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Summary: I have an LED circuit powered by a coin cell. It loses voltage faster than expected and the LED signal becomes too weak to be useful in an unacceptably short time period. Fig. 1

I have a circuit that's running off of a CR2032 (Digikey, P-189-ND). It runs an IR beam break (emitter and sensor) and a comparator that relays the beam break state via another IR LED (transmitter).

I empirically measured the current draw as ~15mA nominally and up to ~20mA when the transmitter LED is active (reasonable?). The battery is rated at 225mAh. So, I expected 10+ hours of constant transmitter illumination. In practice, the transmitter should only be active intermittently, for seconds or tens of seconds at a time. Therefore I really expected 15+ hours of on-time per battery. However, if I could even get 30 minutes of use out of it at this point, that would be acceptable, if not ideal. As it stands, even that doesn't seem to be feasible currently.

When I put the circuit into the entire system, my receiver was having trouble detecting the relay signal after a few minutes. I traced this back to "low" battery voltage (still >2.7V). I actually think that should be a sufficient voltage, but apparently not.

I measured the LED emission with a power meter (Thor Labs S121C) and the transmitter signal power drops at around 0.01mW/s (a fresh battery puts out about 1.7mW). This does not recover after interruptions in the transmission, it is linear and monotonic. I also measured a concurrent drop of about 1mV/s in the battery's output. When I bypass the battery with a bench power supply (set for 3V and 200mA maximum), the transmitter LED emission power remains steady.

Any ideas why the battery seems to be discharging faster than expected? Is this actually in line with the expected behavior? Were my expectations out of line? Is there any way to resolve this without a complete redesign?

I'm looking at other battery options, but the physical limitations for the height profile and weight don't leave much room for anything else.

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CR2032 cells are not designed to sustain currents like 15 or 20 mA. To obtain the capacity claimed by the battery, 225 mAh, you have to discharge it at the nominal discharge rate.

The Panasonic cell you linked has a broken datasheet link, but here is the Murata version:

Murata CR2032 cell datasheet excerpt

Note that the nominal discharge rate is just 0.2 mA or 200 µA. Most devices powered by a coin cell battery either pull very small currents or have a wake/sleep cycle implemented by a microcontroller or similar. (Meaning that they do something a few times per second, and 'sleep' the rest of the time, averaging extremely low current.)

The second page on the datasheet shows discharge rates for 300 and 200 µA (10 and 15 kΩ load, respectively). The current your circuit is using is 75 times as much. Due to cell internal resistance, the cell capacity is greatly reduced as current draw is increased. Instead of realizing the full 225 mAh capacity, you instead get orders of magnitude less.

Recommendations would be to:

  1. Implement a microcontroller that wakes up and drives the circuit at some acceptable interval. It may draw mere microamps while asleep then briefly consume 15-20 mA to "sample" things. Instead of powering the LED/sensor constantly while on, it instead would sample at some frequency that still detects whatever you're trying to detect. This is a more complicated approach but if cell size is critical in combination with extended run time, you need to find a way to reduce current consumption.

  2. Use a larger cell that has a higher sustained current rating. You will likely also find greater capacity ratings that you might not require. Consider a cylindrical CR-type Lithium cell, such as a CR123A or CR2 (camera "battery").

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What can you do?

  • Put a large capacitor or supercapactor across the cell to reduce repetitive peak loads (of course, the initial charging is a heavy load).
  • Use a few cells in parallel, both to reduce peak current drain on each cell, and to provide higher capacity.
  • Reduce current draw: use high-efficiency LED's and larger resistors.
  • Reduce the voltage requirement for the LED. Some IR LED's can operate at 1.2 V, red might need 1.8, and a blue LED might only work at 3.6 V with a brand-new cell.
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While rated in mAH, batteries end up giving less total charge at higher loads. Coin cells also have much higher ESR (equivalent series resistance) than normal cells, on the order of 20 Ohms, vs AAs, which are more like 0.5-1 Ohm. Simply put, they are not designed for sustained loads in the >10mA range for long periods of time. At a certain point, you're depleting electrical energy faster than the chemical reaction in the battery can supply it.

You could go for a small lithium ion battery, but note that this requires charging circuitry. You could also go for a pair of AAAs or even AAAAs, but it may not fit your application.

The upshot is, you're currently trying to take a battery optimized for long standby life and pulsed or very light loads and use it in an application that requires moderate sustained load.

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