# bad cell in a battery pack



## lurch (Dec 27, 2007)

I have 15 NIMH 3500 sub C cells built into a single batter pack. It worked for a few months but now runs down very quickly. Is there a way to find which cell(s) might have failed? I tried to test each with a volt meter but they all are very close in voltage to each other. should I separate them from each other and charge them separately and retest them? HELP!


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## Greg Elmassian (Jan 3, 2008)

You need to test them under load. Voltage will not tell you much of anything unless you have a load. 

The cells are about 1.2 volts... find a nice load like a 6v lamp from a car, and hook that to each battery WHILE measuring the voltage... 

By the way, how do you know that it is one cell that is doing this, since you are checking with a voltmeter? 

It might be that the majority of the pack is worn out.... 

Come on chat and let's talk, there is way too much that needs to be said... 

But to try to be helpful, try doing what I suggested, if you have a properly charged pack, then this will probably show weak cells... 

Regards, Greg


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## lurch (Dec 27, 2007)

I work real odd hours. I will try to get to the chat tonight, Thanks.


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## George Schreyer (Jan 16, 2009)

Let me add that you should check them under load WHEN THE BATTERY IS MISBEHAVING. The bad cell(s) will probably have zero or even a negative voltage across them. 

If you have an IR thermometer, you can check cell temperature under load. Bad cells will run hotter than the others during a discharge, especially after the voltage has dropped due the bad cells.


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## Greg Elmassian (Jan 3, 2008)

And damaged cells will be hotter when charging... sometimes disastrously so! 

Regards, Greg


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## TonyWalsham (Jan 2, 2008)

Lurch. 

Are you sure you have dead cell(s)? 
Conduct the tests as decribed by both Greg and George to see if their is single cell failures. 

I don't like NiMh batteries precisely because they don't last very long. 
Their longevity, or lack of longevity, is determined how they are treated. 
Draw more current than they can give, and they willalso have a seriously reduced life span. 
Leave them on charge too long and they will die. Even a too long tickle charge will do it.


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## eheading (Jan 5, 2008)

Tony, you mentioned that you do not like ni-mh batteries. Forgive me, I could probably dig back into archives and find the answer to this, but which type of battery do you prefer?? I am personally using mostly li-ion batteries, by Aristocraft, and ni-mh batteries. I like the energy/cu. in. of the li-ions, but the cost and charging speed of the ni-mh's. I was just wondering which was your preferred battery type and why??

Thanks,
Ed


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## K27_463 (Jan 2, 2008)

One point that has not been noted: it is possible for a NIMH pack to develop a false peak. What this means is that the charger says it is full, but the pack is not full. Modern smart chargers shut off when they detect a negative Delta V, sometimes of even a few tenths. NIMH can do this during charge. It is likely you do not have a bad cell in the pack at all , if the pack is fairly new as you suggest. This is where having a battery analyzer charger is very helpful. If a false peak shuts down the charger. Wait a while- a few minutes to 30 minutes, and place it on charge again. You may find it will go through the full cycle and charge up far higher than the first time. NIMH batteries should always be conditioned or 'formed" before placement in service. This can mean a full charge discharge cycle up to three or 4 times. This is called forming the pack. Failure to do this can effect the performance of the pack in normal service. I have formed or restored Many a pack that is performing below spec. I use a analyzer that shows the charge level in milliamps. Nimh work fairly well. Store them charged, use a Charger designed for NIMH, and never drain them 100%, such as allowing a loco to run until stopped and then allowing the lights on the loco to go out fully also. This max drain is difficult to recover from in some cases. 

Jonathan/EMW


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## blueregal (Jan 3, 2008)

Believe Tony W. (hope i'm not speaking out of turn) uses Ni cad battery packs as do I. Safer, and most of my run times are less than 1 hour so works for me I use two 7.2 flat battery packs 2300 mah wired in series. You can get everything you need from all battery.com. Hope this helps! Jerry the Regal


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## TonyWalsham (Jan 2, 2008)

Hello Ed.
Jerry is correct.
I have tried LA Gel Cells, Ni Cd and Ni Mh chemistry. Of the three types I have had the most success with Ni Cd.
I have not tried either Li-Ion or Li-Pol. You can get such batteries here but I cannot source a supplier in Australia who will sell me packs of the voltages I want. Let alone chargers for such beasties.
I do have an open mind about such chemistry as my cell phone uses it very successfully.
On the other hand, the lithium batteries that I had in my first digital camera where hopeless.

