0
\$\begingroup\$

I recently made a small coin cell battery powered bluetooth board. I get full voltage when using a dev kits 3.0V output but only 1.5V when using a 3V coin cell battery.

Circuit summed up pic below. To be exact:

  • Using a Nordic NRF52 Dev Kit's 3.0v VDD/GND output pins attached to battery holder + / -, I am getting correct voltage (2.93V) as expected.
  • Using CR927 coin cell in battery holder I am getting 2.93V to the RTC while the switch is off (expected,) but switched on I only get 1.5V everywhere (including the RTC.)

I am a hobbyist and don't know much about power/load/draw, but I am speculating/wondering if these small coin cell batteries must have some sort of maximum load that they can handle at once.

coin cell circuit

All components used are advertised as low power consumption.

\$\endgroup\$
1

1 Answer 1

2
\$\begingroup\$

Batteries have an output impedance and when a battery is not designed to provide high currents it has a high internal impedance.

That means there is basically a resistor in series with the output of the ideal voltage source that is the battery. So the more current you try to draw from it, the more voltage is dropped across the resistance and lost.

\$\endgroup\$
3
  • \$\begingroup\$ Thanks! Googling "coin cell battery impedance" gave me a lot to look over. It looks like I'll either have to draw less power, or (hopefully not) change batteries. Lot's of research ahead, I wasn't expecting the battery to be a challenge. \$\endgroup\$
    – exile97
    Jun 8, 2022 at 5:06
  • \$\begingroup\$ Impedance is not the only aspect to consider. You also need to determine the (average) current draw and the time to last for calculating the battery's minimum energy. \$\endgroup\$ Jun 8, 2022 at 5:34
  • \$\begingroup\$ @thebusybee my solution for my knowledge level was to buy every small battery I could find and test them all. Going with the CR1225, only 2mm larger and it never drops below 3v. Still surprised at how badly the cr927 dropped voltage when the cr1225 isn’t very different at all. Temporarily I’m using 2 stacked SR916SW’s (1.55v/ea) which fit in the cr927 holder and also don’t drop voltage. \$\endgroup\$
    – exile97
    Jun 8, 2022 at 23:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.