How to make lithium-ion batteries last for years

How to make lithium-ion batteries last for years

By Fred Langa

The lithium-ion (Li-ion) batteries used in most of today's tablets, smartphones, and portable PCs require very different care and feeding than with the nickel-cadmium (Ni-Cd) and nickel-metal-hydride (Ni-MH) batteries used in earlier devices.

In fact, proper care of a Li-ion battery can result in as much as 15 times longer service life than with an improperly cared-for battery. Here's how to make sure that your expensive Li-ion batteries last as long as possible in all your portable devices.

I recently had to replace a damaged smartphone — and it was all my fault.

The symptom was ominous: my phone's case became distorted — the body of the phone actually started to bend.

On inspection, it turned out, the phone's Li-ion battery had actually swollen.

At first, I hadn't noticed the change; the battery looked more or less normal when viewed face on (see Figure 1). But when I removed the battery and placed it onto a flat surface, it became obvious that the battery's front and back were no longer flat and parallel; one side of the battery had developed a substantial bulge, as shown in Figure 2. This bulge is what had caused the body of the phone to bend.

Figure 1. The damaged battery looked more or less normal when viewed face-on.

Figure 2. But an end-on view of the battery showed a noticeable distortion. Instead of lying flat, one side bulged out — enough to deform the body of the phone.

The bulge indicated something severe: a high-pressure buildup of toxic gases inside the battery.

The battery's case had done its job — it had contained the gases — but the battery was now potentially a tiny, pressure-cooker bomb — just waiting for something to set it off.

Both the battery and the phone were now irretrievably damaged; it was time for a new phone.

Adding insult to injury, not only was the damage my fault, it could easily have been prevented. (At the end of this article, I'll tell you what I did wrong.)

To avoid repeating my past mistakes with a new phone — or with other Li-ion-powered devices such as tablets and portable PCs — I spent some time researching the care and feeding of Li-ion batteries.

I wasn't looking so much for ways to get more run time between recharges; those techniques are already well known. Most portable devices offer ample manual and automatic power-saving modes and methods such as adjusting screen brightness, slowing CPU speed, and reducing the number of apps running.

Rather, I focused on ways to extend the battery's overall service life — i.e., ways to keep it safe and healthy and able to function as it should for potentially years to come.

In a list at the end of this text, I've included the best references I found; there's a ton of good information there.

But to save you time, I've also distilled some of the best and most interesting take aways below. Follow these five important tips, and you'll help ensure that your Li-ion batteries will deliver long, full, safe service lives in all your portable devices.

Tip 1: Keep your lithium batteries cool

I was surprised to learn that heat is the number-one enemy of Li-ion batteries. Heat issues can be caused by usage factors such as the speed and duration of battery charging and discharging. (I'll come back to this in a moment.)

The external, physical environment also matters. Simply leaving your Li-ion-powered device in the sun or in an enclosed car — even if the device isn't being used at the time — can significantly reduce the battery's ability to take and hold a charge.

Li-ion batteries perform best at about normal room temperature (68F/20C). If the device warms to 86F/30C, its ability to hold a charge drops by about 20 percent. If the battery is used at 113F/45C — a temperature easily reached by devices that are working hard or that are in the sun (say, on a car dashboard), battery capacity can be reduced by half.

So if your device or battery becomes noticeably warm while you're using it, consider moving to a cooler location. If that's not possible, try reducing the amount of power the device is using by turning off unneeded apps, features, or functions; by reducing screen brightness; or by activating the device's power-saving mode.

If that still doesn't help, turn the device fully off until its temperature returns to normal. For fastest cooling, remove the battery (if the device allows that) — the battery and the device will cool off faster if they're physically separated.

Incidentally, although high temperatures are a major issue with Li-ion batteries, low temperatures aren't as much of a worry. Low temps usually won't cause any long-term damage, although a cold battery won't produce as much power as it otherwise would. The power drop becomes very noticeable at temperatures lower than about 40F/4C. Most consumer-grade Li-ion batteries are essentially useless at temperatures around or below freezing.

If your Li-ion powered device becomes excessively chilled for any reason, don't try to use it. Leave it powered off and move it to a warm place (a pocket or a warm room) until the device is at normal temperature. As with overheating, physically removing the battery (if your device allows this) and warming it separately from the device will speed the warm-up process. Once the battery warms to a normal temperature, so will its electrical performance.

