9-volt NiMH Curiosity

[tbone_Ike]

As I have starting looking at 9-volt NiMH; why are the mAh capacities so much lower (vs their alkaline counterparts) in caparison to AA NiMH rechargeables?

Is this because consumer demand (or lack of) has not justified development technology costs?

I was going to buy some of these for our RC controllers, but after seeing the low capacities, don't think that's such a bright idea afterall.

Thanks to everyone who so patiently informs those of us who are new to much of this technology! /ubbthreads/images/graemlins/smile.gif
-jeff

 

[Lurveleven]

You cannot look at the mAh capacity alone, you have to see how many Wh it gives. A 250 mAh 9 V cell will give you 2.1 Wh while a 2500 mAh AA cell will will give you 3 Wh. These are the theoretical numbers, from SilverFox' charger comparision you will see that the actual performance will be lower. When comparing the Wh numbers the 9V cells are not so bad after all. The 9V cells are designed for low drain applications and are where they are used, so I don't see a problem with the low mAh number.

Also note that most 9V NiMH cells are actually 8.4V cells, however Maha have one that is 9.6V.

Sigbjoern

 

[MrAl]

Hi Jeff,

These cells are actually made up of several smaller cells
in order to get to the output voltage required (somewhat
less than 9v). Because the internal cells are smaller, they dont
have as much volume to contain materal that holds charge,
so they cant be rated as high as AA cells, which have
a much larger volume.

If you think the NiMH cells are bad, take a look at the
9v NiCd's. I actually wasted money on one a long time ago
and used it for about a total of two times! It was only
rated at 60mAh, which is basically about the same as
a lemon with two toothpicks stuck in it /ubbthreads/images/graemlins/smile.gif

Take care,
Al

 

[HarryN]

[ QUOTE ]
MrAl said: It was only
rated at 60mAh, which is basically about the same as
a lemon with two toothpicks stuck in it /ubbthreads/images/graemlins/smile.gif

Take care,
Al

[/ QUOTE ]

/ubbthreads/images/graemlins/crackup.gif /ubbthreads/images/graemlins/crackup.gif

 

[Lynx_Arc]

most 9v devices don't require high output batteries. You may find if you need a higher output 9v source wiring external pack of 4-6 AAA or AA may suffice as devices using 9v will probably run off of ~6v as that seems to be close to the cuttoff point under load on spec sheets I have read.

 

[tbone_Ike]

Thanks for the info, but I think I may have been unclear in my question. Although I now understand why there is less mAh in the 9 volt in comparison to the AA cells, there's something I still don't understand:

for example:

current energizer AA:
Alakaline=2850mAh vs NiMH=2500

current energizer 9-volt:
Alkaline=625 vs NiMH=150

Just wondering given the latest NiMH advances why a 500mAh or so 9-volt NiMH isn't typical. Maybe the Wh capicity is a factor and I'm still not getting it.




/ubbthreads/images/graemlins/confused.gif

 

[legtu]

[ QUOTE ]
tbone_Ike said:
Just wondering given the latest NiMH advances why a 500mAh or so 9-volt NiMH isn't typical.

[/ QUOTE ]

Voltage and size. Just like what MrAl said, it's composed of small-sized cells. Given the size of a 9v batt and the current NiMh technology, it's not possible *yet* to put in that much energy.

In order to get to 8.4v, you'll need 7 batts which means cramming in that many cells into a 9v size space. /ubbthreads/images/graemlins/smile.gif

 

[tbone_Ike]

Ok - thinks it's sinking in /ubbthreads/images/graemlins/smile.gif

thanks again for the explanation!
-jeff

 

[David_Campen]

[ QUOTE ]
Voltage and size. Just like what MrAl said, it's composed of small-sized cells. Given the size of a 9v batt and the current NiMh technology, it's not possible *yet* to put in that much energy.

In order to get to 8.4v, you'll need 7 batts which means cramming in that many cells into a 9v size space.

[/ QUOTE ]
You are missing the point of tbone_ike's observation. Just as it takes 6 1.2 volt NiMH cells to produce a (nominal) 9 volt NiMH battery it also takes 6 1.5 volt alkaline cells to produce a 9 volt alkaline battery yet a 9 volt alkaline has slightly more energy content than an AA alkaline battery while a "9" volt NiMH battery has only about 1/3 the energy content of an AA NiMH battery.

From Energizer data sheets:

The energy content of an AA alkaline is approximately
2.85 ampere-hour X 1.5 volt = 4.3 watt-hour

The energy content of a 9 volt alkaline is approximately
.63 ampere-hour x 9 volt = 5.7 watt-hour

The energy content of an AA NiMH battery is approximately
2.5 ampere-hour X 1.2 volt = 3.0 watt-hour

The energy content of a "9" volt NiMH battery is approximately
.15 ampere-hour X 7.2 volts = 1.1 watt-hour

http://data.energizer.com

 

[tbone_Ike]

David,

That's the question I was trying to get accross. In the same given space, with the same number of given cells, why do the 9-volt NiMH's have such a lower percentage of output compared to their alkaline counterparts when the NiMH AA cells have a much closer output in comparison to thier alkaline counterparts?

 

[evan9162]

The real answer is that no one cares about 9V rechargables. The real market is in AAs and AAAs. Spending a lot of engineering R&D to make a 9V rechargable with the same power density as a AA or AAA won't gain you anything, because the market is tiny. The market the last few years has driven NiMH AAs and AAAs up in capacity, mostly because of the explosion of digitial cameras.

