NIMH rechargeable battery question

[Peakbagr]

Some of my NIMH 2700 double A rechargeables are going bad. I use a conditioning charge every 10 times and usually a soft charge inbetween.

I use a Powerx MH-C800S eight cell charger. Are there NIMH batteries with a higher capacity than 2700, and could those be used in my charger?

Thanks
Alan

 

[Kevin Rooney]

Alan -

Have you looked at the Sanyo Eneloop battery? I doubt it's rated much above 2000mA, but ... it has a very slow self discharge rate, so that feature may - in effect - offset the lower milliamp rating.

Kevin

 

[DougPaul]

You might consider getting a Lacrosse BC-900 or Maha-MH-C9000 charger. These chargers have the capability of measuring the capacity of NiMH cells. I have a BC-900 and use it to check the health of my NiMHs every 6 months or so.

A quick check at http://www.thomasdistributing.com/ shows some cells advertised as having 2900 mAh capacity. (The advertised numbers are often measured under conditions that maximize the mAh number which is not necessarily realistic for any specific use*. There is also no guarantee that higher ratings will give you a longer runtime in said specific application.)

An early version of the writeup of some tests of NiMH cells talked about a cell model that had high initial ratings but degraded very rapidly, particularly if it was used at a high current drain. The same write-up described some cells (Sanyo industrial, IIRC) that had modest rated capacity, but were very rugged. The writeup is at http://www.imaging-resource.com/ACCS/BATTS/BATTS.HTM but the current version seems to have deleted the above. The last test was performed in 2005 so it may not be relevant for currently available cells.

As a result of the above, I generally buy cells from well-known manufacturers that are back a bit from the bleeding edge. Of late, I have bought low-discharge cells which have a somewhat lower capacity (eg Sanyo Eneloop), but the charge will last much longer before use. As a result I have a mix of regular and low-discharge cells.


* Since I suspect that your use-of-interest is a Garmin 60CSx GPS, mine draws ~160mA (normal mode, no backlight) from a pair of NiMHs and the GPS shuts down at 2V. A GPS uses a voltage converter so that it probably draws approximately constant power (P=I*V, so more current at lower voltage) from the battery. A test that draws ~160 mA from a cell with a cut-off of 1V would probably be representative.

Doug

 

[Peakbagr]

Thanks, guys. I'm trying to re-condition the batteries that are wimping out.
Just picked up four rated @ 2400 and watch to see the difference, if any, in their performance vs the 2,700s.

 

[DougPaul]

I've recently learned that some LaCrosse BC-900 chargers have suffered meltdowns and I therefore rescind my recommendation. See http://www.vftt.org/forums/showpost.php?p=336792&postcount=49 for more info and please continue any discussion in that thread.

Doug

 

[NewHampshire]

The BC-900's were discontinued and were replaced by the BC-9009. Some think it was the plug unit causing the problem, some not so sure, but it appears that the problem has been fixed with the 9009 model. No melt downs have been reported since, IIRC, about Feb. of this year. If you bought one before then I would be concerned. But now I think things are all set. I recently bought one of the BC-9009's (before reading about the earlier problems) and have not had a problem so far. I would not call that statistical proof of the problem being fixed, but since I have not really seen any problems reported since Feb. I would not be too worried. If you are then simply try and charge your batteries when you can keep a cursory check on them from time to time. If you don't see any problems over a dozen or so recharge cycles I would think you would be fine from then on.

Brian

 

[Remix]

I had a few duracells go bad. I saw discoloration on the contacts. I charge everything in a 8 unit Maha charger and still have old 1600mAH Panasonic cells without problems. This is probably the same charger you have.

If its not a manufacturing defect in the batteries, it could be accumulated damage on the weakest cell. In a product like a flashlight with two or more cells, one cell will typically run out of energy while the other cell works for a short time. In this time interval, the weak cell is being de-polarized and damaged by the energy in the strong cell.

I would not go out and invest in another charger just because a few cells went bad.

Also, I only use conditioning on cells that have not been used over a season.

 

[DougPaul]

FWIW, I had asked that discussion continue in the other thread.

