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Advice on Powering a LED using a CR2032 battery

Cycloner

New Member
Hi, I'm new here. Sorry if this is the wrong forum.
I bought some new CR2032 batteries and tested them on my volt meter and I get 3.36 volts. Why is this? shouldn't it be 3volts?

Also I tested the volts on a socket and I get 124volts and not 120volts.

Is this normal. or is my volt meter not accurate?

if its normal. How do I power an led with this in mind and using the proper resistors.

Thanks in advance.
Chris
 

Diver300

Well-Known Member
Most Helpful Member
The voltage of a battery will reduce when current is taken from the battery, and will further reduce as the battery discharges, and many batteries will be well above their nominal voltage when new, with no current being taken.

There is quite good information here:- https://wisen.com.au/blog/powering-microcontrollers-by-battery/

To run an LED, it depends what colour the LED is, and how consistent you want the brightness to be. The colour effects the voltage. Red LEDs are around 2 V while blue or white ones are nearer 3 V. You can just put a resistor in series, but the current will reduce as the battery discharges, so the light will get dimmer, and most of the time you won't be getting full brightness. There are more complicated circuits that will run the LEDs at constant brightness, until the battery is completely discharged.
 

unclejed613

Well-Known Member
Most Helpful Member
Also I tested the volts on a socket and I get 124volts and not 120volts.
if you are in the USA, wall outlet voltage can be anywhere from 115 to 127 volts depending on location...

the best way to power an LED with such a small battery is to use a charge pump. back in the day i used to use LM3909 ICs for this, but that chip has been discontinued, but chips like this https://www.onsemi.com/products/power-management/led-drivers/dc-dc-led-drivers/ncp5603 are in current production... just do a search for "low voltage LED charge pump" to find others.... running the LED through a resistor will waste some energy as heat, and when the battery gets down to the LED's forward voltage the LED won't light... a charge pump will keep the LED lit for a lot lower battery voltages. white and blue LEDs have a forward voltage of about 3.6V so your 3V battery, if it lights them at all will not light them to full brightness. the charge pump will keep the same LEDs lit at near full brightness all the way down to about 1.25V battery voltage.
 

Cycloner

New Member
Yes I am quite aware of google... that is how I found this site.

Thanks for your input.

So it is normal for the voltage of a battery to deviate from it's specification when new?
I thought maybe my volt meter was not accurate?

Thanks.
 

gophert

Well-Known Member
Most Helpful Member
The little pin batteries have a lot of "internal resistance" Kind of like a resistor already in series with the battery).

Look up (LED "throwie"). Make Magazine was having fun making these, essentially a magnet, a battery that you have and an LED (5mm bullet). They run for 2 to 24 hours (very dim at the end). Have fun. The worst that can happen is you burn out a 5¢ LED.

People that know the right way to do things usually don't try the easy way to do things...
 
Last edited:

rjenkinsgb

Well-Known Member
Most Helpful Member
So it is normal for the voltage of a battery to deviate from it's specification when new?
The stated voltage is either the nominal initial voltage or midpoint between new/fully charged and fully discharged. The voltage changes with the charge state.

eg. A typical "1.5v" dry cell reduces steadily throughout its life and is down to near 1V before it is considered fully discharged. They can also read slightly higher than 1.5V before use.

A rechargeable Lithium cell, nominally 3.6V or 3.7V, is actually charged to 4.2V and discharged down to 3.3V or less.

And just to confuse things more, some power tool makers have started labelling their lithium batteries & tools with other voltages, typically 4V per cell, just to make them look "more powerful" but no actual changes to batteries, chargers or tools. eg. A three cell 10.8V battery is now labelled 12V!
 

Nigel Goodwin

Super Moderator
Most Helpful Member
The stated voltage is either the nominal initial voltage or midpoint between new/fully charged and fully discharged. The voltage changes with the charge state.

eg. A typical "1.5v" dry cell reduces steadily throughout its life and is down to near 1V before it is considered fully discharged. They can also read slightly higher than 1.5V before use.
When brand new they are usually about 1.65V - one that only reads 1.5V isn't new, and is fairly well used - over the decades I've tested thousands of batteries for customers :.

A rechargeable Lithium cell, nominally 3.6V or 3.7V, is actually charged to 4.2V and discharged down to 3.3V or less.

