Topping off a self discharged NiMH

Any reports on if topping off a cell that has been partially self discharged is hard on them? I know NiMH are not supposed to develop memory, but is it better to fully discharge and recharge them in this situation? Just wondering because I still have quite a few regular NiMH and have not fully changed over to the low discharge ones yet.


Thanks

sixfellas said:

Any reports on if topping off a cell that has been partially self discharged is hard on them? I know NiMH are not supposed to develop memory, but is it better to fully discharge and recharge them in this situation? Just wondering because I still have quite a few regular NiMH and have not fully changed over to the low discharge ones yet.


Thanks

Click to expand...

If a cell is almost full, "some" chargers may have a problem terminating properly and you may overcharge the cell. Many chargers include a warning about inserting fully charged cells.

A smart charger should help you avoid any overcharging issues. My choice is the Maha MH-C401FS. It is an independent four channel charger, and is microprocessor controlled to avoid overcharging even when topping off batteries. It all the years I have used it, it has NEVER overcharged a battery.

the section of chemicals that self-discharged, would not be "cycled" without the self discharge, until you charged it up totally, then discharged and recharged. because the part of the chemicals that self discharged, is the part that will not get a full "cycle" until you recharge and discharge that section, without the self discharge.

So the simple "discharge before charge" will not have discharged the section that SELF discharged. (the part most in need of a cycle)

somone was testing if the self discharge from the high-caps was "fixed" by cycling them.
that information would show if the self discharge itself additionally adds to self discharge issues.

i have found that self discharge is not a "cycle" so to speak.
that if a cell totally self discharges, then you recharge it, it does little to improve its "performance", VRSES discharging it in a device then recharging it.
(use it or lose it)

anyways, as you can see my talking about it is just a mess.

if you have one of them high end chargers, and wanted to "cycle" the SELF discharged section, something like the "TEST" mode of the lacross would do that, because it would:
Charge the chemicals that had self discharged
Discharge the WHOLE thing, including the section that had self discharged.
Re-charge the whole thing

when they come back with that data about the self discharge , it would kinda indicate how important it would be to do that.

Your WILL waste cycles in general to discharge the whole thing, when its unnessisary, so you will ruin the cell faster to keep cycling it for no reason. there is no doubt that the cell will live longer if you just top it off , before using it.

I was just trying to point out that until you cycle the PART that self discharged, your JUST going to be cycling the part that is still very vibrant, and working well. that part is less in need of a cycle of any sort.

Note: the chemicals arent in "parts and sections" like my talking indicates, but the vibrancy of the chemicals is relative to the chemical components that HAVE been cycled. which esoterically acts like a part or section of the cell.

frank777 said:

A smart charger should help you avoid any overcharging issues. My choice is the Maha MH-C401FS. It is an independent four channel charger, and is microprocessor controlled to avoid overcharging even when topping off batteries.

Click to expand...

It really doesn't matter if it's microprocessor controlled, it can still miss termination.

Hello VidPro,

You are close, but not quite there...

When a cell is charged, a number of small crystals form. These small crystals dissolved when the cell is used, giving up the energy we use from the cell.

When the cell is fully charged, the chemistry is not stable. This thread went into the details of this process. Basically, the small crystals join together to form larger crystals.

The only way to break up the larger crystals is through a discharge cycle. Unfortunately, the voltage depression that accompanies large crystal formation often prevents a complete discharge. This means that after the discharge, the cell voltage will rebound to a rather high level. The way around this is to do another discharge at a lower rate. You may have to do several discharges at progressively lower rates to completely break up large crystals that have grown hard.

When trying to recover NiMh cells, a good target to shoot for is a resting voltage of 0.95 volts. Don't leave the cells at that low a voltage for any length of time, but at a resting voltage that low, you have done the best you can to empty the cell.

After completely emptying the cell, charge it back up so your resting voltage is above 1.1 volts.

The way to avoid large crystal growth is to store NiMh cells in a discharged state. This will drastically reduce the self discharge rate and large crystal growth, but the cell will still self discharge a little. You need to check cells in storage to make sure their voltage remains above 1.1 volts.

If you want vibrant cells, the recommendation is to store them discharged, then to perform a charge/discharge cycle every 30 days.

