Does NiMH self-discharge when in use?

[OceanView]

I'm wondering if NiMH batteries self-discharge even while in use, or is that a phenomenon of unused cells?

I have a portable weather radio that takes 2AA. Alkaline cells will run the radio for 10-14 days before the radio dies completely. NiMIH cells will only last 3-4. I have tried different sets of NiMH and they're about the same, so there's definitely a large disparity in performance here based on battery chemistry and I'm just curious what the reason might be.


I suppose that maybe the radio is tolerant of a steadily diminishing current from the alkalines, and can therefore last a couple weeks, while the NiMH produce a relatively flat output and expend themselves relatively quickly in comparison and then quickly "die". However, that seems contrary to my understanding of NiMH being better able to wring more energy out of each cell versus alkalines which sag quickly and become unusable even if they have remaining capacity.

Any ideas?

 

[LuxLuthor]

That doesn't make sense to me. What brand and mAh rating are the NiMH batteries, and how are you charging them? Are they sitting around before you start using them after charging? How old are they?

It may be that you have not properly conditioned the NiMH cells. There are ways to get them back into shape if not properly cared for.

 

[OceanView]

Currently, I have 2 pairs of 2300 mAh Energizer AA's. I'm just using the wall wart charger that they were sold with. Another brand I tried for a while were similarly disappointing. I think I used the same charger for those.

Since a pair only lasts a few days per charge in the radio, the freshly charged cells don't sit around much before it's time to swap them back into the radio.

Hopefully it's mostly just a matter of getting a decent charger, because I've always been hesitant to regularly use NiMH instead of alkalines based on my experience with this radio.

 

[defusion]

nimh's arent too good for low current applications (efficiency-wise, it wont hurt them)
use nicd's
same reason they use nicd in cordless phones

 

[clintb]

Something to consider when choosing batteries is not all devices have been made with rechargeable batteries in mind. My Garmin 60cs GPS has a setting for alkaline or NiMh. Think of where the two are starting, voltage wise.

 

[OceanView]

nimh's arent too good for low current applications (efficiency-wise, it wont hurt them)
use nicd's
same reason they use nicd in cordless phones

Hmm, that's an interesting angle. I knew that NiCd's are good for very fast charge/discharge, but didn't know they're better than NiMH for low current applications.

I just looked for AA NiCd's on the web very quickly. Unfortunately, they only range from 600-1200 mAh in capacity. I say this half jokingly, but maybe I'd end up still changing them every 3-4 days with modest capacities like that.

By the way, Amondotech had the highest cap I could find--1200 mAh. :rock:

Clintb, I'll see if I still have the instruction booklet and whether it says anything about input voltage. Thanks for the tip.

Good ideas, folks. Keep 'em coming if you have any more!

 

[Lobo]

Interesting and usefull thread.
A slight change of topic. But how does Eneloops fair in low current applications like this?

 

[VidPro]

yes they SEEM to continue to self discharge at the same rate they would have.
so if you put them in something that takes month, or months to depleat its total capacity, the self discharge hits sooner than the battery has been discharged by the device. EX: clocks and thermostats, that would run for about a year on a alkaline are a terrible choice for ni-mhy , discounting the new lower self discharge things.

if you wanted to find out what was actually occuring in this situation, you would do 2 things. test the ni-mhy for self discharge. test the radio at about 2.4V
with only 2 cells the thing might be highly voltage dependant, but low consumption. only way to tell is meter it.
a easy way to meter it, with JUST a voltmeter , would be to put the alkies in, probe the total voltage when the radio starts going out of tune. if the voltage it fails to operate is around 2.4-2.5v then the ni-mhy wont work.
or measure a set of alkalines after the radio stops working with them, do they still have more voltage than the ni-mhy flattens out at.

i assume the device is a very low draw, so low that the alkaline thrives. or that the ni-mhys are self discharging rapidly.

Other notes: if you had an adjustable volte power source, you could just put it in place of the batterys and find out quick its operational voltage limits.

 

[LuxLuthor]

His post says his alkaline last 10-14 days; whereas the NiMH only last 3-4 days...so this must be a continuously operating weather radio. I don't see it being a situation that would deplete the NiMH according to its known discharge over time issue.

