Battery Holder/NiMH Close Call

[Alan B]

Background and Observations

I put together a 8xAA 2D holder and a bored 2D M@g driving a WA1185 today. All new parts, except for the switch/base/binpin socket/bulb that was moved from another working M@g. The 8 AA NiMH cells were well rested fairly new cells. Seven Enduro cells and one Hybrio, all 2000/2100 mAH LSD types.

Everything seemed fine, the light works well. Took it with me this evening and used it a couple of times in our dark yard when I went to run an errand. As I was about to put the light away I noticed significant localized heat coming from the middle of the battery portion of the body. The light still worked, it was turned on/off just seconds before. The head and bulb area was not hot. The heat was local, not overall, and greater than expected so I quickly removed the battery holder. Two cells in one column were VERY hot. I removed them from the holder in case there was still current flowing, keeping their orientation for reference. They were hot but could be held in the hand. It took them quite a while to cool off.

I measured their voltage, both were in the neighborhood of 1.27 (and their polarity is correct - they were not reverse charged). The rest of the cells were not warm. The light had not been on more than a minute within the previous ten minutes. No anomalies were noted in the operation of the light. The batteries were installed correctly in the holder. Resistance checks have shown nothing out of the ordinary.

At this point I am not sure what happened yet. I am planning to measure the charge required to bring all the cells back full to see where the energy came from. Perhaps it is just a bad cell or two, or high resistance between the two positions for some reason. I should check the holder's impedance between the two positions, but it may be too low for my meter to measure accurately enough.

Hypothesis

Possibly inadequate connection pressure, impedance in the holder or bad cells may have caused significant heating in the two cells. (Turned out not to be the case).

The Rest of the Story

See below. Post #5 in particular.

Lessons Learned (or Reinforced)

Battery holders require constant vigilance to insure safe operation.

Pay attention to the heat coming out of the battery area. If it does not fit your expectations, take action.

Make sure all cells in a holder are equally tight - lower contact pressure on some cells can cause significant heating due to increased contact resistance. This is especially important in cell holders that have four columns of cells - two or three columns tends to equalize force between the columns, whereas four columns have a tendency to have more force on two opposite columns and less on the other two.

Make sure the interior of the battery tube is smooth enough to avoid damaging the cell's external insulation.

 

[LuxLuthor]

Sounds like a short developed. What kind of cells & holder?

 

[Alan B]

Sounds like a short developed. What kind of cells & holder?

The 8xAA 2D series holder is FM. The cells are AA NiMH low self discharge type. Seven of the cells are Titanium Power Enduros, and one is a Uniross Hybrio. The WA1185 draws about 3 to 3.5 amps at these voltage levels. The cells are similar in type, chemistry and capacity.

The cells were all fully charged and rested, and there is zero sign of shorting that I can find. No melting, smoke, fully discharged or swelled cells, light failing to operate, etc. To short out these two cells is difficult, there is no easy path to short just these two. The two cells were the lowest in the voltage stack, starting at the negative terminal.

I have reassembled the pack and left it out of the flashlight all night. No problem. I am most interested in the contact resistance, but my meters are not really capable of detecting that level of resistance accurately. The contact between the two cells is a rivet type appearing to be made of brass and it has considerable cross sectional area. I doubt if the problem lies in the contact itself.

However there is an inherent problem in the contact pressure distribution in a four column battery holder. I now believe this is the root cause of the problem here, potentially exacerbated by a slight difference in the length of the cells.

The two cells that heated up were the Hybrio and one of the Enduros. They are slighty less tight in the holder than the other cells. They are not loose but they are not as tight. I am recharging them now, I will measure the length of the cells later.

One might immediately say that mixing cells caused the problem, but what is really important here is cell length (even cells from the same manufacturer may vary slightly in length), and the length of the cell channels in the holder between the contacts. This holder is very stiff (as many are), and there is very little compliance for cell length difference. Even the manufacturing tolerances on the holder itself will make the stacks have slightly different compliance ranges.

This is much less of a problem in a three stack type holder design since the forces tend to equalize pressure on all three stacks. In a four stack design the pressure can rest on twp of the stacks and the other two have much less, a teeter-totter effect.

