9v ni-mh 7.2, 8.4, 9.6 ?

[Brighteyez]

Sorry if I walked into the middle of your 'pissing' contest. I do understand from your postings that you seem to have some sort emotional interest in this.

But with regards to the original poster, the rechargeables that are commonly sold on the retail market are 7.2V. 8.4 is most likely okay, though 9.6 may be pushing it depending upon the sensitivity of the equipment, since 9.6 may actually be closer to about 10.4 when there is little to no load on it (like from a smoke detector.)

Please feel free to continue with your arguments, my apologies if I interupted.

Your just rehashing everything that's been posted already, most of which has nothing to do with the original poster's question which is, what voltage of rechargable to use.

 

[wptski]

Sorry if I walked into the middle of your 'pissing' contest. I do understand from your postings that you seem to have some sort emotional interest in this.

But with regards to the original poster, the rechargeables that are commonly sold on the retail market are 7.2V. 8.4 is most likely okay, though 9.6 may be pushing it depending upon the sensitivity of the equipment, since 9.6 may actually be closer to about 10.4 when there is little to no load on it (like from a smoke detector.)

Please feel free to continue with your arguments, my apologies if I interupted.

It's not a "pissing contest" at all but has drifted off from the topic of what to use to wether to use! Emotional interest? Nada! All the OT statements above about having to recharge every two weeks because of the self discharging of Ni-MH cells didn't prove out here.

If you don't use them, don't buy just for your smoke alarm, if you use them elsewhere, plug one in and try it. Your runtime may vary!

 

[Brighteyez]

I'm not sure I understand what you're trying to say there.

But I do use rechargeable batteries. And like others here, I probably also use too many of them in applications where they're really not warranted.

Devices that I have gone back to alkaline batteries on are:
smoke detectors, remote controls, wall clocks, and anything else where a device is active and an alkaline battery is going to last 6 months or more. And now the most recent return to alkaline batteries is going to be D cell LED flashlights, like the Mag LED (they're more of a backup light for be to begin with.) Additionally, may even consider that route with the C cell Luxeon lights like the LEDBeam, were it not for the fact that I already have a bunch of C sized NiMHs.

There is no way that I would even consider using a rechargeable in either a smoke detector or a garage remote.

I think a good rule of thumb for a rechargeable battery usage application is if you're not re-charging it once at least once a month, there's probably no benefit in using rechargeables. Pagers, may be an exception here, in my case they ran longer on a NiMH battery (about 5 weeks) than the 3 weeks that it runs on an alkaline. But I took care of that by getting rid of the pager. The thought of remembering to recharge a battery at intervals of 6 or more months just sounds impractical for all but those who might be obsessed.

If you don't use them, don't buy just for your smoke alarm, if you use them elsewhere, plug one in and try it. Your runtime may vary!

 

[wptski]

I'm not sure I understand what you're trying to say there.

But I do use rechargeable batteries. And like others here, I probably also use too many of them in applications where they're really not warranted.

Devices that I have gone back to alkaline batteries on are:
smoke detectors, remote controls, wall clocks, and anything else where a device is active and an alkaline battery is going to last 6 months or more. And now the most recent return to alkaline batteries is going to be D cell LED flashlights, like the Mag LED (they're more of a backup light for be to begin with.) Additionally, may even consider that route with the C cell Luxeon lights like the LEDBeam, were it not for the fact that I already have a bunch of C sized NiMHs.

There is no way that I would even consider using a rechargeable in either a smoke detector or a garage remote.

I think a good rule of thumb for a rechargeable battery usage application is if you're not re-charging it once at least once a month, there's probably no benefit in using rechargeables. Pagers, may be an exception here, in my case they ran longer on a NiMH battery (about 5 weeks) than the 3 weeks that it runs on an alkaline. But I took care of that by getting rid of the pager. The thought of remembering to recharge a battery at intervals of 6 or more months just sounds impractical for all but those who might be obsessed.

Your missing the point. It's a possible addtional application of a 9.6V Ni-MH battery. A alkaline "may" last longer. What's the capacity of a alkaline 9V battery? There may not be any benefit at all just another application. It's all up to the user!

I'm thinking that for some strange reason the self discharging that is seen in moderate capacity AA cell doesn't apply to a 220mAh cell which makes up this battery.

 

[bob_ninja]

I'm not sure I understand what you're trying to say there.

But I do use rechargeable batteries. And like others here, I probably also use too many of them in applications where they're really not warranted.