The BC-900's were discontinued and were replaced by the BC-9009. Some think it was the plug unit causing the problem, some not so sure, but it appears that the problem has been fixed with the 9009 model. No melt downs have been reported since, IIRC, about Feb. of this year. If you bought one before then I would be concerned. But now I think things are all set. I recently bought one of the BC-9009's (before reading about the earlier problems) and have not had a problem so far. I would not call that statistical proof of the problem being fixed, but since I have not really seen any problems reported since Feb. I would not be too worried. If you are then simply try and charge your batteries when you can keep a cursory check on them from time to time. If you don't see any problems over a dozen or so recharge cycles I would think you would be fine from then on.

The BC-9009 has the same electronics as the BC-900 (only the color of the plastic is different). If you followed the link in the other thread (http://www.candlepowerforums.com/vb/showthread.php?t=253907), you would see that BC-9009s have been reported to have the meltdown problem. And there are reports where the charger worked properly for an extended period and then melted down. The problem may be a function of the firmware version as well as the basic charger design. It may be that some specific set of conditions are required to trigger the meltdown--the charger will operate normally until the trigger and then overheat. In some cases where the power was removed before fatal damage, the charger again worked normally after it had cooled down.

SilverFox (a moderator at candlepowerforums) has recommended that one place the charger on a fireproof surface (eg ceramic tile) and place a smoke detector near it. The chemicals released from an overheated cell (potassium hydroxide) can also cause corrosion and skin burns.

Keeping a fan on the charger is probably also a good idea.

Doug

 

[DougPaul]

I had a few duracells go bad. I saw discoloration on the contacts. I charge everything in a 8 unit Maha charger and still have old 1600mAH Panasonic cells without problems. This is probably the same charger you have.

If its not a manufacturing defect in the batteries, it could be accumulated damage on the weakest cell. In a product like a flashlight with two or more cells, one cell will typically run out of energy while the other cell works for a short time. In this time interval, the weak cell is being de-polarized and damaged by the energy in the strong cell.

I would not go out and invest in another charger just because a few cells went bad.

Also, I only use conditioning on cells that have not been used over a season.

No it is not the same charger.

The problem reports center on LaCrosse BC-900s and BC-9009s with suspicion cast upon the BC-700 because it is a very similar design. (The 700 is essentially the 900 without the 1000 mA charging rate.) I have seen no reports that any Maha chargers are at similar risk.

The meltdown is a problem with the charger, not the cells. The exact factors which trigger the meltdown are not publicly known--damaged cells may or may not be a factor, but the the meltdowns have only been observed in the specific LaCrosse chargers. (Certainly the meltdown can result in damaged cells, but that doesn't mean that the cells caused the problem. See http://www.candlepowerforums.com/vb/showthread.php?t=250545 for some pictures of damage to both the cells and charger.)

Doug

 

[Kevin Rooney]

FWIW - I've been using the BC-700 for about 6 months, and it's one of my better gadget investments. Initially I had many older batteries to condition, so the 4 bay unit meant it took an extra week longer to cycle them all thru, but no big deal. Now, I rarely have more than 2 to charge at a time, so the smaller unit is not a factor. Mostly I use the 500mA setting (the default is 200).

I also invested in a dozen AA and 4 AAA's Sanyo Eneloops. Have found I use them almost exclusively now, given their the ability to hold a charge for long periods of time. Plus, they're very well constructed, so that creates an aura of reliability. My old stash were made by Kodak, and while they still have a usable life, they are slowly relegated to powering the solar lights outlining our driveway/parking area/etc.

 

[DougPaul]

FWIW - I've been using the BC-700 for about 6 months, and it's one of my better gadget investments. Initially I had many older batteries to condition, so the 4 bay unit meant it took an extra week longer to cycle them all thru, but no big deal. Now, I rarely have more than 2 to charge at a time, so the smaller unit is not a factor. Mostly I use the 500mA setting (the default is 200).

I was also happy with my 3 BC-900s until I discovered the meltdown reports*... Now I'm scared and plan to take some of the precautions suggested by SilverFox (of candlepowerfourms). I've already been using a fan because the cells got too hot during charging and the charger paused to let them cool. (This pausing is normal.) It may not happen very often and I've seen no reports of fires yet, but I don't want to be the first...

