Charging Batteries Isn't That Hard

Confused about how, and how often, to recharge your gadgets' batteries? It's not as big a deal as some people make it out to be. Read on, and I'll dispose of the clutter.

Myths, Urban Legends, and Truths

Some people consider a failure to practice good "battery hygiene" to be akin to neglecting your teeth.  You may have heard some of this advice about how to get the most out of your newest gadget's rechargeable batteries:

• Always run your battery almost dead before you recharge it
• Never just partially charge your battery
• Never let your battery run all the way down
• Never overcharge your battery
• Always unplug your charger as soon as your battery is full
• Run your battery way down and recharge if fully once a month
• Charge your battery whenever you can
• Remove your laptop battery pack if you are using your laptop on AC power

The Short Answer: You can ignore all but the last two of those rules.  And it won't make that much difference if you ignore those, too.

A Closer Look: Not All Batteries Are The Same

Over the years, electrochemical scientists have given us an assortment of rechargeable battery technologies.  Popular ones have names like:

• Lead Acid
• Nickel Cadmium
• Nickel Metal Hydride
• Lithium Ion (and Lithium Polymer)

Lead-acid batteries are cheap and reliable.  You've got one in your (gasoline fueled) car.  But
they are heavy and bulky.  Nickel-cadmium batteries were popular in early rechargeable pocket calculators, and some early laptop computers.  They may still be found in some small rechargeable appliances, such as vacuum cleaners, electric toothbrushes and hair trimmers.  Nickel-metal-hydride batteries have replaced nickel-cadmium batteries in many applications.
For the rest of this article, I'll be talking about how to get the most from your Lithium-Ion batteries, and their close cousins, Lithium-Polymer.  (Everything I say about Lithium-Ion batteries can be applied to Lithium-Polymer batteries.)  All modern laptop computers, cellphones, cameras, e-book readers and tablet computers use Lithium-Ion batteries.  Lithium-Ion batteries are lightweight, and pack a lot of juice in a small space.  Lithium-Ion is the king of battery electrochemistry - for now.

The old nickel kings are gone, but the memory lingers on.  Nickel-cadmium and nickel-metal-hydride batteries suffered from what's called the "memory effect":  If you partially charge a battery, or charge one that hasn't been fully discharged, the battery will lose its capacity to charge or discharge beyond that point.  So, to prevent the memory effect from sending these batteries to an early grave, people were advised to use their batteries until they were completely flat, and then recharge them fully.  Partial charge and discharge cycles were frowned upon.

Some people still remember the old kings' rules.  But they don't apply to the new king.  Lithium-Ion batteries don't suffer from the Nickel batteries' memory effect problem.  Lithium-Ion batteries are fussy in their own way.

Rules Not To Live By

Lithium-Ion batteries can't stand being overcharged, and can't stand being fully discharged, either.  But that's not your worry.

Every laptop, every cellphone, every e-book reader, camera and tablet computer on the market contains excellent charge management circuitry.  Your gadget will stop charging the battery when it's full.  Your gadget will shut itself off before you can run the battery down to the damage point.  The charge management circuitry will do a better job of preventing overcharge / overdischarge damage than you can possibly do yourself.

As far as running your device down to prevent the memory effect, forget about it.  Lithium-Ion batteries don't have a chemical memory like that.  But they do age, every time they go through a discharge and recharge cycle.  So, forget about a full discharge / recharge every month, too.

Rules To Live By

Lithium-Ion batteries age with use.  Partial cycles are less aging than full cycles, so recharge your gadget when you can.  It's fine to charge your gadgets every night, and start the day with a full charge.  You can't overcharge them.

One thing that does matter, especially for laptops, is heat.  Heat is the great enemy of Lithium-Ion battery packs.  And the hottest place that most people keep a Lithium-Ion battery pack is... inside a laptop.  If you tend to use your laptop for extended periods on AC power, you can preserve the life of your battery pack by removing it from the laptop before starting it on AC.  Any laptop with a removable battery pack will run just fine on AC power with the battery pack removed.

If you're going to remove the battery pack and stick it in a drawer for a month or more, leave it about half-charged.

