Lithium battery breakthrough means your phone will charge in 10 seconds? Not so fast.

[Update: now featuring Actually Correct Math. Somebody stop me before I late-night-blog again…]

Recent news coverage mentions a badly-needed breakthrough at MIT in battery technology. Using a slight variation of existing lithium materials, much faster charge and discharge rates are possible. The money quote is that

[Professor Gerbrand Ceder and graduate student Byoungwoo Kang] went on to make a small battery that could be fully charged or discharged in 10 to 20 seconds.

News outlets seem to have latched on to this part of the announcement and hinted that all kinds of battery-powered devices will soon be chargeable in ten seconds. I don’t think it will be likely to see, say a cell phone (much less a vehicle, as some stories hint) that can fully charge in 10 seconds. Here’s why: A typical cell phone battery might be rated at 800 mAh. It’s not perfectly linear, but you can think of it as being able to deliver 800 milliamps for an hour, or 10 milliamps for 80 hours, and so on. You could approximate the energy storage of the battery by multiplying volts x amps x hours, giving a figure in watt-hours (in this case 3.6 x 0.8 x 1 = 2.88 watt-hr). To charge it in 10 seconds, all that energy would need to be delivered within the 10 seconds, which is a sixth of a minute, or a 360th of an hour. So the charging current would need to be 0.8 x 360 = 288 amps, not counting any efficiency losses in the form of heat.

What’s the big deal about pumping out 288 amps? Ohm’s law gives some idea. To push 288 amps through a complete circuit of one ohm (this includes the internal resistance of the battery), you would need to apply 288 volts, with a resulting power consumption of 288 squared, or just under 83,000 watts. That kind of current is more suitable for an industrial arc welder than a household battery charger. Even if the resistance can be made smaller, the benefit is only linear. To compare, my laptop, which dissipates 65 watts over several square inches, gets uncomfortably hot. Or see how long you can hold your hand on a lit 100 watt incandescent bulb. I can’t imagine packing that much energy into a “small” battery. It’s also hard to imagine a safe charging circuit that uses voltages that much above the nominal voltage of the battery.

So professor Ceder’s breakthrough looks great, and probably will be in iPods in a few years, but take the media coverage with a large grain of salt.

-m

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