* I was helping a friend choose a good charger. She is buying a Maha MH-C9000...

BTW, the best charging rates are from .33C to 1C, where C is the capacity of the cell (Ref: http://www1.duracell.com/oem/Pdf/others/TECHBULL.pdf). 500mA is .25C for a 2000mAh AA Eneloop. 700mA (.35C) is just barely adequate. A number of meltdowns have occurred at a 200mA charging rate so it does not provide safety. Lower charging rates make it harder for the charger to detect the end-of-charge and if the cell is severely overcharged, it will overheat and be damaged. (One of the theories floating around is that a low charging rate makes the meltdowns more likely.)

So I recommend that you charge AA cells (2000+mAh) at 700mA (1000mA would be better, AAA (800-1000mAh) at 500mA) and put a fan on the charger. And use SilverFox's safety recommendations.

I also invested in a dozen AA and 4 AAA's Sanyo Eneloops. Have found I use them almost exclusively now, given their the ability to hold a charge for long periods of time. Plus, they're very well constructed, so that creates an aura of reliability.

I buy my cells in groups of 12 because it divides down into sets of 2, 3, and/or 4. I probably have ~48 active AA cels and ~36 active AAA cells in a variety of types and brands purchased over the last seven years. Most of my purchases for the foreseeable future are likely to be Eneloops.

BTW, while Sanyo Eneloops appear to be a good brand, they have been involved in quite a few of the meltdowns. (I believe this is most likely the fault of the charger, not the batteries. Since Eneloops are popular among the more advanced users, they are simply likely to appear frequently in the reports.)

Doug

 

[DougPaul]

I just found a post describing a recall for a new AC adapter on BC-9009 chargers sold between Oct 1, 2009 and Jan 15, 2010 http://www.candlepowerforums.com/vb/showthread.php?t=260099&p=3274503#post3274503 from 2/10/2010, but I can find no evidence of it on the LaCrosse website. http://www.lacrossetechnology.com


I do, however, find the BC-900 currently listed with an arrival of "TBA" http://www.lacrossetechnology.com/900/index.php. So it presumably isn't discontinued, but is hopefully being redesigned to eliminate the problem. I hope it is marked in some way to show that it is a "fixed" model.

Doug

 

[Remix]

Doug, I think you misunderstood.

I probably have the same charger as the original poster who created the thread. The point I was making that a few cells going bad does not really justify getting a new charger.

Another way the batteries can go bad is from dropping them. If the seal around the cap goes bad then there is a slow degradation until you or the charger find the problem.

 

[DougPaul]

Doug, I think you misunderstood.

Sorry--the references are sometimes hard to figure out.

I probably have the same charger as the original poster who created the thread. The point I was making that a few cells going bad does not really justify getting a new charger.

Another way the batteries can go bad is from dropping them. If the seal around the cap goes bad then there is a slow degradation until you or the charger find the problem.

Agreed.

NiMH cells have a finite lifetime of ~500 cycles if treated well.

There are a number of ways to damage NiMH cells:
* mechanical damage (as above)
* overcharging (often by using a poor quality charger or leaving the
cells in the charger long beyond their being full). Trickle chargers are OK for NiCad, but not NiMH.
* overheating (particularly when charging and fast chargers (ie <1hr) are particularly dangerous)
* complete discharge (best not to discharge below 1V)
* reversing the charge (typically occurs to the weak cell in a series connected battery). Preventable by turning the device off before the battery voltage drops by 1 cell-voltage.
* drawing too high a current or short-circuiting

Weak cells can often be rejuvenated by several full discharge (to ~1V) and full charge cycles. If you have a charger such as the Maha MH-C9000 (or a LaCrosse BC-700/BC-900/BC-9009 if you are willing risk meltdown with possible damage to both the cells and charger) you can use several capacity measurement cycles. If the measured capacity comes back up to a reasonable value, then you can continue to use the cell. If not, discard it.