Tom

Tom:

If I'm reading correctly, a normal discharge cycle on a C9000, etc. isn't correct or enough because the cell's voltage rebounds? If so, cycling a cell on a C9000 is a waste of time then??

Hello Bill,

If you have "crap" cells, this is true. If your cells are healthy, you can do the same thing by selecting a discharge rate of 100 mA and repeating the discharge until the discharge completes in 1 minute.

To try to recover "crap" cells, it is best to spend a lot of time with the CBA. At the end of a 20 mA rate discharge, you can see fluctuations in voltage. I believe those fluctuations are related to the break up of the larger crystals.

To put some perspective on things, a 2000 mAh cell discharged at 20 mA can take around 100 hours to complete the discharge. To completely recondition the cell, you may have to do this at least twice. This means that you will have over 200 hours invested in saving a $3.00 cell... with no guarantee of success.

Tom

FWIW, anyone who wants a device to do a complete discharge, I came up with this one a few years ago before I had a charger that would do any type of discharge.

It puts a (100ma) load on each cell until voltage hits threshold (.7). There is no latching or hysteresis (just a comparator and a PNP switch) so the activity LED (on when discharging) will get dimmer over time once the cell initially hits the threshold as the voltage oscillates just above it (i.e., average current decreases to whatever the cell can support at the threshold).

Really crappy cells may have the light just shut off. Most it just gets dim. I guess some with large crystals would more gradually dim.


http://personalpages.bellsouth.net/r/a/radellaf/battdisc.html

the cell is PARTLY self discharged in this situation, i was just indicating that you would be as well off just topping it off and using it, as you would to discharge once before charge. only because of the part that SELF discharged.

of course i assumed that the cell is one that does get used, and that its been a few weeks. just like what always happens.

do we fully cycle the cell every 2 weeks just to be able to use it?
leaving the cell discharged is rarely an option, as a alkaline would be charged ready to go, if all your cells are discharged, what are they there for? what purpose does a discharged cell serve in life, to sit on the shelf , and exist?

so i was indicating that the TEST mode, for example would actually cycle the self discharge, i am sure the option can be cycle the things all day and night forever so you can use them when you want, but that aint very practical.

so i was indicating, that you would be better off to just top the thing off and use it, or by using the test mode you could actually cycle the part that self discharged.

silver, within your esplaination, you didnt mention what was occuring in SELF discharge, because it SEEMS that in self discharge different things happen than when you discharge it normally.

so if the question was , do i Top off the partly self discharged battery, or discharge it first, then i indicated that you should charge it before discharge , IF your going to bother to discharge it or try and cycle it.
otherwise its a waste of time, because the SELF discharged section is not cycled.

doing multiple cycles of it , would not only be a waste of time, but here we couldnt even keep up with doing that , with the number of cells vrses chargers.

if you do 2 cycles for every USE cycle, every 2 weeks, then you wasted the whole cell on the charger not the light. with these selfdischarging cells, were already wasting all the time on the charger as it is.

what Options modes do we get there to do any of that, the 900:
Charge only
Discharge before charge
Refresh , discharge and recharge endlessly
Test, charge before discharge and recharge

what options are on the 9000 (say) for doing just 2 cycles, then stopping?

or what options are there on your average consumer charger with discharge:
Charge only
discharge before charge
Put it back on and do any of that again

what options are there for SERIES packs?
discharge and reverse charge
recharge and try and balance

i think the real solution would have been to toss these POS batteries when i realised they are nothing but trouble.

well some of the cells i tried to "cycle" and discharge low, as per something alex mentioned, didnt change doodley for self discharge.

i was testing for a few weeks now what alex was testing.

i dont have any actual numbers, because i had to use them in between and all, but some of the bad ones took ~1700 to recharge after 7-10 days, and some STILL DO. this whole rack was over 400, and it still is.
cycling them didnt make any vast change to the Self discharge PROBLEM these cells had :-(

One thing I have done to make my life easier is to recycle any AA 2500 mAH battery that has the "HR" stamp on the bottom. Once I got rid of the Energizer, Powerex and Sony 2500 mAH cells, my life has become much easier.

