Refreshing stored batteries questions

777qqq

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Nov 29, 2008
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I have many C, D, AA, and AAA NiMH and NiCd batteries that I am refreshing after being in storage for several years. I know most knowledgeable people would say to just take them to a recycling center, but I want to see how much I can revive them in the name of science and to avoid waste.

I have a BC-700 charger that I am using for the AA and AAA, and a Radio Shack model 23-425 for the C and D. I am charging the AA and AAA at 700mA and discharging at 350mA. The Radio Shack charger defaults to 900mA charge rate for C and D and discharges at 400mA. I have been using the Refresh function on the BC-700 for the AA and AAA cells, and charging then discharging 3-4 times for the C and D cells. None of the batteries I have cycled so far have reached 80% of their labeled capacity. After cycling the cells, I am going to let them sit for a couple weeks and discard the ones that have dropped in capacity too much.

I have a GB Instruments model GBT-500A battery tester. It only displays a range from "Replace" to "Good" and does not display capacity, so how can I accurately determine which batteries have held their charge? And from what I understand it's important that batteries used together should be within 5% capacity of each other. When charged batteries are placed back into the BC-700 the mA reading defaults to zero.

After cycling the NiCd cells should I discharge completely the ones I plan on storing? NiMh cells should be stored at .9V or 1.0V for storage, so could I determine that the cells are at .9V or 1.0V with my battery tester? The BC-700 displays voltage, but is that reading under load or OC? Thanks for any replies.
 

moldyoldy

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Maybe Wisconsin, maybe near Nürnberg
I have many C, D, AA, and AAA NiMH and NiCd batteries that I am refreshing after being in storage for several years. I know most knowledgeable people would say to just take them to a recycling center, but I want to see how much I can revive them in the name of science and to avoid waste.

I have a BC-700 charger that I am using for the AA and AAA, and a Radio Shack model 23-425 for the C and D. I am charging the AA and AAA at 700mA and discharging at 350mA. The Radio Shack charger defaults to 900mA charge rate for C and D and discharges at 400mA. I have been using the Refresh function on the BC-700 for the AA and AAA cells, and charging then discharging 3-4 times for the C and D cells. None of the batteries I have cycled so far have reached 80% of their labeled capacity. After cycling the cells, I am going to let them sit for a couple weeks and discard the ones that have dropped in capacity too much.

I have a GB Instruments model GBT-500A battery tester. It only displays a range from "Replace" to "Good" and does not display capacity, so how can I accurately determine which batteries have held their charge? And from what I understand it's important that batteries used together should be within 5% capacity of each other. When charged batteries are placed back into the BC-700 the mA reading defaults to zero.

After cycling the NiCd cells should I discharge completely the ones I plan on storing? NiMh cells should be stored at .9V or 1.0V for storage, so could I determine that the cells are at .9V or 1.0V with my battery tester? The BC-700 displays voltage, but is that reading under load or OC? Thanks for any replies.

For any AAA/AA cells using the BC-700, be they NiCd or NiMH, to determine the capacity after any rest period, use the Refresh function, select your discharge rates, and watch for the first completed refresh capacity numbers. normally when starting the Refresh mode, the BC-700 will initiate a discharge, then when 0.9V is reached, a charge is started. Look at the capacity numbers at that point and decide. Use the standard 80% of minimum mah rating for recycling, or if you value the cell for some reason, you might drop down to 50%. However keep in mind that the capacity varies with discharge rate. Actual use in an electronic flash or higher-output flashlight may well be inadequate even though the capacity number at a relatively low discharge current seem OK.

I had that RadioShack charger, but it fried itself and the cells in it.

The old NiCD cells will probably self-discharge so fast that any question of storage voltage is meaningless.

The BC-700 displays the voltage under discharge or charge depending on the mode. Look at the display and decide which you want to follow. I would use the voltage during discharge.
 
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777qqq

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Thanks for the response. So I can determine the capacities and voltages of my AA and AAA cells with my BC-700, but to get the capacities and storing voltages for my C and D cells I would need a tester like the ZTS or a charger that displays capacity and voltage, correct? I cannot accurately determine where .9 or 1.0 volts is on my battery tester's displayed range for storing my C and D NiMH, correct?
 

SilverFox

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Hello 777qqq,

In order to determine the capacity of a battery pack or individual cell you need to expose it to a constant known load and measure the amount of time it takes for the voltage to drop to a minimum value.

The ZTS tester is great for checking cells, but it does not give accurate capacities. If you have several cells you can use the ZTS to group them by charge levels.

A work around for your C and D cells is to use Ohms law to determine a resistance that can simulate a known load. You apply a resistor across the cell and also have a voltmeter hooked up and use a timer to determine how long it takes to drop to a minimum voltage. This is a little labor intensive but it is inexpensive.

If you have funds available for test equipment the West Mountain Radio CBA will allow you to fully evaluate the condition and capacity of your cells under a wide variety of loading conditions.

Tom
 

Aahhyes68

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What is the "best" or most accurate rate in C, to discharge cells to try and check capacity ? I've read varying rates from .2C to 1C to the consumption rate of the device you'll be using it in.

I've harvested a bunch of laptop cells and separated the cells that wouldn't hold over 4v's. I thought I would test capacity on them and make a few notes for myself. Pair them up, etc..


What do you think , Tom ?

Thanks, Steve
 

Etsu

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What is the "best" or most accurate rate in C, to discharge cells to try and check capacity ? I've read varying rates from .2C to 1C to the consumption rate of the device you'll be using it in.

Obviously, "the consumption rate of the device you'll be using it in" is the best. However, since the device sometimes changes, consider whether you're using it in a high-drain device or low-drain device, then test capacity accordingly. Something like 1C for high drain, and 0.1C (or perhaps even less) for low drain.

The reason is because I've noticed that capacity very much depends on the drain, especially as NiMH cells get several years old. I have some 10 year old NiMH cells that still have most of their original capacity, but only when used in low-drain devices. Their internal resistance has made them unsuitable for anything else, and they die in minutes if I use them in anything demanding.

Whether this is also true for LiIon, I don't know. In my experience, LiIon suck after a few years, regardless of how well they've been treated, and no matter what kind of load you put on them.
 

SilverFox

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Hello Steve,

As Etsu pointed out the best indicator is using a discharge current that mirrors you use.

Most capacities are based upon a 5 hour discharge or a 0.2C discharge rate. This allows you to compare what you have now with what the label on the cell says it supposedly started with. The problem is that many cells are labeled "optimistically..." I find that if you take 90% of the labeled capacity you will come close to what the cell originally had.

I use the 0.2C discharge capacity to do an initial binning of cells. I recycle cells that fall below 80% of their initial capacity. After this first check I then move on to higher discharge rates to determine suitability for various applications. What I am looking for is the cells ability to hold voltage under higher loads as well as the change in capacity under higher loads.

Tom
 

Aahhyes68

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Oct 31, 2004
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Thanks to the both of you for answering my question. I've been reading and taking notes for weeks. I think it's finally all starting to come together for me.. I'm a little slow..lol..

Thank you, gentlemen. :)

Steve
 
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