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What You Need To Know About Lithium Batteries Aug 22, 2012

Electric batteries have come a long way since some unknown Parthian first stuck some iron and copper cylinders into a jar of grape juice (or maybe it was lemon juice… or vinegar?). The things that power our iPhones today, 2,000 years later, work on the same basic principle, though they bear little resemblance to a clay jar. The clay has been replaced by plastic, the grape juice has been replaced by an electrolyte gel, and the iron and copper are now lithium, cobalt, or carbon.

There’s a lot of misinformation floating around about batteries. Sites like Battery University can tell you more than you ever thought there was to know about batteries. But here, in a nutshell, are the 4 things you need to know about the battery inside your iPhone, iPod, or iPad.

1. It’s a lithium-ion polymer battery, though it sometimes goes by its shorter name, lithium-ion, or Li-ion. Lithium-ion technology isn’t new; even the “polymer” part goes back to the 1970s. Most of the devices we use today that say “lithium-ion” are really talking about Li-ion polymer batteries.

2. Temperature: Lithium battery technology got a bad rap in 1991 when one of the old-style solid-metal batteries caught fire, causing a batch of recalls. Old-style batteries made with solid lithium metal were subject to “thermal runaway” after repeated discharges. Modern lithium-ion batteries do not contain solid lithium metal; instead, they use lithium ions, suspended in a polymer-and-electrolyte gel (hence the name). These days, devices that use lithium-ion batteries employ protection circuitry that prevents overheating, so you can rest easy. In reality, you’re more of a danger to your battery that it is to you, heat-wise: using or charging your iPhone or iPad in environments hotter than 95° Fahrenheit (35° C), like the inside of a car with the windows up, can shorten your battery’s lifetime (but it won’t make it burst into flame). Don’t worry about cold, either. Freezing cold may diminish charge life temporarily, but it won’t have a permanent effect on the battery.

3. Memory effect? In a word, Nope. The phrase belongs to nickel-cadmium (NiCad or Ni-Cd) batteries. If you charge a Ni-Cd battery before it’s completely discharged, it will lose capacity to charge fully the next time… as if it remembers the smaller charge capacity and regards it as the new norm. Hence, Ni-Cd batteries have to be completely discharged before charging again, to retain their full lifetime between charges. This is a peculiarity of Ni-Cd batteries, not shared by Li-ions. In fact, your lithium-ion battery likes it better if you don’t discharge it all the way between charges. This is why Apple recommends that you go ahead and plug in your charger when the device gives you its low battery alert. When you charge, the battery doesn’t really mind if you don’t top it off. Li-ion battery lifetime is all about charging cycles. One charging cycle is the time between one full charge and one full discharge. You can partially charge or discharge your battery many times during a single charging cycle. In short, your lithium battery has no memory effect, and you can recharge it, in whole or in part, whenever you feel like it.

4. So how long does charging take? Apple says the first 2 hours of charging takes your battery to 80% of its full capacity. During the next 2 hours, the battery will trickle-charge slowly to top off. But since there’s no memory effect, you don’t have to devote 4 hours to topping off the battery. Give it a quick snack whenever it needs it.

Convenience is the watchword with lithium-ion batteries. They’re low-maintenance, long-lived, lightweight, and very easy to live with. They’re one of the technologies that make your iPad or iPod or iPhone such a pleasure to use. And, speaking of pleasures, as a reward for your having read this far, we won’t belabor the technical differences between “batteries” and “cells” (and we can all thank Mr. Benjamin Franklin for the b-word).

Parthian Battery photo by Stan Sherer. The actual working model in the photo was constructed by Danielle Downes and Ava Meyerhoff, of Smith College’s Museum of Ancient Inventions.  Battery icons are creations of bluefaqs.com.