Umbilical canister became pressurised during dive - why?

[little nick]

Why would the canister pressurise?
The light worked fine, no signs of a problem or overheating of the battery. Canister was dry - no leaks?
The torch is a Diveright slimline canister, 12v, 4.5Ah NiMH to a converted XML head?

The toggel boot was blown up like a balloon, & canister hissed when I undid latches.

I could understand it if there was a short - but there did not appear to be?

The battery was flooded many months ago - but it has worked fine since (100 times)

Any ideas?
Nick

[LocoDuck]

Change in ambient pressure from elevation or weather? Change in internal pressure from temperature?

[Packhorse]

What Loco said,
+
Off gassing of the batteries.
My understanding is that they (NiMh/NiCd) often do this when being charged. I'm not sure if they do this on discharge but if you do a deep discharge then it is possible for one or more cells to drop so low that they in fact start "reverse charging" from the other cells. This is not good. A good test for this is to measure the voltage of each cell after use and compare the voltages.

[350xfire]

You can always do a dive without batteries and see what happens.

[little nick]

Well - I ran it for 90 minutes out of the water, and it does the same. No temp change & it built up enough pressure to swell toggle boot like a balloon!

as pack said - never heard of batteries off gassing during use - so I guess one of the pack is shot.

Hopefully I can change the single cell.

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[DIWdiver]

Agree with previous posts.

Unless changing elevation, ambient pressure change of 0.5 psi is a lot without a major storm system wandering about. I doubt that would bulge the boot much. Must be offgassing. Check each cell voltage after discharge; if one is less than 0.8V, suspect that cell and check it during discharge. If it is being reverse charged it will be less than zero, but may be at or above zero if you stop the reverse charge.

Also, if it is an offgassing problem, it will only start after the offending cell is fully discharged. So if starting from a charged pack, it should take a while before the pressure starts to build.

Also, be careful as that gas coming from the cell will probably be perfectly explosive 2H2+O2 mixture (though mixed with the air in the canister). You don't want to create a spark inside the canister, but as soon as it's open and the battery pulled out, the gas will quickly dissipate.

[little nick]

Looks like a number of cells are shot
I did 4 sets of voltage tests, fully charged, no load, fully charged - loaded, discharged - loaded, discharged - unloaded (when I say discharged - not fit was not fully, the LED started to flicker!)

It is a 10 cell, NiMH pack, 12 months old, flooded once. Fully charged: 13.77v, discharged 7.2v (5 hour run time)

cell	V (charged)	V (charged)	V (discharged)	V (discharged)
	no load	load	load	no load
1	1.38	1.31	1.10	1.12
2	1.37	1.23	0.89	0.37
3	1.38	1.30	1.09	1.11
4	1.38	1.30	1.00	1.01
5	1.38	1.29	1.09	1.10
6	1.38	1.31	1.06	1.08
7	1.38	0.91	0.06	0.93
8	1.38	1.13	0.11	0.91
9	1.37	1.27	0.10	0.80
10	1.38	1.34	0.08	0.97

So I guess cells: 2, 8,9 & 10 are dead.

It's probably a complete new battey pack then!

[DIWdiver]

Those cells aren't really 'dead', just weaker than the rest. This is the way packs fail - gradually.

It sounds like you use the pack often, so there isn't an issue of the cells needing to be cycled, but maybe they need to be balanced. That isn't a term normally applied to NiMH, but what I mean is that if your charger cuts off early because some cells are fully charged and give the -dV/dT signal, the other cells may not ever get fully charged. A cycle or two at C/10 charge for 16 hours, then discharge might restore some capacity to your pack. You mention a 5 hour discharge, is that less than you are used to?

Since none of the cells were reverse polarity under load after a long discharge, there aren't any seriously bad cells. Sounds to me like this pack is tired, not dead. It will probably never be restored to its previous glory, but it might be perked up a bit.

[little nick]

Since none of the cells were reverse polarity under load after a long discharge,

Thanks for a very usefull post. I must confess - I did not check the polarity during the test - so there might have been reverse polarity going on. I can re do the test.

Please excuse my ignorence though ( I am mechanical - not electrical) - the cells must be quite bad for them to off gas like this during use? If I charge as you suggest C/10 ( I assume this meens 1/10th of the normal charge rate) do you think this would stop the off gassing during use?

[yorkie_chris]

If I charge as you suggest C/10 ( I assume this meens 1/10th of the normal charge rate)

I believe it means 1/10th capacity... so a 7000mAh cell, you would charge at 700mA.

[AnAppleSnail]

Thanks for a very usefull post. I must confess - I did not check the polarity during the test - so there might have been reverse polarity going on. I can re do the test.

Please excuse my ignorence though ( I am mechanical - not electrical) - the cells must be quite bad for them to off gas like this during use? If I charge as you suggest C/10 ( I assume this meens 1/10th of the normal charge rate) do you think this would stop the off gassing during use?

Cells* discharge gas when reverse-charged. The gases released from NiMH come from the water in the cell (Danger: Oxygen and hydrogen!). During normal charging and use at normal temperatures, they will not release much gas. Normal charging keeps the oxygen and hydrogen contained, and normal use does not release them. As mentioned above, you charge at 1/10th of the cell capacity for 16 hours to do a 'trickle charge.'

