9V battery weirdness in smoke detector - Page 1
Forum section: electronics / general

author: typoknig
author_rank: Regular Contributor
timestamp: September 06, 2019, 04:27:07 am
content: Hi all. I purchased the house I'm living in about 1.5 years ago. I have never changed any batteries in any of my smoke detectors (First Alert 9120B). Last night at 1:30 AM one of my smoke detectors started beeping once per minute. The manual indicates that means the battery is low or missing. I took out the battery and performed the tongue test and my highly calibrated taste organ indicated the battery was good. I got out my meter and tested the battery and to my surprise it tested at 9.502V. This seemed strange to me, I was expecting 9V or under since the battery is an alkaline Eveready Super Heavy Duty (markings "PD0117" and "ED0120" printed on the bottom) that is presumably several years old. Still a voltage of 9.502V on a 9V battery seemed acceptable so at that point I was suspecting that maybe the smoke detector had just gone bad. I blew out the smoke detector even though it wasn't dirty, held down the test button with the battery out to drain the caps, then put the battery back in and it still beeped once every minute. I then hooked up the smoke detector to my bench power supply and fed it exactly 9V and found the beeping stopped. I rummaged around and found a new 9V Duracell battery (which is much heaver than the Eveready) and it tested at 9.002V. I put that Duracell battery in the smoke detector and it seems to be working correctly again.So it seems weird that the smoke detector is that sensitive to voltage since a 9V rechargeable battery is often as high as 9.6V. But what is really perplexing to me is how can an alkaline battery increase in voltage over time? I measured the circuit voltage when the battery was attached to the smoke detector and it still read a solid 9.5V. So is that Eveready battery bad? What is happening here?

author: sleemanj
author_rank: Super Contributor
timestamp: September 06, 2019, 05:20:22 am
content: Check the target battery under load of a few mA, I expect you will find the voltage will collapse rapidly. Checking just the open circuit voltage of a battery is not a good way to judge it's health.Your multimeter is unlikely to be able to show you the voltage collapsing when in circuit from a periodic current draw that the smoke detector might be doing, a DSO would if it captures that moment.

author: fourfathom
author_rank: Super Contributor
timestamp: September 06, 2019, 06:37:35 am
content: Are you sure it wasn't corroded contacts that finally wiped themselves clean? Put the original battery back in and see what happens. Or perhaps the contact rivets on the original battery are corroded so the voltage drop only occurs under load?

author: capt bullshot
author_rank: Super Contributor
timestamp: September 06, 2019, 06:48:34 am
content: I've seen a 9V alkaline battery that had a perfect open circuit voltage, but sometimes the device (a garage opener remote control) didn't work. Replacing the battery solved the issue. Since I wanted to know what's going on I took the battery apart, and: There's six small alkaline cells inside (nothing new to me), connected by some spot welded strips. The cells started leaking and at least one of the spot welds lost its properties, giving a weak contact. Luckily, the leakage was still kept inside the outer battery casing.

author: BravoV
author_rank: Super Contributor
timestamp: September 06, 2019, 06:52:49 am
content: Not sure if its related, 9V does leak from the terminal, not from the body like cylinder cell does, its slowly venting out the nasty stuff and went unnoticed, details ->YARA

author: typoknig
author_rank: Regular Contributor
timestamp: September 06, 2019, 02:37:49 pm
content: Quote from: sleemanj on September 06, 2019, 05:20:22 amI used a 1K resistor to load the battery this morning and the circuit drew 7.7ma (steadily dropping) instead of the expected 9.0ma. I checked the voltage of the circuit and it was 7.7V (and dropping). So it seems you are correct, but I'm still confused how this battery can have a voltage higher than 9V.

author: typoknig
author_rank: Regular Contributor
timestamp: September 06, 2019, 02:38:35 pm
content: Quote from: fourfathom on September 06, 2019, 06:37:35 amThe battery is 100% clean and could pass for new. The contacts on the smoke detector are also clean.

author: magic
author_rank: Super Contributor
timestamp: September 06, 2019, 03:29:25 pm
content: "Super Heavy Duty" is zinc-carbon, not alkaline.That being said, these also should be roughly 1.5V per cellTear it down

author: ejeffrey
author_rank: Super Contributor
timestamp: September 06, 2019, 04:47:42 pm
content: Depending on the particular cell design, open circuit voltage for a new battery can be as high as 1.6 V/cell which would give you 9.6 V out of a "9V" battery. It is a little weird to me that the open circuit voltage is still that high on what is obviously a cell at the end of its life, but I am mostly used to alkaline batteries so the heavy duty zinc chloride might behave differently.

author: xmetal
author_rank: Contributor
timestamp: September 06, 2019, 09:58:02 pm
content: They always seem to die in the early hours. I'm sure it's due to the ambient temperature dropping which then causes the battery voltage to drop. I would use alkaline cells, not zinc-carbon.

author: amyk
author_rank: Super Contributor
timestamp: September 07, 2019, 01:31:28 am
content: As heavily discussed (and debated) in the Batteriser thread, a battery can be basically dead yet show perfect open-circuit voltage, since as long as there is any material left to react, there will be a potential formed, and the ultra-high internal resistance doesn't matter to a high-impedance meter.Quote from: typoknig on September 06, 2019, 04:27:07 am

author: magic
author_rank: Super Contributor
timestamp: September 07, 2019, 06:15:47 am
content: I think I have seen depleted zinc-carbon batteries which showed nominal open circuit voltage but immediately collapsed under load.Alkalines don't exhibit such behavior IME. For some reason their open circuit voltage falls permanently as they are used.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 07, 2019, 10:25:21 am
content: 9V is a nominal nameplate value (1.5V nominal per alkaline cell) - the open-circuit voltage of a full cell is actually a bit more, around 1.6V/cell. 1.5V is just kind of arbitrary, it's a nice number. In reality, it's a range: 1.6V for a full cell, and around 1.0V would be considered empty - below that, very little capacity remains.But, a typical wear mechanism for most battery types is the increasing internal resistance - so much, that this often dictates the end-of-life.Open-circuit voltageusuallydrops as well, but as you have found out, not necessarily. The increased internal resistance dominates in your case.The smoke detector does a load test every now and then (not too often to prevent draining batteries), to verify it can supply the required current at the required voltage to run the beeper in case of fire. It basically connects an extra load and measures the voltage under that load condition.

author: Psi
author_rank: Super Contributor
timestamp: September 07, 2019, 10:30:13 am
content: I had a smoke detector that would do this from time to time.Sound the low battery beep even when the battery was brand new.It seemed to do it after installing a new battery, but if you left it alone beeping it would eventually come right in a few hours.I did wonder if it was trying to do an initial calibration to get a baseline measurement and maybe this was failing due to being outside the valid baseline range.But it annoyed me one too many times so i just replaced the unit.I use those 10 year lithium ones now.They cost the same and it saves having to go around changing battery every year or two.