It's the same reason that odd sized rechargables also don't have the same power density that AAs do - the only ones that are close are 4/3A NiMH.

For instance, a 2/3AA NiMH should theoretically have around 1400mAh of capacity, but the best ones have 650mAh.

C & D cell NiMH also suffer the same fate. With the same power density of current AA NiMH, C NiMH should have around 7Ah of capacity, and D NiMH should have around 16Ah of capacity, but they are at around 5Ah and 11Ah respectively.

 

[IsaacHayes]

I got a question: Say I had 2 of the 8.4v 9V NiMH, wired in series for 16.8, or even more of them. And I used these to trigger an ignition coil, say a small one from a motorcycle. How better would these perform vs 1 9v Alkaline? Since they might be able to give higher peak current, I might get a better spark for longer...

Any comments? BTW, I'm not sure on what my trigger will be, I might change it to a free-running relay, but before I've used a 555 timer tuned to a high frequency to make a constant spark. FYI I'm probably going to dump the output through a voltage multiplier, as using coils in reverse parallel has only worked well on the standard big oil filled v8 car style ones.

 

[wptski]

[ QUOTE ]
legtu said:
[ QUOTE ]
tbone_Ike said:
Just wondering given the latest NiMH advances why a 500mAh or so 9-volt NiMH isn't typical.

[/ QUOTE ]


Voltage and size. Just like what MrAl said, it's composed of small-sized cells. Given the size of a 9v batt and the current NiMh technology, it's not possible *yet* to put in that much energy.

In order to get to 8.4v, you'll need 7 batts which means cramming in that many cells into a 9v size space. /ubbthreads/images/graemlins/smile.gif

[/ QUOTE ]
Not sure about size! Look at a 1/5A Ni-MH at 400mAh. Maybe it's shape but I really don't know.

I had ten IPower 9V Ni_MH cells and was using a MAHA MH-C1090 charger. After a power failure, "I think" but not for sure that was the cause, darn near all failed.

Some 9V rechargables are a lot bigger than a standard 9V and won't fit in the battery holder, pocket, etc. I bought some MAHA 9V Ni-MH and they are smaller than the IPower which I won't buy again!!

 

[Doug Owen]

While it's true that there's little money being thrown into raising the NiMH capacity of nine volters, there's another factor in play nobody's mentioned.

NiMH cells have to made with strong cells to deal with potential pressure. This means thick steel walls. And you need way more walls for the seven or 8 (not six) cells in the nine volter than the single cell in the AA. And each of those cells needs a bulkhead, further eating up the available space. Note that the energy density of alkaline nine volters is low WRT AAs, and they don't need thick cell walls....

This is the very sort of thing that forces small mamals to consume a much larger percentage of their body weight in food each day than the larger ones. The ratio of surface area to volume...

Again, if it had an easy solution (or even a cost effective one) one of the makers would go there and force the others to do so or no longer be competitive. Right?

Doug Owen

 

[tbone_Ike]

Thanks.
I now understand the technology a bit better. I can see there's really not much of a market for these things as you have mentioned as well.
One market I could see them doing well in if they did have some more capacity (other than RC controllers) is in wireless microphone systems. We go through untold boxes of 9-volts between all of the wireless mics that get used around my church!! You never want to risk having one go dead during a service, so it's basically standard procedure to pop fresh ones in constantly in a church (or any live) environment.

thanks again for the insights!

 

[Lynx_Arc]

My guess is the circuitry in the mics are designed to run at nominal 6v levels that is why they use 9v batteries. You could try to use 4x AAA alkaline cells and see if that works but most likely they wont fit, it is a possibility it may not run long if it drops below perhaps 5v or so which basically cuts out use of 4xAAA nimh cells due to lower operating voltage. You could almost fit 3AAA cells in the place of a 9v but my guess is unless they are lithium they wont even get near the low end of operating voltage.

I recall having an odd flat 6v photo battery in an old zenith tv remote that would die about once a year and cost 4-6 bucks each. I got fed up with it and wired 3AAA in series and stuck them in there and they ran 18 months and cost me less than half the cost.

 

[SilverFox]

This is what one looks like when you remove the case.

Tom

 

[tbone_Ike]

An article I found on Shure's website (below) leads me to beleive a 8.4 Nominal 250mAh NiMH, for example the one by iPowerUS, might work well for us in a wireless mic when fully charged, but I'll need to check the spec's on our exact mic models. I would need to know that I can get a very solid 3 hours min of use before I would consider using them.
I need to do further research! This could be a great option using a handfull of rechargeables and keeping a multiple 9-volt charger at the soundboard booth.

SilverFox - thanks for the photo! This thread had me trying to imagine what the inside of the 9-volt configuration looked like & now I know /ubbthreads/images/graemlins/smile.gif

-jeff

Shure's battery article

 

[Lynx_Arc]

The best thing to do is test them. Get one and run it till it dies a few times. My guess is the 8.4v nimh would outrun an alkaline by about 50-100% mainly due to less voltage sag as it discharges. I am guessing mics require a certain minimum voltage to operate and when the alkaline cells drop below it under load they fizzle out.

 

[wptski]

[ QUOTE ]
SilverFox said:
This is what one looks like when you remove the case.

Tom

[/ QUOTE ]
Tom:

Is that one of my IPower cells that I sent you?