[threepointone]

which brand AAA NiMH batteries typically have the highest voltage? How long do they typically last at that voltage? Everything I've seen so far seems to be rated 1.2volts (unlike the 9V NiMHs, which seem to be rated anywhere from 8-9.6 volts), although I read somewhere most of them measure out at about 1.4volts.

 

[NeilR]

An AAA battery is one 1.2V cell.

A 9V may have 7 or 8 cells, each 1.2V nominal charge, which is why some are rated as 8.4V and some 9.6V. If you open up a 9V cell, you can see the tiny cells that make up the "pack". I had a pair melt down in my pocket once, believe it or not, but that is another story

I picked up some Sanyo 900mah AAA's for my PPA battery pack here.

That was the cheapest I found them. Actually another member found them and we split an order to save some $$$$. In looking at the link, I paid $1.50 each for 64; they are now charging $2 each. Price increase?

Regards,
Neil

 

[mono]

Quote:

Originally Posted by threepointone which brand AAA NiMH batteries typically have the highest voltage? How long do they typically last at that voltage? Everything I've seen so far seems to be rated 1.2volts (unlike the 9V NiMHs, which seem to be rated anywhere from 8-9.6 volts), although I read somewhere most of them measure out at about 1.4volts.

They all tend to have about the same voltage, rated for 1.2 but at full charge are roughly 1.45 per

How long they last, how high the voltage curve is as they drain, depends on the actual (vs rated) capacity of the particular cells. Things that can derate them are a very fast drain (moreso for alkalines and other high impedance cells than NiCad or NiMH in a headamp's low current use), or ambitious over-ratings of generic brands, or lifespan depreciation from wear in use, damage (relative) like overheating and worse, venting, or even old stock that sat on a shelf for years.

So 1.2V is a nice number to provide an approximation for some things. Most cells are rated for their capacity down to about 1.0V or lower (.9 or .8). Thus, the best bet for determining the # of cells you would need is to consider them as .9V per cell and how many ( X * 0.9) to stay at your minimum necessary voltage.

The discharge curve for NiMH is good, less linear than Alkaline and drops rather quick past around 0.9V, and it is the same thing with the so-called 9V NiMH cells, they too are just a bunch of tiny cells in series wrapped up in a metal sleeve or plastic tube, so they too exhibit the same kind of voltage drop curve. You may often hear of some speak as to the # of cells IN a 9V battery, as it does directly relate to their actual voltage.

Google Images can find several NiMH discharge curves.

I also like Sanyo AAA, and the Sanyo cell relabels that include Energizer, Panasonic, and others that I can't recall at the moment. Since some opamps might need at least 10V or so, 10 cells can often work. 11 is an odd # and 12 cells would fit 3, 4-cell battery holders. You'll have to try a given design to see how low the voltage can go before it effects sound. I find a variable power supply invaluable for that, just lower the voltage and listen. At a certain point the sound might be worse but not audibly so, which is a whole other can of worms. I tend to want at least a volt minimum, over the level where I can hear any difference but your philosophy may differ.

You can often find the Energizer or Panasonics (relabel above means they're the same thing for any generation of sanyo cell, unless you were seeking the less common sanyo industrial cells) in local stores, for about the same price or even cheaper on sale unless you were buying enough volume to offset the shipping cost.

 

[ATAT]

Another vote for Sanyo, 900mAh cells are what I use for my PPA, amazing compared to some generic Nimhs ive used.

 

[NeilR]

Quote:

Originally Posted by mono So 1.2V is a nice number to provide an approximation for some things. Most cells are rated for their capacity down to about 1.0V or lower (.9 or .8). Thus, the best bet for determining the # of cells you would need is to consider them as .9V per cell and how many ( X * 0.9) to stay at your minimum necessary voltage.

Hi mono,

I don't want to argue religion and philosophy but I think your 0.9V per cell approximation is a bit harsh. Typically these cells will stay at or above 1.2V for 95+% of their useful runtime. For a 10 hour runtime, the cells might only run for 10-15 minutes below 1.2V and that is clear on the discharge curves. Another way to look at it is that you would need 13-14 cells rather than 10 cells to maintain a 12V output in order to squeeze a few more minutes out of them.

Just want to put it into perspective since there is no "true number" here, just different ways to look at it.

Regards,
Neil

 

[mono]

Quote:

Originally Posted by NeilR Hi mono, I don't want to argue religion and philosophy but I think your 0.9V per cell approximation is a bit harsh. Typically these cells will stay at or above 1.2V for 95+% of their useful runtime. For a 10 hour runtime, the cells might only run for 10-15 minutes below 1.2V and that is clear on the discharge curves. Another way to look at it is that you would need 13-14 cells rather than 10 cells to maintain a 12V output in order to squeeze a few more minutes out of them. Just want to put it into perspective since there is no "true number" here, just different ways to look at it. Regards, Neil

Yes it is "harsh" if that's how you want to see it, but it does also give a bit more runtime. I think your 10 hr / 15 minute is off by some though, as the curve usually starts dropping faster around 1.05-1.1V, IIRC. For example if the pack is draining at 0.2C, at 1.2V it might have 20% capacity left which is 2 hours out of ten, not 15 minutes. Not all amps will drain it as high as 0.2C though, many portables probably wouldn't hit the (by today's technology as 900mAH cells) 180mA figure, but even so, dropping down to .05C/45mA probably won't reduce that 2 hours to 15 minutes. That is, unless your cells weren't working at full capacity anymore which brings us to my next paragraph.

What amp and what was it's supposed current usage? 900mAH / 10 hrs = 90mA Was your amp expected to draw that much or were the batteries producing less than 900mAH? If far below 900mAH, that couldl explain why they ran for a shorter period once (after) they hit 1.2V.

So long as the voltage aimed for isn't a bare minimum for proper function, I'd fudge a bit, not get hung up on exactly 12.0V or 15.0V, etc, rather than a grouping of cells that the battery holder and case can accomodate.

 

[NeilR]

mono- I was reluctant to put a number on that (15 minutes). That is just a WAG because I have never measured a battery at that point- the falloff happens too fast and I have never sat around for hours waiting for it to happen. All my numbers (discharge rate) were hypothetical to illustrate that 1.2V is a reasonable value as long as you understand that you lose some runtime. That may be preferable to changing to a bigger case, for example, to house batteries that are only going to extend the runtime for some relatively short period, whatever that number is.

This would be a good application for a high end DMM with data stoorage capability....

I have observed the falloff many times (using NICADS) in a flashlight. I used to do a lot of deep underwater wreck penetration diving. There is a rule there where you never use rechargeables in your lights. Although I followed that rule with my primary and backup lights, I used nicad C's in a modeling light strapped to my camera. That light was good for several hours, but would go from full illumination to dead within a minute or two at most. It was amazing (and scary under the circumstances) how fast it falls off the cliff and was the reason for that rule.

 

[mono]

Quote:

Originally Posted by NeilR That light was good for several hours, but would go from full illumination to dead within a minute or two at most. It was amazing (and scary under the circumstances) how fast it falls off the cliff and was the reason for that rule.

I have experienced a somewhat similar situation with LED flashlights. Many will use 3 x cells to power ultrabright LEDs, but I'd as soon suffer a little loss in a dropping resistor to use 4 x cells and have longer runtime once the cell voltage starts dropping at a high(er) rate. Going from barely perceptible difference in brightness (since it's a gradual decrease) to almost completely dark in a few minutes is not a situation I find desirable. I could put switching regulation to flashlights, but that takes up some space, and time, and wastes current too, so I just prefer using 4 cells on a ~ 3.4V forward series of LED.