Should I Let My Battery Die Before Re-Charging? Dispelling the Myth

Cordless tools, cell phones, electric vehicles and other rechargeable devices have been with us since the 80s. But the batteries they use have evolved, and so has the battery care needed to maintain them. I bought my first cordless power saw in 1990 (it was a Hitachi), and its battery life noticeably deteriorated over time due to something called the memory effect. The general wisdom of the day was that you had to let your battery completely discharge periodically to preserve its charging capacity, but at the time, I didn’t know that.

Old school nickel-cadmium (Ni-Cd or NiCad) batteries use a different technology from the modern lithium-ion (Li-ion) batteries that are common today, and the memory effect is essentially a thing of the past— but people still believe it exists. Steven Athwal, who markets refurbished cell phones, comments, “I’ve heard people say that they fully drain their phone once in a while because it’s ‘good’ for the phone, in the same way as clearing cache or deleting photos to clean up space… but it doesn’t quite work like that.”

So how does it work, and what kind of care do the batteries in your phone, tools and car actually need?

Why People Believe the Myth

For those who have used devices with NiCd batteries in the past, the concept of the memory effect has been ingrained over time. NiCad batteries were invented in 1899 and powered various devices, including cordless tools, electric toothbrushes and even early cell phones from the 1960s to the new millennium, but they began to be phased out in the early 2000s because of environmental concerns and energy density issues.

NiCads were replaced first by Nickel-Metal Hydride (Ni-MH) batteries — which have a greatly reduced memory effect — and ultimately by Li-ion ones, which have no memory effect. Some medical devices, emergency lighting, and backup power systems still use NiCads because of their ability to deliver large currents; however, they are no longer common (except, of course, in older devices).

Modern batteries don’t display the memory effect, but, says Athwal, “people are still going to think it, even subconsciously.”

Older NiCad Batteries and The Memory Effect

The memory effect on older NiCad batteries is due to crystal formation on the anode (the negatively charged terminal). During discharge, cadmium on the terminal reacts with hydroxide ions from the electrolyte to form larger cadmium hydroxide crystals and release electrons, and during charging, this process is reversed. If the battery isn’t fully discharged, some of the larger crystals remain on the anode and take up space, leaving less room for cadmium ions.

If you repeatedly charge the battery fully and then discharge it only partially, more and more crystals form on the anode, and the battery holds less and less of a charge. That’s why it’s good NiCad battery care to let it run all the way down at least once a month. You can also break up the crystals by shocking the battery with an external power source, such as a 12-volt car battery.

How Modern Lithium-Ion Batteries Work

The energy produced by a Li-ion battery is due solely to the movement of lithium ions from the anode to the cathode and back again through a lithium-salt electrolyte. No crystals are formed in the process, so there is no memory effect.

The positively charged anode is typically made of graphite, while the cathode is made of a metal oxide that contains lithium, such as lithium cobalt oxide or lithium iron phosphate. Lithium ions shuttle between the two electrodes through the electrolyte, which blocks the movement of electrons and forces them to flow to the load. The battery also includes a porous separator, which allows lithium ions to pass through but prevents the electrodes from touching and causing a short circuit.

During discharge, lithium ions travel from the anode to the cathode, freeing up electrons that travel through an external circuit to power the load connected to the battery. When you attach a charger, it supplies the energy needed to push the lithium ions back to the anode. This cycle can repeat hundreds of times.

Is It Bad to Let Your Battery Die?

“Li-ion batteries age faster when they’re kept at spectrum extremes (always 100% or frequently 0%),” says Athwal. “Letting your phone die occasionally won’t ruin your device, but doing it often will shorten its lifespan.” This applies to all Li-ion batteries and tracks with the battery care advice Tesla provides to new owners of its vehicles, which use Li-ion batteries.

Optimal Charging Range

“Lithium-ion cells appear to be in the best condition within their middle level of charge, approximately 20-80 percent,” advises solar contractor Ethan Heine. “The material in these cells becomes stressed by extremely low and extremely high levels of charge.” This 20-80 rule also tracks with Tesla’s battery care advice. Athwal also concurs: “The healthiest range is roughly 20–80 percent. You don’t need to micromanage it, but staying out of the extremes gives the battery a much longer life.”

What About Tool Batteries?

The 20-80 rule generally also applies to tool batteries, provided they are Li-ion batteries and not NiCads. If you have older tools with NiCad batteries, you should drain the batteries completely before recharging at least once a month to avoid losing capacity to the memory effect.

Unlike phone and car batteries, tool batteries are frequently stored for prolonged periods. To preserve battery life, Dr. Lee Pan, writing for Large Power, recommends charging them to 50 percent before storage and keeping them in a cool (59-68 F), dry environment.

FAQ

Is it bad to leave my phone plugged in overnight?

No. Although charging your phone to 100 percent does generate heat that can shorten battery life, my iPhone has an adaptive charging feature to prevent this. It learns from my habits and maintains the charge at 80 percent until just before I typically use the phone, at which point it completes the charge. Phones from other companies, as well as some external chargers, also have this smart feature.

Is fast charging bad for batteries?

Usually, no. “Fast charging is fine when it’s needed. It’s the heat that ages the battery, not the speed itself,” says Athwal. Heine adds: “Rapid charging is generally considered safe as long as the device does not get too hot during the charging process. However, repeatedly rapidly charging a device that gets hot will shorten its lifespan.”

About the Experts

• Steven Athwal is the Founder of The Big Phone Store, which recently became one of the UK’s largest sellers of refurbished phones.
• Ethan Heine is the President and CEO of Suntrek Solar and a third-generation solar contractor.

Source

• Tesla: Range Tips; (2025)
• Large Power: Best Charge Level for Lithium Battery; (2021)

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