And just to confuse things more, some power tool makers have started labelling their lithium batteries & tools with other voltages, typically 4V per cell, just to make them look "more powerful" but no actual changes to batteries, chargers or tools. eg. A three cell 10.8V battery is now labelled 12V!
I always consider Li-Ion 4.2V per cell - as that's what you charge them with.

However, regardless of what voltage you choose to specify, it's unlikely to be that when tested, other than for a very short time.

Here's a charging curve, from one of our products (as part of the debugging and testing it squirts the data out of a serial port pin) - 16.8V constant voltage, and 510mA constant current, 4 cell pack:


Charging curve.png
 

audioguru

Well-Known Member
Most Helpful Member
A CR2032 is a "3V" disposable Lithium battery that is not rechargeable. Energizer makes disposable AA and AAA "1.5V" lithium cells.
Modern Lithium cells are rechargeable LiFeP04 types that are fully charged at 3.4V to 3.6V, not the 4.2V of ordinary older lithium rechargeable cells.
 

gophert

Well-Known Member
Most Helpful Member
A rechargeable Lithium cell, nominally 3.6V or 3.7V, is actually charged to 4.2V and discharged down to 3.3V or less.
No reason to talk about rechargeable lithium ion batteries when the user has a lithium metal primary (non-rechargeable cell).

The 2032 has internal resistance at 15-20 ohms at the recommended 0.5mA discharge rate. When you apply a 5 to 10 mA load, the internal resistance climbs to 75-150 ohms - perfect for 5mm LED.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
No reason to talk about rechargeable lithium ion batteries when the user has a lithium metal primary (non-rechargeable cell).

The 2032 has internal resistance at 15-20 ohms at the recommended 0.5mA discharge rate. When you apply a 5 to 10 mA load, the internal resistance climbs to 75-150 ohms - perfect for 5mm LED.
I completely agree, the limitations of the CR2032 mean it's prefect for short term LED driving, no other components required.
 

Cycloner

New Member
Thanks everyone for your input.

So if I understand correctly most batteries deviate from their specification? How can I test my volt meter then?

if you are in the USA, wall outlet voltage can be anywhere from 115 to 127 volts depending on location...

the best way to power an LED with such a small battery is to use a charge pump. back in the day i used to use LM3909 ICs for this, but that chip has been discontinued, but chips like this https://www.onsemi.com/products/power-management/led-drivers/dc-dc-led-drivers/ncp5603 are in current production... just do a search for "low voltage LED charge pump" to find others.... running the LED through a resistor will waste some energy as heat, and when the battery gets down to the LED's forward voltage the LED won't light... a charge pump will keep the LED lit for a lot lower battery voltages. white and blue LEDs have a forward voltage of about 3.6V so your 3V battery, if it lights them at all will not light them to full brightness. the charge pump will keep the same LEDs lit at near full brightness all the way down to about 1.25V battery voltage.
can't I use a voltage regulator?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Thanks everyone for your input.

So if I understand correctly most batteries deviate from their specification? How can I test my volt meter then?
You can pay to have it 'calibrated', but why the urge to test it?. However, batterie's don't 'deviate from their specification', that IS their specification - the 'normal' voltage given is just a nominal value, not a 'specification'.

can't I use a voltage regulator?
To do what?,not much point if just feeding an LED from a CR2032 - you're just wasting battery life.
 

unclejed613

Well-Known Member
Most Helpful Member
Thanks everyone for your input.

So if I understand correctly most batteries deviate from their specification? How can I test my volt meter then?



can't I use a voltage regulator?
you would need a voltage standard to check your meter properly. most modern digital meters are within about 2% or better accuracy, even the crappy ones. it's not like you are making mission critical measurements on a spacecraft (if you were, you would be using a completely different set of test equipment than your run-of-the-mill meter)...

again, a voltage regulator (assuming you meant an analog one) will waste power as heat, and a charge pump will waste a little bit, but not nearly as much, but also keep the LED on even if the battery voltage decays below the LED's forward voltage... however if you are just making a "throwie"... a CR2032 and an LED alone is cheap and easy...
 

Visitor

Well-Known Member
Have you put fresh batteries in the meter? I've had a few meters that start behaving screwy when the batteries get low, before any indication of low battery.
 

unclejed613

Well-Known Member
Most Helpful Member

charlesli

New Member
I purchased an LCD that uses a CR2032 battery, and I use a multimeter to measure the battery, basic in about 3.05 volts, perhaps your multimeter is not quite accurate?
 

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