In most reasonably high output demand applications, NiMH should last longer than Alkaline. This is likely a low amp demanding radio, but I'm betting his NiMH are not charged properly, have reached the end of their chargeable life, or were not good quality to start with. They should not be self-discharging in 3-4 days. More like many weeks to months.

 

[elgarak]

A radio is typically not a high amp demanding application, thus not very suited for rechargeables.

It appears to me that the radio requires a certain minimal voltage to run. NiMH, with their slightly lower voltage compared to alkalines, simply drop to fast below the threshold voltage necessary to run the radio. That also means that they should have quite a charge left. Do you have a tester available, or something like the BC-900 charger with which you can test the remaining charge? That would confirm or deny my theory.

 

[abvidledUK]

I have a radio with LCD clock & frequency display, it drains batteries in two weeks, even if not used.

Just use it on mains now.

 

[cerbie]

nimh's arent too good for low current applications (efficiency-wise, it wont hurt them)
use nicd's
same reason they use nicd in cordless phones

I think they use them on some cordless phones because they're cheap. I haven't seen one using NiCad in over 5 years that wasn't a bargain basement phone. Edit: now that I think about it, make that near 8, as the first using them I got (Radio Shack--I think they were really Unidens) were from 1999.

As to the OP, that seems about right for a device that doesn't pull too much current. Many devices that have no method of regulating the voltage may be designed around Alkaline batteries, counting on their voltage sagging a bit under constant load, but lasting basically forever with those lower voltages.

For a torch analogy, take a cheapie light (right before you get the to cashier--one of those), and run it on NIMHs. Let's say you get 4 hours. It was bright for like 3 1/2 of those hours, and pulling a lot of power (Watts, not Amperes!), but only lasted four hours. With the Alkies back in, it will dim below the NIMHs in minutes...but it may very well last a day on those cells. This sort of thing is likely what's going on with your radio. In the case of NIMH, you can basically figure an amphour is an amphour, within very small tolerances. With Alkaline, it's not so simple.

 

[LuxLuthor]

Well the main point is this is not due to the self-discharge issue with NiMH...not in 4 days anyway...which was his question.

 

[TorchBoy]

same reason they use nicd in cordless phones

NiCd can also cope with constant trickle charging and more abuse. That's also why they're used in solar garden lights instead of NiMH.

A slight change of topic. But how does Eneloops fair in low current applications like this?

I have a Psion 5MX for which I keep a record of the battery runtime capacity. The best results I've got was 9h 10m, 540mAh running on GP Digi1 NiZn AA cells over about two and a half months. The single run I've had with Eneloops gave 5h 5m, 357mAh over a tad more than two months. The only results I have for alkaline are 5h 15m, 271mAh over about two and a half months.

I have no idea how much capacity is being basically evaporated by the device draw when I'm not using it (and thus isn't measured), and I've wondered about self discharge too. The NiMH cells in my shaver certainly seem to last ages - GP 1800mAh - makes me wonder if self discharge is low in <=2000mAh cells generally.

For a torch analogy, take a cheapie light (right before you get the to cashier--one of those), and run it on NIMHs. Let's say you get 4 hours. It was bright for like 3 1/2 of those hours, and pulling a lot of power (Watts, not Amperes!), but only lasted four hours. With the Alkies back in, it will dim below the NIMHs in minutes...but it may very well last a day on those cells. This sort of thing is likely what's going on with your radio. In the case of NIMH, you can basically figure an amphour is an amphour, within very small tolerances. With Alkaline, it's not so simple.

Nice explanation cerbie, you've probably got it there, although I spent several seconds figuring out what an "amphour" is. ("Does he mean ampere or amphora?") Amp-hour or amp hour I understand. Anyway, I wonder if the radio gets hotter because of it when using NiMH cells.

 

[cerbie]

The remaining battery charge, as per elgarak's hypothesis (or mine, too--it would disprove one of them), could also be tested with a light that's regulated. A casual runtime test would be accurate enough for this--if it gets more than 10% or so the run time of a fresh charge of the cells, the radio is requiring too high of a minimum voltage. Maybe 10% is even too high...