Devices that I have gone back to alkaline batteries on are:
smoke detectors, remote controls, wall clocks, and anything else where a device is active and an alkaline battery is going to last 6 months or more. And now the most recent return to alkaline batteries is going to be D cell LED flashlights, like the Mag LED (they're more of a backup light for be to begin with.) Additionally, may even consider that route with the C cell Luxeon lights like the LEDBeam, were it not for the fact that I already have a bunch of C sized NiMHs.
....

That is a fair comment. I keep switching between NiMH and Alkaline for remotes. However, smoke detector and clocks do draw power all the time so I use NiMH for them. Remote which uses tiny bit of power few times per day probably derves Alkaline.

The 2 "camps" are at odds regarding NiMH "self discharge" rate. Handlobraesing cites Twicell doc's 40% charge loss in 2 months. The common wisdom seems to be that NiMH have to be recharged at least every 3 months. In the other camp Bill offers his experiment in which 9V NiMH battery is still working fine after 8 months.

I cannot tell either way, don't know. Which is why I am interesting in Bill's and doing my own experiment. I have a feeling that the self discharge rate for NiMH is in fact very different for different types and brands. Just a guess.

In this particular case, I am using a much higher capacity pack to compensate for self discharge and match longevity of 9V alkaline or better it. Assuming average capacity of 1500mAh for my older AAs, assuming the above mentioned self discharge rate after 2 months they will have 1500 x 0.6 (60%) = 900 mAh (excluding detector power draw)
That is still well above my 9V NiMH's 150 mAh and I am guessing still better than most 9V alkalines. Therefore even if their self discharge rate is very high they should still last a long time, many months.

Regarding safety issues, should I go away for 2-3 months of course I would recharge them before the trip.

Regarding cost, in my case it is effectively zero as I am using older AAs that are no longer suitable for most other applications and would have been discarded.

So it comes back to the self discharge rate. I don't think I lost any batteries due to it. The only exception is a NiCd battery pack for a drill that I left in the garage for entire winter (very cold here around zero F) and thereafter didn't hold charge well. I even dug up C and D NiCds which I didn't use or charge for about 3-4 years, recharged them and they still work fine!?!? Strange.

So to claim that NiMH batteries will self discharge 100% in only 3 months regardless of type or brand is a strech. I admit that some high capacity types may have a bigger problem, but others may not.

 

[Brighteyez]

I'm afraid I may have conveyed the wrong thought. First of all, a NiMH battery will not fully discharge in 3 months. I've pulled some packaged NiMH batteries that we had sitting in a warehouse for almost 5 years to check, just for grins (the rest were being sent off for recycling/disposal.) While about 33% of them were totally shot, I was surprised that the rest were able to accept a charge. As they were 1600 and 1800 mAh batteries, and I didn't didn't have any need to use these batteries for anything, I just left them laying around for use around the office.

My decision to use revert back to alkaline batteries for low draw items like clocks, smoke detectors, remotes, etc. is purely one of convenience and overall economics. After all, if you change a battery in a smoke detector once a year, it's going to take a lot of those 20¢ AA batteries from Costco to pay for the cost of a NiMH battery that you might have to charge even twice a year. And of course, as we all know, and as some corollary of Murphy's Law dictates, when that NiMH battery decides to give up the ghost, you will not have a fully charged replacement standing by and will need to spend the time to charge one up. And even at an average of 40-45¢ for a brand-name AA/AAA alkaline, it's going to take a good 5 years or so before you can recover the 2 bucks that you plopped down for the NiMH battery.

While the 9V batteries cost quite a bit more, it's still going to take quite a few of them before you recover the cost of an equivilent NiMH battery. The good news here is that the 9V NiMH batteries actually have a bit more capacity than the alkalines, and the level of power that is drawn by a smoke detector is so negligible that it may actually be less than the self-discharge rate.

So to claim that NiMH batteries will self discharge 100% in only 3 months regardless of type or brand is a strech. I admit that some high capacity types may have a bigger problem, but others may not.

 

[wptski]

I have three detector. One upstairs, two in my basement. The one by the basement stairs had the PowerEx in it all that time. One close to my furnace, I had problems with it always chirping. Not sure if it was eating batteries or not, so it was batteryless! I plugged in another PowerEx to try out. The upstairs one has a alkaline in it.

 

[john2551]

Handlobraesing,
I cannot find any info on "self discharge" rate for NiCd and NiMH, or even within the same (NiMH) chemistry for AAs from different brands. Another interesting question is if a steady low power draw (such as from smoke detector) affects self discharge rate either way.
Being in the dark, I may try NiCds next time to see how long they last. For now I will monitor older NiMH and see how they do.
Given the benign environment (inside house, steady temps) I should expect minimum 4 month run, which would require only 3 charges per year. That is much better than my NiMH 9V battery which lasted about single month, or 12 charges per year.

