Just completely discharged a NiMH.

[Kraid]

I had two Duracell 1700s in my Turbo Charger (http://turbocellcharge.com/v2/cart.php?action=shop) and fell asleep! When I woke up, it read completely dead. Which was reinforced by my ZTS Tester.

I'm still very new to rechargeables. But this is bad, right? Dimished recharging capabilities or something?

 

[SilverFox]

Hello Kraid,

Not a good idea, but the damage is proportional to the amount of time spent at a very low voltage. You can get away with an excursion, but if you leave them totally discharged for a while, you will do a lot more damage.

Get those cells on the charger!!!

Tom

 

[Kraid]

Sickbay, stat! Got it! Thank you!

 

[Fallingwater]

I was under the impression that NiMH cells didn't much care about being left discharged. After all, with their high self-discharge any ordinary NiMH cell that was left untouched for a few months would go bad if they did care...

 

[VidPro]

i would say the bigger problem would be any reverse charge that occured, more than the deep discharge.
mabey you get luckey and the driver in that thing quits operating at around 1V and nothing bad happened.

 

[SilverFox]

Hello Fallingwater,

Let me see if I can clarify...

Both NiCd cells and NiMh cells prefer to be stored in a discharged state. You can also short out NiCd cells, after discharging them, and store them that way. You can not do that with NiMh cells.

A completely discharged NiMh cell will often have a open circuit voltage in the 1.0 - 1.2 volt range. If the load is heavy, they will show 1.2+ volts. If you leave the cell connected to a circuit, it will not only drain the battery, but also drop the voltage down. This very low voltage condition is one of the things that ruins NiMh cells. You end up with a "completely" discharged cell with a reading of 0 volts.

Self discharge is associated with capacity. Over the course of a year, a NiMh cell will loose nearly all of its initial capacity (healthy cells will loose 75 - 80% of their capacity), but it will still show a voltage of over 1 volt. When you purchase new NiMh cells, check their voltage. If they are below 1.0 volts, send them back. They have either been stored improperly, or they are old stock.

To properly store a NiMh cell, discharge it to 1.0 volts at 1 amp and store it that way. It is best to cycle the cell through a charge/discharge cycle every 30 days, but you can let it go for as long as 6 months in a discharged state before real damage starts to set in.

I just checked some cells that I had in storage for 4 months. The cells were stored at room temperature. I discharged them at 1 amp to 1.0 volts and after 4 months they still have no capacity left, but have voltages in the 1.15 - 1.18 volt range. I feel that 4 months is too long to go without cycling them, but in this case it was unavoidable. I am hopeful that a couple of charge/discharge cycles will restore them to their initial "vibrancy."

Tom

 

[kalkwasser]

Hi Tom,

Hoped you don't mind about me asking something related to charging NiMh batteries. I am new to this flashlights hobby and recently purchase some Sanyo Eneloop batteries.

Just wonder what is the best setting to charge these guys? As in, do I need to always do a discharge and charge cycle whenever I want to charge them after draining using them in flashlights? I am using a BC-6 charger.

Thank you.

 

[WDG]

To properly store a NiMh cell, discharge it to 1.0 volts at 1 amp and store it that way.

I have a set of Powerex 2700's I've been discharging at 100 mA for storage (cycling every month - MH-C9000) - I gather it's better to use 1000 mA instead? Could you expand on this a bit? My thinking was that the slower discharge would be better to get into the "slower" electrolytes. I've also been doing maintenance cycles on other cells using this same slow discharge rate, based upon this thinking. Do I need to stop doing it this way? I'm trying to be kind to my cells and keep them as vibrant as they can be.

 

[SilverFox]

Hello Lalkwasser,

The battery manufacturers recommend charging in the 0.5 - 1.0C range when using a charger with charge termination. You get the strongest end of charge signal using those charging rates and this will minimize overcharging problems. The Eneloop cells are 2000 mAh, so a 0.5C charging rate would be 1000 mA and a 1C charging rate would be 2000 mA.

The other recommended charging rate is 0.1C with a timer that shuts the charger off after 16 hours. 0.1C for the Eneloop cells would be 200 mA.