It is surprising that this would be enough to explain the heating that took place, however perhaps there are other contributing factors such as the internal resistance of the cell increasing with temperature causing a runaway thermal condition. I have not looked into those details. The Hybrio seemed less hot than the Enduro, but no precise temperature measurement was done.

 

[Alan B]

Cell length measurements:

Hot Cells:

50.63 Enduro
50.55 Hybrio

Other Cells (all Enduro)

50.67
50.63

Note: this measurement taken includes heatshrink on one caliper arm to prevent shorting the cell. This adds 0.28mm to each measurement. This extra is included in the measurements above, they are direct from the Mitutoyo digital calipers.

Four of the non-hot cells are charging now. The two hot cells were charged in slots 1 and 4 of a Lacrosse BC-900. They took about 1450 mAH each, but the detailed data will be presented later when the next four cells have completed charging. One of the next four cells has completed at about 500mAH. This is getting interesting...

 

[Alan B]

More data. Four of the six non-hot cells have completed charging:

424 mAH (slot 1)
827 mAH
681 mAH
705 mAH (slot 4)

Hot Cells:

1473 mAH Enduro hot cell
1426 mAH Hybrio hot cell

The cells were fully charged a few weeks ago, not just before use. The Enduro cells were purchased in December and charged then. Some have been recharged since, so there is not a consistent starting charge. It may have differed more than I realized. They were all in my "fully charged" storage. The last two are in the charger now.

I double checked the cell from slot 1 and it had a higher voltage than the other three when they terminated. It does appear to be full. Of all of them so far, it is the closest to what I would have guessed the charge state would be based on the limited used of this newly assembled light.

This does look more like a short of the two hot cells. One mechanism for a short that occurred to me is these two cells were at the bottom of the series stack, so a short from the top of the higher voltage cell to the M@g wall would cause this problem. There are a few flecks of aluminum around, presumably from the boring, but nothing that appears to be large enough to cause a short.

Hmmm. Another thought. The third cell in the series could have had a can to wall short. This would have shorted the first two cells. I can no longer tell which cell this was but I will inspect all cells for outer insulation damage.

Based on this inspection I now suspect that is the culprit. One cell has some cuts in the cell's outer insulation that appear to be slices through to the can. Another has similar marks that are less deep. Perhaps the slight ridges remaining from the boring were enough to damage the insulation. It is hard to reach down that far and feel it to be sure, but the boring is slightly rough, and lathe cuts make tiny ridges.

Length measurements of these four:

50.66 mm Enduro
50.57 mm Enduro
50.54 mm Enduro
50.54 mm Enduro

Dummy cells are often used in these packs, how long are they? Here is one sample:

50.68 mm dummy cell length

Even though the length does not appear to be the root cause of this problem, it is a potential issue in these four-stack holders. Keep a close eye on it.

So at this point it looks like the #3 cell's outer insulation was damaged. The interior of the M@g tube is very slightly rough, but to do this the battery pack would have to rotate in the tube. This may well occur when the M@g tailcap is being installed due to the very strong spring and the single small contact point at the battery positive on the front, so the pack is likely to rotate. I don't believe this damage could have occurred if the battery pack had not rotated. Thee are several small slits "sawed" into the battery outer covering, reaching the negative can underneath.

Luckily, no major damage done. I'll have to figure out how to polish the interior out a bit.

Lessons:

Make sure the interior of the light is smooth enough to avoid damage to the battery insulation.

Make sure the battery insulation is in good shape.

 

[Alan B]

Sounds like a short developed. What kind of cells & holder?

Looks like you were right, Lux. It just was not easy to find this one.

 

[Alan B]

Last two cells completed charging:

419 mAH
396 mAH

This is more consistent with my expectations of a fairly small amount of charge from the batteries. It looks to me like the Hybrio and four of the Enduros were similarly near full charge, and three of the Enduros were were rested for much longer and at a lower charge state to start with.

These last two cells have no indications of insulation damage, so just two cells show that phenomena.

Inspecting the two hot cells shows the Hybrio has some very small scratches that fit the same pattern. So three cells show some sign of this.