Clearly Bill's 9V Maha is superior to the one I tried (forget the brand, maybe Energizer) and it holds its charge very well. Good stuff It is certainly an excellent choice for detectors. I just didn't want to spend 10 bucks and wanted a useful application for older AAs, retirement duty of sorts.

The "self discharge" issue may be a concern. In fact, another problem could be that one of the AAs goes below 0.9V while detector works just fine due to its low power draw. I'll check 'em out in the autumn (they are working for about 1.5 months now).

Bob,

It probably was an Energizer because they are only 7.2v.

John

 

[InfidelCastro]

For that application, I would go with 'heavy duty' carbon zinc batteries.

Second choice would be alkalines.

 

[bob_ninja]

John,
"Xtech" 8.4V 150 mAh
I think I got it in Radio Shack, now The Source
Well I got what I paid for
Anyway I got tired of trying to find a decent rechargable 9V, hence the AA pack using the holder from Radio Shack/The Source.

 

[wptski]

My smoke detector finally started chirping with the PowerEx Ni-MH 9.6V cell at 7.67V. Install sometime prior to 11/05.

 

[abvidledUK]

One of my smoke detectors just chirped last week.

Duracell 9v coppertop, three years use.

5.34v !!

Replaced.

BTW, it then ran in a SafeLight Companion for over 26 hours.....

Not the brightest, but still useable output.

https://www.candlepowerforums.com/threads/132314

 

[bob_ninja]

My 8xAA NiMH pack reads 9.82V
Charged back in the spring, so about 4-5 months.
It uses older AAs that I would have thrown away.

 

[moldyoldy]

Obviously the posters in this thread have some strong opinions on use of rechargeables in low drain applications. Based on my attempts some years ago with Ni-Mh 9v cells in smoke detector-type applications, I can understand the bias against rechargeable cells. However recent production rechargeable cells seem to be quite effective in my usage, which is fairly broad.

Just because an alkaline or lithium cell is deployed in these "standby" applications does _not_ equate to "worry-free" install-and-forget-it usage. I have had alkalines and lithiums fail in a couple months from purchase in such usage, and the cells were purchased from Sams or Walmart, so I have reason to believe that the cells were not shelf-aged.

As for 9V usage, like Bill, I do use the PowerEx 9.6VDC in smoke detectors w/o any problem. I haven't reached a year yet, so I cannot claim success in challenging throwaway alkaline cells which I view as environmentally challenged! Even if they are $0.10 each, throwaway batteries are still a landfill problem.

One usage I have to check the veracity and effectiveness of rechargeable 9V cells is my CO2 alarms. In one case, the Nighthawk CO2 alarm is normally on AC, so the battery does nothing. Then during a power failure, the battery takes over and runs the LED display. I started using Powerex 9.6v rechargeable cells here because a power failure usually meant a discharged 9v alkaline cell. ugh. more landfill issues. Since I switched over, the Powerex 9.6v cells have always been ready and I hear the beeping when I arrive home.

IOW, set aside past experiences if they were more than a year ago. Later rechargeable batteries have been pretty good. Of course, many threads point to production issues as to why both rechargeable and non-rechargeable cells/batteries fail early.

Tim

Tim

 

[Curious_character]

There's a second reason besides self-discharge you shouldn't use a rechargeable battery in a smoke alarm. They're designed to "chirp" for some minimum length of time when the battery gets low. The voltage of NiMH cells drops much more rapidly at the end of life than alkalines, so the "chirp" mode might not last long enough for you to notice it.

You're literally playing with fire when you use any battery in a smoke detector other than the one it's designed to use.

c_c

 

[wptski]

There's a second reason besides self-discharge you shouldn't use a rechargeable battery in a smoke alarm. They're designed to "chirp" for some minimum length of time when the battery gets low. The voltage of NiMH cells drops much more rapidly at the end of life than alkalines, so the "chirp" mode might not last long enough for you to notice it.

You're literally playing with fire when you use any battery in a smoke detector other than the one it's designed to use.

c_c

Depending on how much time your smoke detector without you around governs the frequency on doing function tests which may be daily since nobody knows how long they will chirp which may also different for every unit.

 

[Handlobraesing]

If you really must use NiMH to feel good for yourself, you can start replacing the alarm clock backup batteries with NiMH.

 

[wptski]

If you really must use NiMH to feel good for yourself, you can start replacing the alarm clock backup batteries with NiMH.

Some are still missing the point of this thread. It's like using Ni-MH in a flashlight instead of alkaline which some still perfer, it's your choice. If your stilll worried are self discharge which doesn't seam to be a major problem with these MAHA PowerEx batteries, don't use them.