Tip 2: Unplug the charger to save the battery

Overcharging — leaving a battery connected to a too-high voltage source for too long — can reduce a Li-ion battery's ability to hold a charge, shorten its life, or kill it outright.

Most consumer-grade Li-ion batteries are designed to operate at around 3.6 volts per cell but will accept a temporary overvoltage of around 4.2 volts while charging. If a charger outputs the higher voltage for too long, internal battery damage can occur.

In severe cases, overcharging can lead to what battery engineers delicately refer to as "catastrophic failure." Even in moderate instances, the excess heat produced by overcharging will negatively affect battery life, as you saw in Tip #1.

High-quality chargers can work in concert with circuitry inside well-designed Li-ion-powered devices and their batteries, reducing the danger of overcharging by properly tapering off the charging current. (I'll cover charging devices in a moment.)

But the simplest, can't-fail method is not to leave your Li-ion devices connected to any charger longer than is needed.

These properties are quite different from those of older Ni-Cd and Ni-MH battery technologies, which did best when left on their chargers for as long as possible. That's because those older battery types have a high rate of self-discharge; that is, they start losing a significant amount of stored energy the moment you take them off the charger, even if the device they power is turned off.

In fact, a Ni-Cd battery can self-discharge at a rate of 10 percent in the first 24 hours. The self-discharge curve flattens after that, but a Ni-Cd battery will still lose an additional 10–20 percent charge per month.

Ni-MH batteries are even worse. Their self-discharge rate is about 30 percent higher than that of Ni-Cd.

But Li-ion batteries have a very low rate of self-discharge. A healthy, full, lithium battery will self-discharge at about only 5 percent in the first 24 hours off the charger — with only an additional 2 percent or so per month after that.

Thus, it's simply not necessary to leave a Li-ion device on the charger until the last possible moment. For best results and the longest battery life, unplug the charger when it or the lithium-powered device shows a full charge.

It's also not necessary to give new Li-ion devices an extended charge before first use. (Ni-Cd or Ni-MH devices used to come with warnings to provide an initial charge of anywhere from 8 to 24 hours.) Li-ion batteries are fully ready for use when the charger or the device reads 100 percent charge. No extended charging is needed.

Tip 3: Don't deep-discharge your battery

Not all discharge cycles exact the same toll on a battery. Long and heavy usage generates more heat, putting more stress on the battery; smaller, more frequent discharges extend the overall life of lithium batteries.

You might think that a higher number of small discharge/recharge cycles would eat into the battery's overall lifespan. That was true with older technologies, but it's not the case with Li-ion.

Battery specs can be confusing because most manufacturers count a full Li-ion charge cycle as whatever it takes to add up to a 100 percent charge. For example, two 50 percent discharge/recharge events equal one full-charge cycle. Likewise, three 33 percent discharge/recharge cycles equal one full-charge cycle, five 20 percent cycles equal a full charge, and so on.

In short, a higher number of small discharge/recharge cycles doesn't reduce a lithium battery's total available full-charge cycles.

Again, heat and stress from heavy discharges reduce battery life. So try to keep your deep-discharge events to a minimum. Don't let your device routinely run down to zero charge (where the device turns itself off). Instead, think of the bottom 15–20 percent of battery capacity as a reserve — for emergency use only. Get into the habit of swapping in a fresh battery (if possible) or plugging the device into external power well before the battery is empty.

Tip 4: Slow and steady charge/discharge is best

As you now know, both fast discharging and fast recharging generate excess heat and exact a toll on battery life.

If you've run a device long and hard, let the battery cool to room temperature before recharging it. Batteries won't accept a full charge when hot.

And when recharging, make sure your charger doesn't make the battery become hot to the touch — a hot battery is a sign the charger is pumping too much current, too fast, through the battery.

Overcharging is more likely with chargers that are cheap, off-brand models; that use fast-charge circuitry; or that are wireless (inductive).

A cheap, generic charger could be little more than a transformer in a case with some connecting wires. These "dumb" chargers simply pump out current, accepting little or no feedback from the device being charged. Overheating and overvoltages can easily occur, damaging or even destroying the battery.

Fast chargers are designed to provide a useful charge to a drained battery in minutes rather than hours. There are various approaches to fast-charging technology, and not all of them are compatible with all lithium batteries. Unless the charger and the lithium battery are specifically designed to work together, fast charging could cause overheating and overvoltages. Generally, it's best not to use one brand of fast charger on a different brand's device.

