Low power PIC Battery circuit design

Dear group,

I am currently developing a wrist-worn based device using the PIC18F24J50.

The problem I am having is with the design of a battery circuit and specifying the correct Battery. It requires;

-Charging from USB (5v vbus)

-To last up to one month

The wrist device samples an accelerometer at 10Hz when active. If no activity (acceleration) has been detected for 25 seconds the PIC is put into deep sleep mode.

Can anyone recommend a suitable Battery and charging circuit from USB? The PIC runs on 3.3v

How can I minimise battery usage?

Where can I source a suitable Battery? Does anyone know any good suppliers?

Really appreciate any help here.

Thanks

Reply to
thorin92
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Does "last" mean the interval between charges?
"Up to" one month or "at least" one month?
Reply to
John Fields

thorin92 schrieb:

Hello,

to calculate the size of the necessary battery, some data is needed. If this devive is used over a month, how many hours will it be active and how many hours inactive? What current is consumed in active and sleep mode? Using these numbers, it is easy to calculate the necessary charge in Milliamperehours. Than you may search for a battery with the right size, if possible the battery should have 50 to 100 % more capacity than calculated. If the battery is to large for the device, you have to reduce the current consumption.

Bye

Reply to
Uwe Hercksen

You can run it with rechargeable CR2032. Just charge it with LDO regulator.

Deep sleeping and wake up on pin change interrupt.

Yes.

Reply to
linnix

Use the lowest clock speed and voltage you can get away with. BTW It generates a lot of data for a chip with only 3kb of ram!

And disable everything that is not being used - especially any default pullup resistors and output drivers. Should be able to get down to around 10uA with a 32kHz clock running - perhaps half that if you deliberately drop the PIC voltage from 3v to 2.2v or thereabouts. You have to be a bit more cunning if it will be attached to USB for readout.

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If you are careful a 65mAh battery should only need recharging every 4-6 months maybe even longer if it spends 12hours in deep sleep mode.

Regards, Martin Brown

Reply to
Martin Brown

Without knowing what you're using for an accelerometer it's impossible to say.

Without knowing how accurate the accelerometer needs to be, it's impossible to specify one.

I suspect that just finding an accelerometer that isn't going to demand a 1-pound battery to run for a month is going to be significant, much less what the PIC can do.

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Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

Accelerometer would take 5 to 10mA. So, you can't keep it on constantly either. The best way is to use a tilt switch to trigger and wake up the micro, then enable the accelerometer. Yes, we do need more details from the OP.

Reply to
linnix

10uA typical @ 2.5V for 10Hz data rate for one I see.. that's low enough to perhaps consider a throw-away battery like the lithium 225mAh 3V CR2032.
Reply to
Spehro Pefhany

The piezo accelerometers that I know of take that much. I don't know if there's a chip accelerometer that would work (or if the OP _needs_ a chip accelerometer). I also don't know if there's either a chip or piezo accelerometer out there that could be turned on and come up quickly enough that the current savings from the "off" time would make it worth while.

In principal I don't see why an accelerometer couldn't be constructed that could be powered up for a measurement, then powered down, etc., for low overall power consumption -- but unless someone has developed one already, you'll need one honkin' big battery.

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Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

The accelerometer? What's the manufacturer & part number?!

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Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

ST LIS331DLH; around $5 in quantity. There's a lot of action recently on this type of product because they're being put into MP3 players, phones and such like. Oh, and it's 3-axis (!).

Reply to
Spehro Pefhany

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Cheers! Rich

Reply to
Rich Grise

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