Tom

 

[SilverFox]

Hello WDG,

Your discharge rate is fine, as long as you tend to your cells regularly. I use the 1 amp discharge rate because it is faster, and there are times when I travel and can not tend to my cells regularly. Discharging at a higher rate leaves a little more in the cell, which gives you a couple of extra months of storage should you need it.

As far as a maintenance cycle goes, I charge and discharge my cells at 1 amp when I am using the C-9000. I also try to not have many cells in storage. Secondary cells make the most sense when you are constantly using them.

Also, I am ignoring any conditioning or cycling with the low self discharge cells. I charge them up and let them sit until I use them. I am not sure if that is the best way to take care of them, but they are still quite new.

Tom

 

[WDG]

Thanks, Tom. The time it takes hasn't been an issue, so I'll continue using 100 mA. Sometimes I get to second-guessing myself and wonder if I'm doing more harm than good! Had I understood the advantages of LSD first, I wouldn't even have this set to store, so I keep them primarily for use in photo gear.

Thanks, again. Y'all are a wealth of knowledge, and I greatly appreciate it!

 

[Fallingwater]

Silverfox: got it. Just a question: why would I want to store NiCD cells shorted?

 

[SilverFox]

Hello Fallingwater,

NiCd cells in storage suffer from dendrite growth. Shorting the cell stops, or greatly reduces, this failure mechanism.

Tom

 

[Sub_Umbra]

Great posts, Tom.

I've run across notes about storing NiCD cells shorted here and there before. How is it mechanically done? Is there some metal clip that fits AAs? Is there some improvised clip that may be simply made for this purpose?

Thanks

 

[Fallingwater]

Put them in a holder and short the wires together.

 

[SilverFox]

Hello Sub Umbra,

I am not aware of any "official" shorting device. Most people just "roll their own." The RC people have discharge trays, but I think they are designed around battery packs. You may be able to modify one for individual cells, but most of the cells used in RC are Sub C.

I think Fallingwater's suggestion is good.

I watched a guy make one a while back. He took some large diameter solid copper wire and bent it in a shape that was a little longer than the cell. He then took a hammer and flattened the ends slightly, to give better contact and to "grip" the cell better. Then he fashioned a small bend, kind of like a Shepard's hook, in the ends to add some tension to the clip and attached the clip to the cell. He had obviously done this many times before and completed the clip in well under a minute.

Tom

 

[Sub_Umbra]

Thank you.

If shorted in a holder, should it be a single cell holder?

Wouldn't multi cell holders reverse some polarity? For that matter, I'm suprised that shorting packs wouldn't do the same.

This is interesting -- and mysterious -- well, to me anyway. :sigh:

 

[Fallingwater]

Single cell or you risk reversal.

 

[VidPro]

a 10 Ohm resister comes to mind for doing that trick, you can buy fat 5&10W ones at the radio shack cheap, and do a full discharge without damage or heating up a wire.
the wires are long enough to use as jumpers , to EACH seperate cell even in a holder item, meaning one fat resister per cell, not in series.

after you do any of that to a ni-cd , you might have to "wake it up" again to charge it, you dont want to start a series charge then, without all of them waking back up . ---1.2v---1.2v---0v----1.2v---connected to 6v charger

 

[kalkwasser]

Hello Lalkwasser,

The battery manufacturers recommend charging in the 0.5 - 1.0C range when using a charger with charge termination. You get the strongest end of charge signal using those charging rates and this will minimize overcharging problems. The Eneloop cells are 2000 mAh, so a 0.5C charging rate would be 1000 mA and a 1C charging rate would be 2000 mA.

The other recommended charging rate is 0.1C with a timer that shuts the charger off after 16 hours. 0.1C for the Eneloop cells would be 200 mA.

Tom

Hi Tom,

Thanks for the prompt reply.
There are certain parameter that I am not familiar with indicated on the BC-6. So I have set the BC-6 to discharge>charge at 1 cycle. Parameter for discharging is 0.1A and 0.1V. Charging is 1A. And since I am charging them all in 1 in a 6AA adapter. What is the final value of the battery pack should be when it is charge?

Is my above parameters setting correct or anything I should concern?

Thanks.