Power Managment

Donald Heer11/17/10

Topics

Battery TypesCharging SchemesInstantaneous vs. Average PowerExample Battery Life ExampleBattery Safety

Battery Types

Alkaline- AA, 1.5V Nominal, 2.5Ah at 100mA, RINT ≈ 225mΩ, Digikey N107-ND

NiCad (Rechargeable)- AA, 1.2V Nominal, .7Ah at 100mA, RINT ≈ 16mΩ, Digikey SY114-ND

Lithium Coin Cell- CR2032, 3V Nominal, .24Ah at .19mA, RINT ≈ 15KΩ, Digikey N189-ND

Lithium Ion (Rechargeable)- Prismatic Cell, 3.6V Nominal, 1.8Ah at 3.4A, RINT ≈ 320mΩ, Panasonic CGA103446

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Charging Schemes – Simple

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Charging Schemes - Better

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Charging Schemes – Mo’ Betta’

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Power vs. Average Power

Power is the power used during a single action. Power over time gives you capacity used. - Motor Motion: 12V @ 500mA for .5 Seconds

Average Power is power used over a fixed period of time. - Motor Motion: Door used 20 times per hour

Eg. The motor uses: 20times *12V * .5A * .5sec * 1/3600 = 16mW/Hour

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Example Battery Life Example

Imagine you are building a keypad activated door lock. You want it to run as long as possible. For some reason you are required to use a 9V alkaline battery.

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How long will the system work?Can the 9V system directly drive the Lock? If not how can we fix it?

Specifics for Example

Property Value

KeyPad Use 3.3V @ 5mA

Lock Cycle 200mA @ 6V

Controller Active 3.3V @ 2mA

Controller Stand By 3.3V @ 2uA

System Leakage 2uA

9-6V Regulator Switching Regulator at 95% Eff.

6-3.3V Regulator Linear Regulator

Uses per Day 200

9V RINT ≈ 1.5Ω, 600mAH @ 25mA

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How long will the system work?Can the 9V system directly drive the Lock? If not how can we fix it?

Battery Safety

Short CircuitsIn low resistance batteries, you can quickly create LARGE currents that can easily melt components.

Over HeatingDuring charging (and discharging) battery chemical reactions produce heat. The faster the rate the more heat. The issue is some chemistries (Lithium Ion) can ignite!

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Take Away

Check both your average power for battery life and your instaneous power for intermitent usePick the correct battery type, not just the newestDon’t