NIMH storage?

[sweetlight]

I have a refrigerant leak detector that uses a NIMH recharagable battery stick. This leak detector is now used as backup and thus may not be used for long periods of time. My question is should I just store it, or should I charge the battery every now and then? Thanx for any input.

 

[coppertrail]

First, what kind of NiMH cells are you using? I store my unused NiMH cells in sealed baggies/containers in the fridge.

 

[Eugene]

Since switching to eneloops (low self discharge) I leave them in my devices and just test and top off about every 6 months. Higher capacity NiMH you'll need to top off monthly.

 

[Conte]

Ideally, a Nickel based cell should be stored at a 40% charge and "chilled" for good measure.

As you are using it for backup, I'm assuming you want it ready to go, and not have to charge them before use. If you can, store them outside the unit (not installed). Maybe toss them in the fridge for good measure. The baggie is a good idea too.

NiMH batteries should be maintained every 3 months. For best results research has shown that if you "Excersize" them at those intervals it will help avoid memory effect and maintain maximum capacity.
To excersize them, you run them down to their dead state then hit them with a full (preferablaly rapid) charge.

My information comes from batteryuniversity.com
Very informative.

 

[SilverFox]

Hello Conte,

NiMh cells actually remain more vibrant when they are stored in a discharged state and kept at room temperature. Every 30 days they should be exercised by undergoing a charge/discharge cycle.

Most people find this care too demanding, so they put some charge into the cells, and extend out the interval between charge/discharge cycles. Storing cells in cool temperatures reduces the self discharge rate and allows for the extended interval.

Most people are not running critical applications, so the difference in care is not usually noticed. Those who push things to the limits look at things a little differently and are happy to spend the extra time in care for the extra performance from the cells.

Tom

 

[Eugene]

Ideally, a Nickel based cell should be stored at a 40% charge and "chilled" for good measure.

As you are using it for backup, I'm assuming you want it ready to go, and not have to charge them before use. If you can, store them outside the unit (not installed). Maybe toss them in the fridge for good measure. The baggie is a good idea too.

NiMH batteries should be maintained every 3 months. For best results research has shown that if you "Excersize" them at those intervals it will help avoid memory effect and maintain maximum capacity.
To excersize them, you run them down to their dead state then hit them with a full (preferablaly rapid) charge.

My information comes from batteryunivercity.com
Very informative.

That page was getting outdated even before low self discharge. With low self discharge those storage issues are kind of pointless. You may for from loosing 10% a year to 8 or 9% per year keeping them chilled, I just top off at every 6 months.

 

[Conte]

NiMh cells actually remain more vibrant when they are stored in a discharged state and kept at room temperature. Every 30 days they should be exercised by undergoing a charge/discharge cycle.

This is the Information I have collected for NiCD cells, and I can vouch for that based on personal experience. Even still, a 40% charge is recommended.

The 40% is to compensate for the self discharge between maintenance cycles while not allowing then to fully discharge.

NiMH maintenance cycles are extended to 3 months to avoid hitting them to to many discharge cycles.

References :
http://www.batteryuniversity.com/partone-21.htm
http://www.batteryuniversity.com/partone-19.htm
http://www.batteryuniversity.com/partone-14A.htm

Now mind you, these are the opinions of a single reputable source, and not my own, so I'd be an *** to debate it any further.

Most people are not running critical applications, so the difference in care is not usually noticed.

Ahhh, too true. I forget sometimes not everyone is as hardcore a battery geek as I am. Was just sharing my information.

That page was getting outdated even before low self discharge. With low self discharge those storage issues are kind of pointless. You may for from loosing 10% a year to 8 or 9% per year keeping them chilled, I just top off at every 6 months.

This is also very true. But he stated that he was using a "NIMH recharagable battery stick". Most likely proprietary common tech, so I believed this info to be applicable to him.

 

[SilverFox]

Hello Conte,

Actually the 40% partial charge figure comes from the battery manufacturers...

What we have to determine is if the manufacturers have the same goals as the end users when it comes to storage.

The answer, while somewhat similar, is still no.

The chemical engineers at the battery manufacturing plant understand that NiMh chemistry tends to "go to sleep" with inactivity and self discharge. They also understand the damage that occurs in an over discharge situation.

On the other side of the equation is the fact that production runs may not be done frequently, resulting in storage of the cells. The storage time at the plant is kept to a minimum, but could extend to a year. The storage time in the supply chain is difficult to predict. Some retail sources sell a lot of batteries, others don't.

In order to minimize returns and defective parts, they came up with the 40% charge number, and they hope that the cells will be put into use within 3 years.

In contrast, the goal of the end user is to have a cell that meets its labeled capacity, and is "vibrant" in its performance.

The best way to meet this goal is to store the cell in a discharged state. This eliminates the damage done by gradual self discharge, but you can only get away with this for a limited amount of time. This is where the 30 cycle comes from. In addition, cycling the cell through a charge/discharge cycle exercises the cell and keeps the electrolyte evenly distributed within the cell.

The downside to this is that while your cells are stored, you are using up a charge/discharge cycle every month.

Looking a little closer at this eliminates this concern. Cells are supposed to last 500 charge/discharge cycles. If we are using up a cycle every month, they would wear out after 500 months, or a little over 41 years.

In this case the benefit of having vibrant cells far outweighs the concern of wearing out the cell in storage.

As you can see, cell storage depends on what your goals are. If you are leaving the country for a year or two and have too many cells to take with you and have to put everything into storage, by all means follow the manufacturers recommendations for storage. However, if you find a sale on cells and stock up early planning on putting the cells into service in the next 6 - 12 months, do a proper break in on the cells, discharge them, and store them in a cool environment remembering to do a charge/discharge cycle on them every 30 days.

Tom

 

[DynoMoHum]

I recently pulled about ten 4 cell GP 3300 mAh Sub-C packs out of my old RC storage boxes and cycled them twice, on the second cycle nearly every pack was putting out something very close to 3000 mAh at 30 amp discharge rates. These packs have been sitting in my basement( around 64F year round ) for at least 4 years with no use.

A couple of the packs had been dead shorted (stored with a wire soldered across the main terminals of the pack). This dead shorting was a bit of a trick that some RC car racers used to do in a effort to keep the output voltage of charged packs at their highest levels. This dead shorting was only common in RC racing for about one season that I'm aware of. I had given it a try on a couple packs, and that's why I had some stored like this... I saw no significant difference between the packs that were dead shorted vs the ones that were just left with no charge.

Basically I had stopped racing around 5 years ago and haven't really even touched these cells until a few weeks ago. I have some ideas that I'm going to maybe find some new use for these cells, possibly as a external power source for some camera speedlights... or who knows maybe I'll get one of my RC cars out in the spring and take it for a spin.

So... at least in the case of these GP 3300 cells I have, they seemed to hold up reasonably well after having been stored for 4+ years with no charge in them. That is I consider putting out 90% of their rated charge (at nearly 10C discharge rates even) after just once complete charge/discharge cycle, a pretty good result for NiMh cells stored for this long.

I think, what the best way to store NiMh cells is, probably depends on the type of cell... Each and every different brand and model of cell likely has some of it's own unique characteristics.

In the end, I suspect that charging and discharging most non LSD cells about once every few months is probably better then leaving them sit for 4+ years without any attention... but then, at some point you have to ask your self how much work you really want to put into something.

I've got little experience with LSD cells... but my suspicion is that they wouldn't mind having a bit of exercise a couple times a year rather then just sitting unused for years on end. But then, they may very well work pretty well even if you let them sit for several years...