It appears that the insulation on these cells is quite thin. I'll have to keep a close eye on this in the future.

 

[Alan B]

I have polished out the bore with 400 grit wrapped around a 1" pvc pipe chucked in the lathe, holding the M@g by hand. We'll monitor it closely and see how it goes.

The insulation on these NiMH cells is surprisingly thin. Keep an eye on it.

Be Safe!

 

[SilverFox]

Hello Alan,

I am glad you were able to figure this out. I also have had some close calls with some cells having damaged heat shrink. I think they are using thinner heat shrink because they are trying to get a little more inside the cell and the thinner heat shrink allows them to slightly expand the diameter of the can.

Tom

 

[Alan B]

Thanks, Tom. The insulation is thin, and not particularly sturdy.

Last night I cut some 24 pound paper and rolled a tube to fit in the body tube for additional protection. Perhaps I'll see if I can find some mylar to put in there, or similar. That would be easy and provide some real protection.

 

[Mr Happy]

I think an unbored Mag, in addition to having a very smooth interior, also has anodizing on the inside that insulates it. I think the removal of this anodizing could be noted as a general risk from boring out lights.

 

[Alan B]

I think an unbored Mag, in addition to having a very smooth interior, also has anodizing on the inside that insulates it. I think the removal of this anodizing could be noted as a general risk from boring out lights.

I would agree, in certain circumstances. The rough surface finish of a bored out body tube is a threat to SOME battery pack cells (the ones that touch it). The insulation from the anodize is a small insurance policy in case a scratched cell touches the wall.

Many battery packs hold the cells away from the body tube, such as 3-stack AA holders, and these have no risk of cell insulation coming into contact with the inside of the tube. Therefore the surface finish or interior anodize do not matter with them. Others like the 4-stack unit here place the cells into contact with the inside of the body tube. Then these factors matter.

The risk comes from those adapters that place the cells into contact with the inside wall of the body tube. The risk is increased when the battery carrier can rotate and grate the cell against the interior wall.

In these cases it is important to provide a good surface finish and prudent to add a replacement insulating layer such as mylar.

 

[Alan B]

This is probably the cell that was in position #3, so the current went through the can to the flashlight tube. There appears to be some sign of heating in the scratches:

 

[SilverFox]

Hello Alan,

Right through the warning on the label...

"Do not ... mutilate batteries..."

That does look like the problem.

Tom

 

[LuxLuthor]

Right through the warning on the label...

"Do not ... mutilate batteries..."

The irony is palpable.

Personally, I think it is a mistake to rely on the electrical insulation of anodizing--too many scratches, nicks, abrasions. I also find that the slotted grooved mags (like FM & others have done) often have sharp edges on sides and/or ends that can damage cells even if carefully inserted. Tightening down the tailcap introduces a new potential tearing force. Most battery labels are 1 mil thick, where most shrinkwrap is 3-5 mil. In addition, some battery holders are not properly designed.

 

[hopkins]

Glad you caught the problem.

Someone in the past said they had cut up a 2 liter soda bottle to get a piece that would slide in to insulate the inside of their mod'd Maglite.

Not sure if you have the clearance for that.
Maybe paint the inside with some Varathane?

 

[Alan B]

Glad you caught the problem.

Someone in the past said they had cut up a 2 liter soda bottle to get a piece that would slide in to insulate the inside of their mod'd Maglite.

Not sure if you have the clearance for that.
Maybe paint the inside with some Varathane?

Thanks for the comments.

I finally cut a rectangle from an inkjet overhead projector slide and put it inside. Come to think of it, I should print something on it. Anyway, it is working well. Now it is waiting for a new hotwire regulator prototype to complete the light.

Soda bottle plastic would work fine if it is not too thick.

Varathane would also work pretty well but it would be a challenge keeping it in the right place and out of the wrong places. The threads and the switch area would be interfered with if the coating went there.

I'm quite satisfied with the slide material. It is pretty thick and tough and very easy to use. It has one surface that is slightly rough to take inkjet ink, and this goes on the outside to grip the surface helping to keep it in place. The slick surface goes in and the battery pack fits well.