretly killing your batteries. This costly mistake is easily avoidable with a simple change.

Temperature is a critical factor. Storing lithium-ion batteries in a cool, dry place, ideally around 15°C (59°F), significantly slows degradation. High temperatures accelerate chemical reactions inside the battery, leading to rapid and permanent capacity loss.

Think of a battery like a piece of fruit. You can leave it on the counter, and it will be okay for a few days. But if you put it in the refrigerator, it will last much longer. Batteries work in a similar way. Heat is the enemy of battery chemistry. It acts as a catalyst, speeding up all the unwanted chemical reactions that cause a battery to age. This includes the growth of the Solid Electrolyte Interphase (SEI) layer, a film that forms on the anode. While a stable SEI layer is necessary, excessive growth caused by heat consumes lithium ions and increases internal resistance, permanently reducing the battery's capacity. I once had a client who couldn't figure out why their battery stock was performing so poorly. It turned out their warehouse was not climate-controlled and would reach over 40°C (104°F) in the summer. That heat was destroying their inventory before it ever got used.

The Relationship Between Temperature and Capacity Loss

The combination of high temperature and high state of charge is especially damaging. The table below shows the estimated permanent capacity loss of a battery stored for one year under different conditions.

As you can see, simply storing a fully charged battery at a warm room temperature can cost you 20% of its life in just one year. This is why proper, climate-controlled storage is non-negotiable for anyone handling batteries professionally.

Should batteries be removed from tools for long-term storage?

Leaving the battery in the tool seems convenient. But this small habit could be draining its life and posing a safety risk. Learn why separating them is a crucial step.

Yes, you should always remove the battery from the power tool for long-term storage. Even when the tool is off, it can draw a small amount of power, leading to over-discharge. This separation prevents this parasitic drain and protects the battery.

This is a lesson I try to share with all our clients, especially those developing new electronic devices. The electronics inside a tool or device are rarely ever completely "off." They often enter a standby mode, where they draw a very small amount of current to be ready for the next use. We call this "parasitic drain" or "quiescent current." While this drain is tiny, over weeks or months of storage, it can completely deplete a battery. As we discussed earlier, a fully depleted lithium-ion battery is at high risk of being permanently damaged. Removing the battery from the tool physically disconnects the circuit and stops this slow drain in its tracks. It's a simple habit that acts as a powerful insurance policy for the battery's health.

Understanding Parasitic Drain and Safety

For our B2B customers, this is an important design consideration. When we at Litop develop a custom battery solution for a medical or IoT device, we work with their engineers to understand the device's quiescent current. If it's too high, it could mean the product has a very short shelf life before the battery is dead. We can then help them optimize their power management or recommend a battery with a lower self-discharge rate. Beyond preserving the battery's health, separating it from the tool is also a fundamental safety practice. It eliminates any risk of the tool accidentally turning on during transport or storage, which could cause injury or damage. It also reduces the risk of a short circuit if the tool's electronics were to fail. It’s a simple, zero-cost action that provides both longevity and safety benefits.

How can you tell if a power tool battery needs replacement?

Your battery charges, but dies quickly on the job. This frustration disrupts workflow and costs time. Know the clear signs that it's time for a replacement before it fails completely.

Key signs a battery needs replacement include significantly reduced runtime, failure to hold a charge for long, and taking much longer to charge fully. If the battery feels very hot during charging or use, or if the casing is damaged, replace it immediately.

Recognizing a failing battery is crucial for both productivity and safety. Continuing to use a degraded battery can be frustrating, but forcing a damaged one to work can be dangerous. The signs are usually quite clear if you know what to look for. The most common indicator is simply that the tool doesn't run for nearly as long as it used to on a full charge. This is a direct result of capacity loss from aging. Another tell-tale sign is when a battery that used to charge in an hour now takes two or three hours, or the charger's indicator lights flash an error code. This often points to high internal resistance or failing cells that can no longer accept a charge efficiently. At Litop, our Battery Management Systems (BMS)⁴ are designed with protective features to detect these issues, but it's always important for the user to pay attention to the battery's performance.

Key Indicators of a Failing Battery

Physical inspection is just as important as performance monitoring. Any change in the battery's shape is a serious red flag.

For our clients in fields like medical devices, battery reliability is not just about convenience; it's about patient safety. That’s why we implement rigorous testing and quality control to ensure our batteries perform reliably until their expected end of life.

Conclusion

In short, your power tool batteries do degrade in storage. To maximize their lifespan, store them at 50% charge in a cool place, separate from the tool. These simple steps protect your investment and ensure your tools are always ready when you are.