You charge a tablet or a battery pack for your power drill to 100%, put it in a drawer, and forget about it. The next time you pull it out, the battery is dead. What gives? Here’s why batteries don’t (and can’t) stay charged.
All Batteries Lose Charge Over Time
Before we dig into the different kinds of batteries, let’s look at the biggest overarching concept related to this topic.
Energy doesn’t want to stay in one place, it wants to move to reach equilibrium. Take the simple example of heating and cooling your home. In the winter, you must continuously add heat as your home releases heat energy into the cooler environment. And in the summer, you must continuously remove heat, fighting against the energy outside your home.
Although we take them for granted, batteries are a bit of a technological miracle. With batteries, we’ve managed, improbably, to create a system where we can temporarily store electrical energy in a compact container and access it on demand—and, for the most part, it stays put without escaping into the environment.
But the electrical energy we stash away in batteries is not entirely unlike a bunch of school children all squashed into a classroom. The children fidget about, full of energy, really wishing they could be outside the confines of the classroom, racing about the playground. You could easily argue that it is not the natural state of children to stay calm and still in neatly organized rows.
The electrons packed away in your battery are like those fidgety kids, practically dying to be free and bouncing around again. The natural organization of the chemical compounds in the battery is not calm and neatly organized rows, so to speak—which is why batteries can be quite dangerous when things go wrong.
Even when your device is completely turned off or the battery is disconnected entirely, as is the case with power tool batteries removed from the tool, it’s not truly off on an atomic level. The chemical reaction inside the battery that makes the battery possible is still active, albeit in a much more subdued way than when you’re actually using the battery.
This continued low-level activity within the battery slowly depletes the stored energy. It’s called self-discharge—electrical discharge in the absence of an external load placed upon the battery—and it’s unavoidable.
Different Battery Types Discharge at Different Rates
If you’ve paid attention to the kind of batteries your different devices use and how often they seem to run down when left off the charger for too long, you’ve likely noticed that not all batteries are created equal.
While all batteries suffer from self-discharge as a fundamental side effect of their design and, you know, obeying the physical laws that govern the universe, the rate at which they discharge is significantly different. Here are some common rechargeable battery types and how quickly they discharge.
|Battery Type||Self-Discharge Rate Per Month|
|Nickel-Metal Hydride (NiMH)||25-30%|
|Low-Discharge Nickel-Metal Hydride||0.25-0.50%|
Lithium-ion batteries are the kind of batteries most of us have the most frequent experience with. That’s the kind of batteries used in smartphones, smartwatches, tablets, laptop computers, and a wide variety of other consumer electronics like drones, action cameras, Bluetooth speakers, and so on. They’re also the kind of batteries found in electric vehicles.
Nickel-metal hydride (NiMH) batteries are rarely used in portable consumer electronics these days but are used frequently in power tools as they cost less than lithium-ion batteries.
While the self-discharge rate of NiMH batteries is high, there is a variation of called low-discharge NiMH. The discharge rate is as low as 0.25-0.50% per month but is used primarily in small rechargeable batteries. The popular line of Eneloop rechargeable batteries from Panasonic is perhaps the most widely used example of low-discharge NiMH.
Nickel Cadmium (Ni-Cad) batteries are an older technology, now improved upon by NiMH, that was the original portable rechargeable battery type. While the discharge rate was better than NiMH, Ni-Cad suffers from a memory effect and requires more maintenance than NiMH and lithium-ion batteries, making it a less preferred battery type today.
Lead-acid batteries aren’t used in portable devices because of their high weight and safety issues stemming from the sulfuric acid bath the lead electrodes sit in. The lead-based design ensures even small lead-acid batteries weigh as much as a modest dumbbell which makes them impractical for anything but stationary applications.
The majority of lead-acid batteries are used for things like automotive starters, off-grid power storage such as you’d use with solar panels and uninterruptable power supplies for computers and other equipment.
How to Slow Battery Self-Discharge
You can’t fully stop batteries from discharging, but you can do one simple thing across all battery types to lower the discharge rate: keep them cool.
Whether you’re trying to keep a lithium-ion or NiMH battery topped off longer, do your best to keep the battery cool.
Cool within reason, of course. Don’t put your batteries in the freezer (condensation issues taking them in and out of the freezer can cause serious problems for the advanced internal circuitry found in modern rechargeable batteries), but do everything you can to avoid heat.
To do so, you might consider charging and storing your power tool batteries in your cool basement instead of leaving them out in your sweltering detached garage. You’ll also want to keep electronics out of hot cars and store them in the coolest part of your home if they won’t be in use for a while.
If your batteries are discharging rapidly enough that you practically need to leave them on the charger to ensure they are ready to use when you need them, then it’s time to replace them. Batteries degrade over time, even with the best of care, and if a battery is no longer properly holding a charge, it should be recycled and replaced.