Extending Battery Life

Yesterday I was researching some old computers, machines that required substations' worth of power. Today I'm thinking about systems that run for years off a single coin cell. How much things have changed!

Ultra-low-power embedded systems can run off a CR2032 for 10 years if they spend most of that time in a deep sleep, waking only occasionally and for brief periods to do something useful. But it's not easy to squeeze the last coulomb from these small cells.

We know that a processor's power consumption is proportional to the voltage squared times the operating frequency. But we're not really interested in power, since battery capacity is rated in amp-hours. Current is the issue, and current in a resistive network is linear with voltage.

Is it linear with an active (very active) component like a microprocessor? Here's what the datasheet says about Microchip's PIC18LF46K22, a typical very low-power part:

TI's MSP430F2013 also exhibits the same behavior, though it's unclear if this is the pretty-much-meaningless “typical” data or worst case.

This linear Vdd vs. current data is a bit puzzling. Depending on clock rate, running at a lower Vdd results in substantial energy reductions. A number of vendors are locked in a battle for dominance in the ultra-low-power space. One would think they'd put a regulator on board to operate the chip at the lowest possible Vdd, optimizing their current numbers. Obviously, there may be some pin issues, though those could be powered pre-regulator.

The processor will, in these applications, mostly be sleeping. But we really don't care much about the relationship between Vdd and current when it's in a snooze mode. For a 10-year life from a CR2032, the average current draw over that decade cannot exceed 2.5 uA. With sleep currents of very-low-power MCUs in the tens to hundreds of nanoamps, sleep is practically irrelevant to these issues.

To reduce the processor's needs, why not add an external LDO (low-dropout) regulator?

Unfortunately, during the processor's long periods of sleep, the typical LDO will require tens or more of microamps. I can't find one that is frugal enough with coulombs.

Touchstone Semiconductor has a boost converter that looks like a perfect solution. Its TS3310 will do all sorts of cool things to crank low-voltage sources up to MCU levels. But it will also regulate a battery down to lower levels. When the processor is sleeping, it only needs 180 nA from the battery, which is just 7% of the 2.5 uA budget. Regulate from 3 to 1.8 V, and you'll gain, even accounting for the regulator's needs, around a third more energy. Using the regulator will boost the amount of current a system can consume by around a third for the same battery lifetime.

The 2mm square IC itself is $0.99 in 2,000-unit quantities. It does require a small inductor, but those are about seven cents in volume. A couple of capacitors are needed, as well, which would also be the case with an LDO.

It may be tempting to assume one can use this trick to build a system that will run for more than 10 years off a coin cell. Alas, that's not to be. The vendors specifically claim a 10-year life on their batteries. It was just last summer that Duracell changed its ratings from seven to 10. If a part supplier says, “Don't use this part in this mode” (in this case, for more than a decade), it's poor engineering practice to ignore the warning. Sure, the system might work, but it's a crapshoot.

Of course, a cynic might note that he will probably not be employed at the same place after all those years go by, so he can't be held accountable…

(Note: Touchstone has fallen on hard times. Silicon Labs recently acquired its IP and will hopefully continue selling this part.)

This article was originally published on EBN's sister publication Embedded.

4 comments on “Extending Battery Life

  1. Anand
    March 25, 2014

    This is a common rule that relative costs come down with advancements in technology. E.g. The HTC One V released in 2012 was a mid ranged phone with 512 megabytes of RAM but now, we get twice the ram and a jelly bean update for half the price of the phone, and this happened in 2013, only a year after the release of the phone. Same is with battery backup. Batteries have really come a long way, and they would still have to go a long way, and find compatible grounds with processor technologies and give better results. For a more powerful processor, the power usage should be intelligent enough to run the battery for quite some time without having to compromise on its resources.

  2. Himanshugupta
    March 25, 2014

    Until there are technology advancements in battery life extension, the smart way is to carry a portable battery charger which weighs less that a pound and cost less than 50USD if shipped directly from China. This way i can charge my smartphone for 6-8 times which means that i am not worried about lot of things.

  3. Hailey Lynne McKeefry
    March 25, 2014

    There's some really cool and interesting tech on the horizon with the batter industry. One that i am particularly interested in is charging over WiFi: Doing away with a charger altogether would be a real game changer.

  4. Eldredge
    March 26, 2014

    @Hailey – Great point. It is a real nusiance to fimd mulitple places to plug in all of the battery powered devices we have  – and not just phones and tablets. Tools, home telephone handsets, battery powered toothbrushes, and on the list proliferates. If the industry could standardize on a wireless recharging ssytem that could work for all of these devices, that would be a step forward.

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