IE: 2 amp-hour Eneloops trickle-charge at 200 mA for 16 hours. A 10-amp-hour D cell NiMH would like to charge at 1 amp for 16 hours, but 500 mA for 32 hours would work almost as well if your power supply is limited. NiMH cells top off at about 1.4v in trickle charging, and charged cells in a pack will dissipate the extra current as heat, while the less-charged cells will continue charging. If you wind up with a high discharge rate, monitor cell heat - your strong cells CAN be hurt if they cook in a trickle charger. A room fan is usually plenty to cool off charging NiMH packs - just be sure of it.


*Li-Ion cells do more exciting things when reverse-charged, or trickle charged.

[fyrstormer]

It seems odd that a sealed battery canister wouldn't have an over-pressure vent. A simple pinhole covered by a rubber flap would prevent water leaking in while allowing positive air pressure to leak out.

[Packhorse]

It seems odd that a sealed battery canister wouldn't have an over-pressure vent. A simple pinhole covered by a rubber flap would prevent water leaking in while allowing positive air pressure to leak out.

A proper OPV ( Over Pressure Valve) is a lot more complicated than a pin hole and a rubber flap.

[little nick]

A proper OPV ( Over Pressure Valve) is a lot more complicated than a pin hole and a rubber flap.

True

Untill recently the canister never pressurised, when taking the lid off to charge - it was always fine! Although I have not had time to test for reverse charging - this must be the case, to cause the gas build up. I have ordered a new battery pack (£££)

Is it wise to replace a couple of the tired cells in the exsisting pack (cell 2 & 7-10) or would the old cells drag the new ones down - ie I'd be throwing good money after bad?
I can probably trickle charge the old pack: 12v, 4.5A, so 0.5A for 16 hours, I think I have something that will do that, as the OEM charger finishes after a few hours.

This will / could increase the capacity of the pack - but I guess won't help if any of the cells are that bad they are reverse charging? Have I assumed right?

[fyrstormer]

A proper OPV ( Over Pressure Valve) is a lot more complicated than a pin hole and a rubber flap.

I was merely describing the simplest solution.

[Packhorse]

I was merely describing the simplest solution.

Its not really a solution. It may solve the symptom but not the cause.


But your standard cannister that has a clamped on lid does have a OPV. The higher the internal pressure the more the lid is pushed off the can. At a certain point it will extrude the Oring.

[little nick]

I don't have a smart charger, but I have found an old wall wart, 12v, 500mA - designed for a car battery booster pack. I just need to change the connector & we will see if I can get life back in the old pack!

Thanks

[little nick]

Its not really a solution. It may solve the symptom but not the cause.


But your standard cannister that has a clamped on lid does have a OPV. The higher the internal pressure the more the lid is pushed off the can. At a certain point it will extrude the Oring.

Of in my case - blow up the toggel boot!

[DIWdiver]

True

Untill recently the canister never pressurised, when taking the lid off to charge - it was always fine! Although I have not had time to test for reverse charging - this must be the case, to cause the gas build up. I have ordered a new battery pack (£££)

Is it wise to replace a couple of the tired cells in the exsisting pack (cell 2 & 7-10) or would the old cells drag the new ones down - ie I'd be throwing good money after bad?
I can probably trickle charge the old pack: 12v, 4.5A, so 0.5A for 16 hours, I think I have something that will do that, as the OEM charger finishes after a few hours.

This will / could increase the capacity of the pack - but I guess won't help if any of the cells are that bad they are reverse charging? Have I assumed right?

I would recommend replacing the whole pack. You've got a lot of cycles on those cells, you wouldn't want to rebuild it just to have more cells fail in a little while.

Yes, 0.5A/16 hrs charge may add some capacity. Or it may not. It will at least get you the best possible charge. In any case, if you are discharging it to the point where cells are offgassing, it's not long for this world.

Normally during charge and discharge there is a small amount of gas produced, but this is re-absorbed and never vents. When the cell is fully charged, but still being charged, the rate of gas production increases a lot, but at C/10 it is still low enough to be re-absorbed. That's what should happen during toward the end of a 16 hour charge. If you fast charge, or reverse charge, then the rate of gas production exceeds the cell's ability to re-absorb it and it vents. This is bad because it's the water from the electrolyte that's escaping. Remember when we had to 'top off' our car batteries occasionally? That was the same thing happening. That's why proper charge termination is so critical when fast charging, and not when slow charging.

Proper charge termination depends on the cells being reasonably well matched. That's another good reason to replace the whole pack. If you were talking one cell, maybe. But several? Not for my time/money.

[H20doctor]

ive had the same thing happen in my lights , the culprit was not letting the battery pack cool off after charging up.... i put a warm pack into the canister and went diving.. so now I make sure the pack has cooled down for several hours before i use it for diving..

[little nick]

Well good news!

c/10 for 16 hours worked - it ran on the bench for 2 hours with no pressure build up!

Thanks to all your advice !

[DIWdiver]

That's great!

Now as long as you don't discharge it too deeply, it should last a while longer. If you have time, that's the best way to charge it in general.

[little nick]

That's great!

Now as long as you don't discharge it too deeply, it should last a while longer. If you have time, that's the best way to charge it in general.

This is teh way forwards for me - I'll only use the fast charger if I need it (1-2 times a year) & use the wall wart (.5A) for the rest!
Now I have 2 battery packs - so it should never be a problem.

The wall wart - although it claims 12v, is putting out 18v which seems a little high to me. I might need to invest in a smart charger.

[DIWdiver]

If it's really only putting 1/2 amp, it doesn't matter what the voltage is. The cells may be as high as 1.5-1.6V each during charge. But if the current is limited to C/10, you're good until the cells fail.

[little nick]

Thanks - you have been a great help. I assume if will be about .5A - but I can't test it.

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