CR 2032 battery question

[Kdog44]

question: I want to use two CR 2032 batteries in series for a project. The load the batteries will see is about 580 Ohms. The current draw from the batteries is about 10mA. On this datasheet (http://data.energizer.com/PDFs/cr2032.pdf) It gives a voltage reduction curve over time for a 0.19mA draw, my circuit needs to consume 50 times that. So will the batteries voltage decrease approximately 50 times quicker? I know that it is a non-linear relationship between voltage and time for a set load, but is that a good approximation? Also is there anyway better I could get about 5-6 volts to my circuit that I want on a 5X5 cm PCB? I have most of the bottom layer clear, and I want it to be compact.   

background: I am designing a miniature version of the "Simon Says" game, for practice in PCB prototyping. The base of the game is the PCB itself, and it will have all four LEDs and buttons mounted onto it. In my eagle file I have the two CR2032 battery holders on the bottom of the pcb. I need at-least 5 volts for the micro I am using.

[IanB]

The internal resistance is listed as between 10 and 40 ohms, so with a 10 mA draw you will see a voltage drop of up to 0.4 V per cell. (If the current has peaks higher than 10 mA the situation will be worse.) Taken with the discharge profile of the cell it won't take much to fall below the 5 V minimum.

How about the classic solution of a 9 V battery with a voltage regulator?

[mazurov]

It gives a voltage reduction curve over time for a 0.19mA draw, my circuit needs to consume 50 times that. So will the batteries voltage decrease approximately 50 times quicker? I know that it is a non-linear relationship between voltage and time for a set load, but is that a good approximation?

If your draw more current than specified the capacity of the cell decreases in addition to "run time".  The relationship is complex and for 50x overload the only sure way to find out is to test.

[Kdog44]

How about the classic solution of a 9 V battery with a voltage regulator?

Hmm I was hoping to keep the overall profile of the board and the battery(s) slim. But I might have to go in that direction if I can't find a better solution. 

[Psi]

You should be able to do a Simons said game for less than 10ma continuous battery draw.
Use the right LEDs and you will get plenty of brightness from 0.6mA
The mcu can sleep most of the time.  Use a big cap to buffer the peaks.
Sound might be an issue depending on how loud you want. But shouldn't be too bad.
If money isn't a prob put a small Maxwell supercap on the PCB. They're like $10-20
, look like a regular electrolytic cap but have huge capacity to buffer high current peaks lasting even a few seconds

[Kdog44]

You should be able to do a Simons said game for less than 10ma continuous battery draw.

You are right, I have a couple of 0805 LEDs that can be operated at 0.1mA, and I could put the MCU into sleep mode and being triggered by the buttons as interrupts, but I think I might play it safe for this first revision.  

[Ice-Tea]

CR2032 are about 200mAh, so 10mA will give you about 20h or so

[Kdog44]

CR2032 are about 200mAh, so 10mA will give you about 20h or so

I am not concerned about the capacity of the battery, I am concerned about the voltage dip because of the drop across the internal resistance.

[tszaboo]

CR2032 are about 200mAh, so 10mA will give you about 20h or so

I am not concerned about the capacity of the battery, I am concerned about the voltage dip because of the drop across the internal resistance.

Technically, this type of application should be possible without any voltage regulator with a single 3V battery, if you use red leds, and the correct microcontroller. It would even increase your battery life.

[Zero999]

if you use red leds, and the correct microcontroller.

Or blue LEDs can be used with a simple voltage doubler.

[Stonent]

CR2032 are about 200mAh, so 10mA will give you about 20h or so

I am not concerned about the capacity of the battery, I am concerned about the voltage dip because of the drop across the internal resistance.

Technically, this type of application should be possible without any voltage regulator with a single 3V battery, if you use red leds, and the correct microcontroller. It would even increase your battery life.

I was thinking about MSP430 or some of the ATTINY V series, run the whole thing from a 32K watch crystal.

• Low Supply-Voltage Range: 1.8 V to 3.6 V • Universal Serial Communication Interface
(USCI) • Ultra-Low Power Consumption
– Active Mode: 230 µA at 1 MHz, 2.2 V – Enhanced UART Supporting Auto Baudrate
Detection (LIN)
– Standby Mode: 0.5 µA
– IrDA Encoder and Decoder
– Off Mode (RAM Retention): 0.1 µA
– Synchronous SPI • Five Power-Saving Modes
– I2c
• Ultra-Fast Wake-Up From Standby Mode in C™
Less Than 1 µs

• Speed Grades
– ATtiny2313V: 0 – 4 MHz @ 1.8 - 5.5V, 0 – 10 MHz @ 2.7 – 5.5V
– ATtiny2313: 0 – 10 MHz @ 2.7 - 5.5V, 0 – 20 MHz @ 4.5 – 5.5V
• Typical Power Consumption
– Active Mode
1 MHz, 1.8V: 230 µA
32 kHz, 1.8V: 20 µA (including oscillator)
– Power-down Mode
< 0.1 µA at 1.8V

Then you may be able to get by with running the coin cells in parallel or just use 1.
In active mode the Attiny at least on paper seems to use less current. But you do get the addition of the ADCs in the 430 which the 2313 does not have. The whole thing could be interrupt driven with sleeps in between.  With the 430 you could use the ADCs to sample the voltage of the battery presumably and alter a PWM signal to the LEDs to keep the brightness current. Maybe even switch off to a voltage doubler if you get to a point where the voltage is too low to keep the LEDs visible.