Car key fob lithium coin battery check voltage before replacing it

[zillah]

Dear Experts

Sometime I worked with my friend in his car wrecking yard , and sometime we need to check car key fobs that are not working if thier batteries are dead or not ,,,using Fluke 117 multi-meter and KYORITSU

Most batteries are Lithium Coin Battery size 2023, 2016,2025 (3V)

Some remotes fluke shows voltage for their batteries are very low obviously need to be replaced once we replace them they would work no drama

And other remote fluke and KYORITSU show voltage for their batteries almost 3V or 3.06V, 3.16V and those remote are not working but when I replace their batteries they would work !!!!

I want to know why those remotes work for new batteries while they won't work with the old batteries although voltage are still around 3v or +3V ? Isn't measuring a voltage should be a reasonable evaluation to check a battery if it good or not ? ?

Thanks

[Jeroen3]

Measure the voltage, while it's transmitting.
Sometimes the batteries are depleted, but the unloaded voltage is high enough for the meter to show 3V.

Dave did a video on this.

[sokoloff]

You need to place a small load on the battery (larger than the infinitesimal load a voltmeter applies).

The battery’s float voltage could be high enough, but as soon as you draw off any current, it drops.

Cheap battery testers are readily available and easier to use (no skills and faster). I’d get your buddy one.

[zillah]

Measure the voltage, while it's transmitting.

Thanks for that tip
I am thinking how can I do that with a key fob

[zillah]

Cheap battery testers are readily available and easier to use

Would a tester put a load on a battery while it is testing it ?
Thx

[andrewjessop]

Would a tester put a load on a battery while it is testing it ?
Thx

That is the whole purpose of battery testers... to put a load on the battery while measuring the voltage.

[zillah]

That is the whole purpose of battery testers... to put a load on the battery while measuring the voltage.

Noted
Yes I would search for one on google
Are you aware of any decent one for lithium coin battery ?

[Fungus]

Isn't measuring a voltage should be a reasonable evaluation to check a battery if it good or not ? ?

No.

Short version: Imagine your battery has a resistor on the output and that resistor gets bigger and bigger as the battery is used up. It starts as a couple of Ohms and ends up as thousands of Ohms.

Ohm's law tells us that there's a voltage drop across that resistor when current is flowing. This is what appears to cause a voltage drop on your battery but it's an illusion. The battery voltage is the same internally, you just can't see it because the resistor is in the way.

But ... there's only a voltage drop when current is flowing. If you measure with a multimeter then there's almost no current flowing and you don't see reality, you see a much higher voltage then when the battery is in use.

Bottom line: You can't measure a battery without a load on it. Put a 1k resistor on the battery when you measure it, eg 1.5k.

[Fungus]

Are you aware of any decent one for lithium coin battery ?

For 3V batteries you can just use a LED, compare the LED brightness with a new battery. Simple, cheap. You can get a while bag full of 'testers' for under $1.

[MosherIV]

Some dmms have a low Z mode which adds a load resistor when measuring voltage.

The one I am thinking of is the Keysight U1232a
I think some other have something similar.

[sokoloff]

That is the whole purpose of battery testers... to put a load on the battery while measuring the voltage.

Are you aware of any decent one for lithium coin battery ?

Any of the cheap battery testers that look roughly like this are likely to be fine.

https://images-na.ssl-images-amazon.com/images/I/41gI3OXVecL.jpg

$6 on Amazon, $2 on Aliexpress.

[oldway]

For non-rechargeable batteries, I use a fairly radical process: I measure the short-circuit current.

With a cr2032, a new battery will provide an initial current of about 50mA.
This value will fall quickly.
Use the multimeter on the 400 mA scale.
If the current is zero or less than 1mA, the battery is out of order.

Be careful, never use this with rechargeable batteries !!!!

[Fungus]

For non-rechargeable batteries, I use a fairly radical process: I measure the short-circuit current.

That's basically what the LED method does, but much faster/easier than messing around with a multimeter and an LED fits in your pocket.

nb. LEDs are only suitable for 3V batteries that put out a few mA, eg. CR2032 coin cells.

[oldway]

I use this method (measuring short circuit current) with all kinds of non rechargeable batteries...but using the 10A range.
As well 1.4V, than 1.5V and 9V non rechargeable batteries....

[zillah]

Thanks guys for the inputs