battery power supply for prototype circuit

Thread Starter

Z'YonG

Joined Feb 2, 2017
63
Hi, I have designed and built a circuit, the original power supply that I used is 6V DC voltage from a bench top power supply. However, when I try to power up my circuit with two lithium ion button battery (CR2025, 3V each), my circuit cannot turned on. Once the batteries connected to the circuit, the voltage dropped from 5.98V to around 2 to 3V.
The resistance of my circuit is around 358 ohms.
I have connected my circuit to the bench top power supply again, and connected a 0.22 ohms resistor between the output of my circuit and ground, an oscilloscope is connected with both ends of the resistor to see the voltage when I turn on the circuit. There is a voltage spike when I just turned on the circuit.
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When the circuit just turned on, the voltage jumps to around 1V and then dropped rapidly to around 8mV. Based on my own calculation, when the circuit just turned on, the current is 4A and then dropped to about 0.036A, which matches the current output from the bench top power supply (0.02A to 0.04A).
However, I am not reapply familiar with power supply, Can anyone please explain to me that why my circuit can be power up by an 6V voltage from a bench top power supply but not a 6V from a battery? How can I make it to be able to supply by the battery?

Thanks a lot in advance!
 

#12

Joined Nov 30, 2010
18,210
Coin cells just don't have enough surface area to deliver more than a few milliamps. Another way to say it is, too much internal resistance.
 

wayneh

Joined Sep 9, 2010
16,390
How can I make it to be able to supply by the battery?
Without the schematic it's impossible to say, but there are "soft start" circuits that limit current in-rush, stage the turn-on of various ICs, pre-charge larger capacitors, and so on. The strategy you need depends on what your circuit looks like.

You can't squeeze 4A from a coin cell!
 

dendad

Joined Feb 20, 2016
3,519
You could try adding a large electrolytic capacitor directly across the coin cells so it gets a chance to charge up before you connect your circuit to it. The cap then supplies the start up pulse. But coin cells are a pretty poor power supply unless you are really pressed for space and your load has very small current requirements. I'd go with 4 AA cells myself. Coin cells also generally are pretty expensive.
40mA is a pretty large load for a coin cell.
 

crutschow

Joined Mar 14, 2008
24,947
According to the data sheet (it's amazing what you can learn from one), the CR2025 has internal impedance when fresh of 20Ω each, so two in series would have 40Ω, and that goes up as the cell discharges.
It's capacity is 163mAh so, even if you could get it to work, they would last, at most, about 4 hours.

So, as dendad said, you need larger batteries.
 

Thread Starter

Z'YonG

Joined Feb 2, 2017
63
Thank you for all your help. Now I understand what is going on.
I will try it with 4 AA batteries and see how it goes.
but just curious, if I want to use the button cell battery, what direction can I look into?
 

crutschow

Joined Mar 14, 2008
24,947
I mean what are the possible solutions for using button cell? what should I look into and how can I approach it?
I see two choices:
  • Reduce the circuit current draw to the point where the battery you have is adequate.
  • Look for a larger button cell with sufficient capacity to operate your circuit for the length of time you want.
 
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