In the coming months I'm going to get a C9000 analyzer and two dozen Eneloops. My address is changing again and I'm not sure if a Cadex is around for me to play... err, analyze things with.

Just started to get into Eneloop type cells but still have some normal ones (ratings between 2000 and 2700 mAh). When testing, I test them all.

With more than 2 weeks after the charge (mostly more, 1-2 months) there are some of the high power Ni-Mhs fully depleded, all show less charge than the Eneloops (thats why they are quite good for seldomly used gadgets).

If I put all cells into the charger (normal one time dischage/recharge cyle) and have them charged, wait about 1 day and do a discharge cycle again,
even the totally dead cells show almost the same mAh than the best ones.
Just yesterday, when the best of the batch came up to about 2400 mA, there was one of the dead cells with them.
No several charging cycles, just one.

PS: I feel lucky, if the mAh the charger measures is just 20 % below the number printed on the cells, any maker.

Hello VidPro,

Here I thought I was being perfectly clear...

Let's look at this again.

As a NiMh cell is charged, crystals are formed. When the cell is discharged, the crystals dissolve and give off energy.

In frequent use, the cell is discharged and charged in roughly a month or less. People who fall into this category of use have very few problems with their cells.

The problem comes when you have more cells than you can use. Some people stock up on batteries and end up with many more than they can actually use. Now they have to figure out how to store them. This is where the issue of self discharge comes in.

The first thing we need to establish is that there are two causes of self discharge. Small crystals joining together to form larger crystals is one cause. The other is when the separator loses its dielectric strength. Large crystal formation is considered natural self discharge, and it can be reversed if caught early on. Separator damage is irreversible.

Now let's consider a situation with healthy cells, and we will ignore damaged cells for the time being.

Let's suppose that I have 10 lights that use AA cells. A Mag85 that takes 9 cells, 3 one cell lights, and 6 lights that each take two cells. To get everything up and running I need a total of 24 cells. Thinking that I need to have a back up, I purchase 50 cells.

I find that I am frequently using one of my one cell lights, and two of my two cell lights. The nine cell light gets used often, but not frequently.

When reviewing battery usage, I find that with my frequent used lights, I am recharging every two weeks. The nine cell light gets recharged about once a month. The others are infrequently used and may go several months without being recharged.

As you can see, I am basically using 14 cells, and storing the rest.

I decide that I need to rotate my cells through my lights. Six months later, I get around to doing it and notice that the voltage is down on the cells I am rotating into my lights. I charge the cells up and put them in my lights.

After a while I notice that my direct drive lights are no longer as bright as they used to be, my run times are shorter, and my batteries are heating up more when charging. My cells are on their way to becoming "crap" cells.

This is my understanding of the problem...

Now, what can be done to improve things.

Let's start with charging.

The crystal size is related to the charge rate. Higher charge rates produce smaller crystals.

Crystal stability is related to crystal size. Small crystals are less stable than larger crystals.

Cell stability is related to its state of charge. Fully charged cells are less stable than fully discharged cells.

When we get a new cell, it is usually mostly discharged. The first thing we need to do is a standard charge (16 hours at 0.1C charge rate). This is followed by a few charge/discharge cycles to make sure the cell is good, then we put the cell into service.

When our usage is frequent or often, the cells perform well. Trouble starts when we store the cells with infrequent use.

The best way to store cells, is to store them in a discharge condition. It is also the best option to keep the cell vibrant. You asked what is the purpose of having a discharged cell on the shelf, and that is a good question. I hope the bargain price you got for the cell offsets the storage costs and the efforts required to maintain that cell. I look at the discharged cell sitting on the shelf as raw material. I have tested it and know that it is ready for use once I charge it.

While trying to portray issues as black and white, we always notice that there are far more shades of gray. Battery storage is one of these gray areas.

Going back to our original example, my philosophy would be to put the cells that I am not using into storage. I have 14 in use, and let's pull another 5 for back ups. The other 31 cells would be discharged and stored.

With this inventory allocation, I have my main lights that I am using, and back up cells for my single and double cell lights. I also have a charger that is capable of charging my cells faster than I can use them. To keep the storage cells vibrant, I do a charge/discharge cycle on them every 30 days.

If vibrant cells are not a high priority, the cells can be kept alive with a charge/discharge cycle every 90 - 180 days, and it may be possible to stretch this out to once a year.

