Car key fob lithium coin battery check voltage before replacing it - Page 1
Forum section: electronics / testgear

author: zillah
author_rank: Contributor
timestamp: May 07, 2018, 11:16:10 am
content: Dear ExpertsSometime 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 KYORITSUMost 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 dramaAnd 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

author: Jeroen3
author_rank: Super Contributor
timestamp: May 07, 2018, 11:21:39 am
content: 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.

author: sokoloff
author_rank: Super Contributor
timestamp: May 07, 2018, 11:22:23 am
content: 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.

author: zillah
author_rank: Contributor
timestamp: May 07, 2018, 11:25:10 am
content: QuoteThanks for that tipI am thinking how can I do that with a key fob

author: zillah
author_rank: Contributor
timestamp: May 07, 2018, 11:25:56 am
content: QuoteWould a tester put a load on a battery while it is testing it ?Thx

author: andrewjessop
author_rank: Newbie
timestamp: May 07, 2018, 11:29:11 am
content: Quote from: zillah on May 07, 2018, 11:25:56 amThat is the whole purpose of battery testers... to put a load on the battery while measuring the voltage.

author: zillah
author_rank: Contributor
timestamp: May 07, 2018, 11:31:20 am
content: QuoteNotedYes I would search for one on googleAre you aware of any decent one for lithium coin battery ?

author: Fungus
author_rank: Super Contributor
timestamp: May 07, 2018, 11:37:52 am
content: Quote from: zillah on May 07, 2018, 11:16:10 amNo.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 whatappearsto 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'sonlya voltage dropwhen current is flowing. If you measure with a multimeter then there's almost no current flowing and you don't see reality, you see amuchhigher 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.

author: Fungus
author_rank: Super Contributor
timestamp: May 07, 2018, 11:39:08 am
content: Quote from: zillah on May 07, 2018, 11:31:20 amFor 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.

author: MosherIV
author_rank: Super Contributor
timestamp: May 07, 2018, 11:48:55 am
content: Some dmms have a low Z mode which adds a load resistor when measuring voltage.The one I am thinking of is the Keysight U1232aI think some other have something similar.

author: sokoloff
author_rank: Super Contributor
timestamp: May 07, 2018, 11:54:16 am
content: Quote from: zillah on May 07, 2018, 11:31:20 amAny 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 onAliexpress.

author: oldway
author_rank: Super Contributor
timestamp: May 07, 2018, 11:56:18 am
content: Fornon-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 !!!!

author: Fungus
author_rank: Super Contributor
timestamp: May 07, 2018, 12:16:20 pm
content: Quote from: oldway on May 07, 2018, 11:56:18 amThat'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.

author: oldway
author_rank: Super Contributor
timestamp: May 07, 2018, 01:00:18 pm
content: 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....

author: zillah
author_rank: Contributor
timestamp: May 07, 2018, 08:21:09 pm
content: Thanks guys for the inputs