Wireless (inductive) chargers use a special charging mat or surface to restore a battery's power. It sounds wonderfully convenient, but inductive charging always generates excess heat, even when it's working normally. (Some hi-tech kitchen stove tops actually use induction to heat pots and pans.)

Not only is the excessive heat produced by a wireless charger not good for lithium batteries, it also wastes energy. By its nature, inductive charging's efficiency is always going to be significantly lower than a standard charger's. To me, higher heat and less efficiency easily outweigh convenience. You might feel differently.

In any case, the safest approach is to use only chargers sold by the OEM of your lithium-powered device. It's the only way to be sure that the charger will keep temperatures and voltages within specs.

If an OEM charger isn't available, use a low-output charger that's unlikely to pump damaging amounts of power into the device you're charging.

One source of low-output, non-OEM charging that's often available is the USB port on a standard PC. A typical USB 2.0 port provides 500mA (.5 amps) per port; USB 3.0 provides up to 900mA (.9 amps) per port. In contrast, some dedicated chargers will output 3,000-4,000mA (3-4 amps). The low amperages offered by USB ports will usually provide cool, safe charging of almost any Li-ion device.

Tip 5: Whenever possible, carry a spare battery

If your device allows for easy battery replacement, carrying a spare battery is cheap insurance. It not only gives you twice the run time but also helps you avoid the need to fully discharge a battery or use a quick charge. When the in-use battery approaches 15–20 percent charge, simply swap out the drained battery for a fresh, cool one — you get instant full power, with no heat worries.

A spare battery also allows for other benefits. For example, if you find yourself in a situation where the installed battery is running hot (say, because your device was working extra hard at some long task or because the ambient temperature is exceptionally high), you can swap out the hot battery to let it cool.

Having two batteries should also eliminate any need to use fast chargers — you can charge the spare at a safe, slow rate while the other is in use.

Post mortem: My battery-killing mistakes

Looking back, I believe I damaged my smartphone battery on a long road trip. I was using the smartphone's GPS to navigate on a beautiful, sunny day. The smartphone was sitting in the sun, in a dashboard-mounted cradle, and I had the screen brightness turned all the way up so I could see the moving maps in the bright sunlight.

Moreover, all my normal background apps — email, etc. — were running. And I was also streaming music via 4G and piping the audio into the car's sound system via Bluetooth. That phone was working hard.

To keep the phone powered, I'd plugged it into a generic 12V adapter I'd bought because it had the right plug — and was inexpensive.

The combination of solar heating, high CPU use, full-on screen, and poorly regulated electrical power made the phone get exceedingly hot. I remember being surprised at how toasty it was when I removed it from its cradle. This severe overheating undoubtedly started the battery's death spiral.

I apparently compounded the problem at home by routinely leaving the phone plugged in overnight, letting it needlessly sit on a generic charger for hours after reaching full charge.

With my new phone, I'm now using only OEM chargers and carrying a spare battery. I expect to get long — and safe — service from both the batteries and the phone by following the tips and steps mentioned above.

But those are the basics; there are more ways to extend battery life. I suggest checking out the extensive resources listed below — the best sites I found in my research — and you'll have all the information you need to get the most battery power and life from all your lithium-powered devices.

Much more information on modern batteries

If you want to know everything about caring for Li-ion batteries, see the following:

• "Lithium-ion battery" – Wikipedia entry
• "Batteries in a portable world" – buchmann.ca website
• "Battery University" – Cadex Electronics website
• "HP Notebook PCs — Understanding lithium-ion and smart battery technology" – HP Customer Support website
• "Tips for extending the life of your smartphone battery" – Samsung website
• "Maximizing battery life and lifespan" – Apple website
• "ASUS battery information center" – ASUS website
• "Charging Li-ion batteries for maximum run times" – Power Electronics Technology PDF
• "Capacity fade study of lithium-ion batteries cycled at high discharge rates" – Journal of Power Sources PDF
• "Battery performance characteristics" – Electropaedia article
• "Basic understanding of LiPo, Li-ion and LiFePo4 battery, its use and care" – COBOX article
• "Lithium-ion battery and lithium iron phosphate battery charging basics" – PowerStream website
• "Proper lithium-ion battery charging and safety" – EDN Network article
• "3 easy ways to extend your lithium-ion battery life" –RELiON Batteries article
• "Ask Ars: What is the best way to use a Li-ion battery?" – Ars Technica article