Let's take a look at battery packs. Power tools work well when they are used daily, but after a period of storage when you go to grab your drill, it seems that the battery is always low or dead.

If you follow the same procedure and store your packs discharged with a refresh charge/discharge cycle every 30 days, your packs will last a long time. However, when you grab your drill you need to charge the battery first. This can be a problem.

You can get around this by storing the pack partially charged. This gives you the ability to use the pack immediately if needed. A partially charged pack will still self discharge, but the effects will be reduced. You will find that the cells will stay in balance better with a partial charge than when stored fully charged.

Getting back to the original question of whether it is better to top off a self discharge cell, or do a discharge/charge, or a charge/discharge cycle on it, my answer is to use the cell before it has a chance to self discharge...

There are several chargers available that have a discharge function. Some stop after discharging, so discharging a cell is easy with them. Others charge after the discharge, so you have to watch what is going on and stop the cycle after the discharge.

Power tool battery packs are harder to discharge outside of the tool. The advanced RC chargers allow you to do a discharge, but not everyone has on of those. The best I can do is to recommend watching as the end of a project draws near and charge the pack up before your done with it. This will allow you to partially deplete the pack before putting it away for storage.

Here is an example. My X990 runs for around an hour. I find that I use it more in the winter than in the summer. As summer approaches, I charge the pack up, then run the light for 20 minutes. This is how the pack is stored over the summer. If I need the light, there is still some charge left, and I am minimizing the self discharge influence on the pack.

Yes, there is some effort required to keep your cells vibrant, but I have found that running things on "crap" cells is very frustrating, and I am willing to put in the effort.

The new low self discharge cells seem to be the answer to this problem. They have done something to stabilize the cell in a charged condition. This should make it a lot harder to end up with "crap" cells due to ignoring them and just letting them sit on the shelf. Plus, when they are sitting on the shelf, they are charged and ready for use.

It will be interesting to see how well they actually do. We may still have to do a discharge/charge cycle every five years, or so, but I think most people can manage that.

Tom

An interesting update...

I ran across some cells that are not up to par. I decided to do a deep discharge on them to see if that will help things out. I ran them at 0.01 amps with a 0.5 volt cut off. During the discharge, the voltage was bouncing around and it reminded me of the pen on a seismic recorder. Well, not quite that bad, but it was bouncing around a lot.

After the discharge, the voltage jumped back up to 1.2 volts.

I ran the test 5 times and each time the voltage would rebound to 1.2 volts.

I then ran a test with the voltage cut off reduced to 0.1 volts. At the end of the test, the voltage rebounded to 1.2 volts.

I took a piece of wire and dead shorted the cell and left it shorted for 2 hours. When I removed the wire, the voltage of the cell climbed back to 1.2 volts.

The interesting thing is that when I got these cells, they were at 0.97 volts. I can't seem to get them back down that low.

Oh well, I will charge them back up and see if it helped.

Tom

SilverFox said:

I decided to do a deep discharge on them to see if that will help things out. I ran them at 0.01 amps with a 0.5 volt cut off.

Click to expand...

SF, not being very experienced at this point with rechargeable cells, what kind of charger gives you the flexibility to do the types of things you describe here? I have an Elektrolumens light on order whose performance will be optimized with the use of NiMH C cells. In searching for a good charger I've run across a few that seem promising, including the BC1HU Quick Smart Charger, the Intelligent Battery Charger w.LCD T-6278, and the Ansmann Energy 16 Charger. Is there any one of these (or perhaps another that I'm missing altogether) whose performance and ease of use clearly stands out above the rest?

Hello Bill,

The discharging that I am doing is on a CBA. You can find information on it a www.westmountainradio.com .

My favorite charger for C and D NiMh cells is the Maha C808M.

Tom

Thanks much, Tom.

I see that Thomas Distributing has those for sale. They'll also give you a price break on Powerex C cell NiMH if you order at the same time. Looks like the price comes out to about $7 per cell. Is that price high or are they of higher quality?

Wow, Tons of great information. I do have a few too many cells sitting around and turning to crap it sounds like. I just need to get them into my gadgets I guess. Thanks for all your input.