 

[OceanView]

Wow, woke up today to find a lot of interesting responses! That's the great thing about CPF.

Sounds like simple self-discharge isn't likely to explain the disparity when used for just a few days. I just threw that question out since it was the only possible reason that I could think of. I see now that there are other factors that could be in play.

The thought just occurred to me--my weather radio does have a recharging jack, although I never used it because the radio did not come with an AC cord. But, I guess that means that the radio was designed for some sort of rechargeable battery. Maybe for NiCd. (I remember from the manual that it's a recharging-only jack. The radio can't be powered through the AC cord.) Although the radio itself isn't terribly old (just a few years), I think this model has been around for a long time, so it was probably designed when NiCd were the dominant rechargeable chemistry.

I would test out these other hypotheses that people have put forward, but I don't have a voltmeter or a good charger that could tell me about capacity. I would try Cerbie's casual runtime test...but I don't have a regulated 2AA light to try.

Based on Lux's comment, plus other threads I've read recently, it's probably time to get a good charger. And based on Torchboy's experience (those GP's were NiCd, I assume, that got 9 hours), I should try some NiCd AA's in this radio and see if the performance improves.

Another side note, but I also got to thinking about all these different hand-cranked/solar-charged radios, like the Grundig FR series or Freeplay models. They all come with NiMH battery packs. Based on what people have said in this thread, I wonder if that's the best choice for a radio. Well, I wouldn't be surprised if the disposability issue between NiCd and NiMH is why NiMH are the batteries of choice in consumer products, rather than based on performance alone. For example, can NiCd-containing consumer products be sold in the EU? I get the feeling that they're banned.

Thanks, everyone! Great input! :rock:

 

[MrAl]

Hi there,

There are other technical aspects of NiMH cells that could come into play here
besides the ampere hour rating. Namely, the nominal cell voltage. The cell
voltage of a NiMH cell is 1.2 volts while that of an Alkaline is 1.5 volts. This
difference in terminal voltage is probably what is causing the 'problem'.
Yeah, the NiMH can put out current longer than the alkalines (or at least for the
same time) but they will do so with a lower terminal voltage.
I think VidPro was talking about this same thing.
As the voltage of the alkaline decreases while the device is running it doesnt
decrease as fast as the NiMH cell does so the radio runs longer. Even though
the NiMH cell has lots of energy left in it, it cant run the radio because the
radio can not run on that low of a voltage.
One way around this, if possible you can build a battery pack with one more
cell in it (3 cells instead of 2) and put a silicon diode in series with this pack.
This will provide a higher voltage for a longer amount of run time so the radio
runs longer before the cells die down.
The nominal voltage of two alkalines starts out at 3.2 volts, while the nominal
voltage of two NiMH cells starts out around 2.6 volts or so. By connecting three
NiMH cells in series the nominal voltage will be about 3.9 volts, and the diode will
subtract about 0.7 volts, leaving the radio to run on a nominal 3.2 volts again.
This should be able to run the radio for a longer time than the two alkalines.

If you try it post results here if you can...thanks.

Of course you can also use a boost circuit to boost the output of
two NiMH cells to 3 or so volts, regulated, so the radio can use more of the
stored energy in the NiMH cells.

 

[defusion]

think that might be a bit overkill
and if you are thinking about it, consider 2x14500 li-ion batteries aswell, run then in parallel, and use a resistor to bring down the voltage.

 

[Tritium]

Lots of electronic devices do a voltage check of the cells. If the device was built for alkalines then the Microprocessor in the device will possibly see the NiMh as discharged batteries. My digital camera will not work with some NiMh but it will work with others. The only difference is the peak voltage of the cell fresh off the charger. My NiMh that come off at 1.4vdc work while those that come off at 1.2 will not.


Thurmond

 

[MrAl]

think that might be a bit overkill
and if you are thinking about it, consider 2x14500 li-ion batteries aswell, run then in parallel, and use a resistor to bring down the voltage.

Hi,

Ha ha, well, that all depends what you are into...for me, all in a days
work.
I like the Li-ion idea too though, if he's up to using those cells.