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Battery Life

Dan Nelson
Intellectual 480 points

Other Parts Discussed in Thread: CC2541, TPS62730

I'm trying to calculate how much power the cc2541 consumes so that I can determine what the battery life will be.  The application itself is very light and only operates for a short time each day.  Most of the power will go to waking up,say every second, and listening for a connection and then going back to sleep.  The datasheet gives current draw, but I need to know what the duty cycle would be in order to find average power consumption and hence battery life.  I need to get 3 years life out of a battery.

In short, what is the background power consumption of the cc2541 if it's waking up every second to listen for a connection.

Thanks

  • 0
    TI__Guru 57865 points

    Hi Dan,

    Have a look at Measuring Bluetooth® Low Energy Power Consumption found at http://www.ti.com/lit/swra347

    Best Regards

  • 0
    TI__Expert 4060 points

    Hi Dan,

    Please have a look at this application note: http://www.ti.com/general/docs/litabsmultiplefilelist.tsp?literatureNumber=swra347a

    This is in the process of being updated for CC2541, but the new version is not out yet. Roughly speaking, you should expect something close to 20 uA when you wake up once a second to listen for a connection.

    Best regards,

    Karl

  • 0 in reply to Karl T.
    Intellectual 480 points

    20 uA is a very good figure.  Thanks.

    Dan

  • 0 in reply to Karl T.
    Intellectual 480 points

    I know we are talking rough figures here, but is that 20 uA that you quoted based on using the TPS62730?

    I'm trying to budget for power and choose the right battery, but I haven't got as far as getting any software up and running except for what's in the mini dev kit.  It will be a while before I have anything that I can do some real measurement on. So I'm hoping that I can get a very rough estimate as to what sort of power consumption I should budget for.

    The application will be very light weight, mostly sitting around waiting for the master to connect.  The amount of data that is transferred will be very small, say in the order of 200 bytes per day.  I need to get 3 years battery life.  I've found a 7 Ahr lithium-ion "C" cell, but it has transport restrictions.  I suspect that it will be overkill, but at this point I don't have anything to go on.  I've found a similar cell but with 3.6 Ahr capacity.  That would give me roughly  125 uA over 3 years.  I would be interested to know what power a typical light weight application would use.

    Maybe as a starting point we could look at the keyfob in the mini dev kit.  Are there any power consumption figures for it?

    Sorry for such a nebulous question, but I'm being asked to give commitments for battery life without being in a position to measure it.  So any kind of rough figures or comparisons would be appreciated.

    Thanks

    Dan

  • 0 in reply to Dan Nelson
    TI__Expert 4060 points

    Hi Dan,

    Yes, the 20 uA would be with CC2541 and the TPS62730. The TPS gives around 20% improvement in the active currents (MCU and/or RF active), so we are talking about a ~4 uA improvement for these kind of connection times.

    Very difficult to estimate how much current your application will use without knowing the exact code to run. As a rough figure, the MCU part of CC254x will draw around 7 mA when running at full speed, but this is also code-dependent. If we do assume 7 mA, then after subtracting the 20 uA for connection, you are left with 100 uA. This would enable you to run around 1.5% duty cycle; ie. 15 ms of MCU operating time per second. If you're not doing a lot of processing, then I think that would be plenty.

    If you're aiming for a three-year life, you don't need to go lithium; good-quality alkaline cells are rated for 5 years shelf-life these days. A pair of AAs would give you around 2.3 Ahr capacity, can be bought anywhere and do not have transport restrictions.

    Best regards,

    Karl