Or... just don't worry about it too much.  Manage your battery pack perfectly, and you might get 30% - 50% more life out of it.  That might mean three years, instead of two.  How much effort is that worth to you?

Battery Meters: Calibrate Your Battery?

There is one good reason to let your Lithium-Ion gadget run its battery down occasionally.  If your gadget has a "fuel gauge", or battery life meter, it also has a "smart battery pack".  That's a battery pack with some electronic circuits built-in.  These circuits monitor the way you charge and use your gadget in an attempt to provide an accurate picture of the battery pack's state of charge for the fuel gauge.

As it turns out, that's hard to do.  You'll get better readings if, on rare occasions, you use your device until the battery is near-exhausted, or the device shuts off automatically.  That process lets the smart battery learn how much power the battery can deliver from full charge to exhaustion.

It's called "calibrating the battery".  For best results, start with a full charge, and use your device the way you normally use it until the battery is exhausted.  There are apps you can get for some gadgets that will run the battery down automatically - but that's not a normal usage pattern.  Those apps will calibrate the battery, but it may not reflect your normal usage pattern.

Fuel gauge technology is a long way from perfect.  Don't depend on it, and don't waste your time with frequent battery calibrations trying to get perfection.  Remember, full discharges age a lithium-ion battery faster than partial discharges.  You can try a calibration cycle when your fuel gauge is just too weird to be useful.  If it doesn't help, there's probably something else wrong.

A Difference of Opinions

Does the charger turn off when the fuel gauge says 100%?  Not necessarily!  The fuel gauge reports charge levels based on usage patterns and a track record.  The charge management circuit monitors battery voltage and current flow during charging, and stops the charge cycle when the voltage and current reflect the chemical state of a fully-charged battery.

If you see your laptop's battery charge state at 95%, plug in the AC adapter, and find that it's still saying 95%, and reporting "Plugged in, not charging" an hour later, what's wrong?  The smart battery might need a calibration cycle, but nothing more than that.  The charge management circuit started to charge the battery, and stopped almost immediately, when it detected the voltage / current levels of a full battery.  The battery has as much charge as it could hold, but the fuel gauge is reporting that charge level as 95%.  With the AC adapter connected, the laptop will run on AC power, so the battery charge doesn't drop.

I'll stress the point that the battery has as much charge as it can hold.  It's just that the fuel gauge is off.

You might be able to get better agreement between the fuel gauge and the charge management circuit by running a calibration cycle, as I described earlier.  Or maybe not: as battery packs age, they lose their ability to accept a full charge.  The deteriorated electrochemistry stops the charge cycle short of where the smart battery has placed the 100% charge mark.

If the fuel gauge is way off when charging stops, and a single battery calibration cycle doesn't improve matters, your battery is seriously deteriorated.  (Or perhaps some other component has failed.)  It might be time to replace the battery pack, or the whole device.

Only You Can Prevent Laptop Fires

One last thing.  Some battery pack failures, age related or otherwise, can produce a lot of heat.  Battery packs contain safety features to prevent fire or explosion, but don't depend on those features.  If your gadget is getting very hot while charging, unplug it and replace the battery pack.  (Laptops will run just fine on AC power, without the battery pack.)  Here's a short video of a laptop on fire at Los Angeles International Airport:

Wrap-up: Myths, Urban Legends, and Truths - Revisited

Nickel-cadmium & Nickel-metal-hydride legacies to ignore:

• Always run your battery almost dead before you recharge it
• Run your battery way down and recharge if fully once a month
• Never just partially charge your battery

True, but the charge management circuitry takes care of it for you:

• Never let your battery run all the way down
• Never overcharge your battery
• Always unplug your charger as soon as your battery is full

True, but won't make your battery last a lifetime, either:

• Charge your battery whenever you can
• Remove your laptop battery pack if you are using your laptop on AC power for extended periods

In conclusion: practice good dental hygiene and your teeth will last a lifetime.  But even the best battery hygiene won't go very far, so that's not where to put your time and energy.

To learn more:
The Battery University is excellent: http://batteryuniversity.com/

--Joe Levy

Next Post: "Can Magnets Hurt Your Electronics?" (with "Breaking Bad" spoilers).  There's a "no spoilers" version here.