Unlike in the USA where there are many different sized battery packs, living in Australia I can only get SubC, AA and AA size battery packs from local suppliers at a reasonable price.
Air freighting batteries from the USA is not an option based on cost.
I use Sub C size cells in 6 cell twin stick formation that are readily available for anything that they will fit into.
For a loco where they won't fit I have to use AA or AAA sizes.

From *my* past experience I only ever use Ni Cd for SubC size. 
Ni Cd's last twice as long as NiMh cells in terms of the number of recharges that they can achieve. 
Ni Cd's are more forgiving than NiMh when recharging. You absolutely *must* use a peak detection charger otherwise even an hour so too long on trickle charge can cook them.
Where I cannot fit SubC cells I use AA size SANYO ENELOOP batteries. These are a hybrid NiMh/Alkaline cell designed for digital cameras and are guaranteed to hold 85% charge for 12 moths. In other words, unlike NiMh cells they do not go flat when left alone. They must use a peak detection charger as well but so far they have lived up to the hype.

Ni Cd cells do have an alleged failing. The, so called, *Memory Effect*. This is where a battery pack will not take a full charge. As long as you don't try and rapid charge a Ni Cd when it is partially discharged, the *Memory Effect* will not happen.

What Jon describes about NiMh charging seems to me to be the NiMh equivalent.

The above has been my experience, your mileage may vary.


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## K27_463 (Jan 2, 2008)

It is not really the same thing as NICAD memory effect. Nicad memory can be difficult to recover from. NIMH packs retain their full capacity, the charger just interprets it wrong. Nimh Reamins able to employ partial charges and discharges with no ill effect as long as the pack has been formed before initial service has started. I have been putting NIMH packs through forming exercises for many years, typically three full cycles before I deliver them to a client. While it is true that Nicads are typically specified at 1000 cycles lifetime, and NIMH at 500, that still equates to 10 years of use at 1 cycle per week. 

Care must be exercised in all cases, with all chemistries. Both in charging and in use.There are lots of misconceptions. Since there is reluctance to take my word for it, I will defer to another authority. Google battery University.com. 
The Cadex corporation. 
A commercial grade industrial supplier of custom chargers and other hardware. Read what it says on that web site. Note what it states about fast charging vs slow trickle charging for example. You can check the Archives for this site to see certain folks have long advocated only slow trickle charging. Oddly now however, those same folks offer and sell a fast smart charger of advanced capability that they learned about from another certain MLS poster. Sometimes things change and actually get better with new technology. I have begun employing Lithium based packs for the last 1-2 years after carefully watching the industry and the early rest results. As new technology for chargers ans come available , the incidence of issues with current generation lithium have become exceedingly rare. The simple fact is as the industry shifts forward, the cost of Nicad is now going higher than NIMH, and Lithium is coming down to get closer to NIMH. Unfortunately, Cadmium remains extremely toxic and poisonous for landfill waste. Lithium, also has its environmental issues of course. 
Jonathan/EMW


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## TonyWalsham (Jan 2, 2008)

Jon. 
I have no dispute with any of what you say. 

I am only giving an answer from my experience of using NiMh in AA and SubC size. 
I am well aware that NiCd will eventually be completely banned. 
Quite possibly it was something I did or didn't do when the NiMh packs that failed were originally sold. Anyway, fail the did. More than once. 
I admit I am biased. NiCd cells never let me down.


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## lurch (Dec 27, 2007)

Thank you everyone for your help. Testing them under load showed the defective ones. The packs that I have had problems with just the ones I have built for others that like to leave the battery on the charger and I have no control over this. The packs were previously NI Cd and I'm sure the chargers are not very forgiving to NIMh cells. I am going to rebuild them with NiCds, For my trains I use D, C and AA NIHM cells that I snap into a holder and when I need to charge them I just pop them out and put them in a Smart charger.I have learned my lesson about "upgrading a pack" with NiMh batts that I have no control over how they will be charged or cared for. Live and Learn. I will cost me a few dollars I don't have but hey I have always stood behind whatever I have built or done. My name and word is worth a lot more than the batteries. Thank you all again. It is good to know that there is "friends" here that are always willing to help and share. Johnny (Lurch)


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