NiMH cells can exhibit memory (loss of capacity due to frequent shallow discharges) if charged at a rate below ~.33C. Rejuvenation should fix it.

Doug

 

[DougPaul]

Back to the OP's question? What a novel concept!

Some of my NIMH 2700 double A rechargeables are going bad. I use a conditioning charge every 10 times and usually a soft charge inbetween.

Hard to tell from here. Try a couple of discharge-charge cycles to rejuvenate.

I use a Powerx MH-C800S eight cell charger. Are there NIMH batteries with a higher capacity than 2700, and could those be used in my charger?

Check the specs. http://www.thomasdistributing.com/s...ybrwith-bag-and-two-8-cell-holders-p-424.html There is also a link to the manual.

If it doesn't say, then I'd guess 3000mAh is the max, otherwise the 1A charging rate is below .33C.

As for larger capacity batteries, check http://www.thomasdistributing.com/shop/batteries--nimh-rechargeable-c-122_104.html. Pushing for the latest-and-greatest capacity cells can be expensive and can result in fragile cells with short lifetimes. You may ultimately be happier with a bunch of 2500 mAh cells or 2000 mAh low-self-discharge cells (eg Sanyo Eneloop).

Also, manufacturers' ratings are not always reliable--they have incentive to inflate their capacity ratings (for instance, by using strategies that greatly reduce the cell life) or tricks that you cannot do with your charger. In addiiton, their test discharge currents may not be representative of your application.

Doug

 

[Kevin Rooney]

...BTW, the best charging rates are from .33C to 1C, where C is the capacity of the cell (Ref: http://www1.duracell.com/oem/Pdf/others/TECHBULL.pdf). 500mA is .25C for a 2000mAh AA Eneloop. 700mA (.35C) is just barely adequate. A number of meltdowns have occurred at a 200mA charging rate so it does not provide safety. Lower charging rates make it harder for the charger to detect the end-of-charge and if the cell is severely overcharged, it will overheat and be damaged. (One of the theories floating around is that a low charging rate makes the meltdowns more likely.)

So I recommend that you charge AA cells (2000+mAh) at 700mA (1000mA would be better, AAA (800-1000mAh) at 500mA) and put a fan on the charger. And use SilverFox's safety recommendations.

That's interesting. The BC-700 has 3 settings - 200, 500 and 700. Am quite sure the manual recommended 500 so will check it again.

I've not had any issues with battery overheating, unlike the chargers which sometimes come with the batteries. Mostly the batteries are slightly warm if I catch them immediately after the cycle completes. If I leave them overnight, they're always room temperature.

 

[DougPaul]

That's interesting. The BC-700 has 3 settings - 200, 500 and 700. Am quite sure the manual recommended 500 so will check it again.

Read http://www1.duracell.com/oem/Pdf/others/TECHBULL.pdf section 6.3. .33C to .1C is not recommended. See also http://batteryuniversity.com/learn/article/charging_nickel_based_batteries. Best rates are .33C to 1C. Use a fan above ~.5C (good to use a fan over the entire range).

I've not had any issues with battery overheating, unlike the chargers which sometimes come with the batteries. Mostly the batteries are slightly warm if I catch them immediately after the cycle completes. If I leave them overnight, they're always room temperature.

Sometimes (particularly older or somewhat damaged cells) develop quirks that cause the charger to miss detecting the end-of-charge and keep on charging a full cell which can make it get pretty hot.

I've had a few hot cells before I used a fan--hot to the touch, but no damage to the cell. And a few cases where the charger did not detect the end-of-charge and kept on charging...

I also had a Ray-O-Vac PS4 charger (2A, ~1C) overheat and destroy some cells (including the ones that came with it...). Also before I started using a fan.

My MH-C401fs at its 1A charging rate makes the cells a bit warm to the touch at the end-of-charge but AFAIK has never harmed any cells.

It is generally a good idea to keep an eye on the charger so one can catch occasional malfunctions. And, of course, a fan is a good idea too. I read of some people orienting the charger so that the cells were vertical to improve the (presumably fan-less) convection. They claimed that it kept the charger and cells cooler.

Doug