Are Nihm batteries dangerous?

[Arnaud]

I'm reading PADI's manual for night diving and, in the light section, they mention that using Nihm batteries in a dive light is dangerous because of the hydrogen that's produced. Yet, it seems that UK, among others, recommends them.

Does anyone have an explanation (in layman terms, please)?

Also, Nihm batteries seem to be the best rechargeable type out there. Do you guys agree?

[Dryglove]

I use a stealth light with a halcyon new style 10w head.My light uses NIMH batteries.Anyways here is an article about nimh batteries.

http://www.halcyon.net/lights/helios.shtml

[TwoBitTxn]

That actually should be Nimh, or Nickel metal hydride. The best thing about Nimh batteries is they don't take a memory, and they have a longer life span than NiCd (Nickel Cadmium). They are the preferred battery for rechargable lights.

Thats the most help I can offer.

TwoBit

[Arnaud]

Thanks, LAL. I've read the article but it doesn't mention anything about hydrogen emissions. I appreciate the upside of Nihm batteries, but how do they deal with the hydrogen being produced in a water/air proofed space? Unless Nihm don't produce Hydrogen and that was just some BS from PADI?

[madmole]

Just BS from Padi. I could understand it if they sold Torches

ALL batteries produce a gas when charging (lead acid, Nicad and NiMH). Most sensible manufacturers deal with this in by including a very small catalyst inside the torch that converts the very small amount back to water using the O2 in the torch. (thats why UK torches sometimes rattle)

As the torch can withstand 10's of bar pressure, the 0.001 bar of H2 inside wont make a difference

Anyway, unless you have a Kowalski the battery is charged outside of the torch, this is when the gas is given off

[Dryglove]

The primary gas emitted from the Nickel-Metal Hydride cell when subjected to excessive overcharge or overdischarge is hydrogen. Do not store a Helios light for extended periods of time with the battery pack in place and the lid closed. Always open the lid of the light to vent any ambient gasses after storage and before you operate the light.

Thats about all i could find .They dont seem to be to dangerous to me.At least i hope not since im using one ......lol.

[FLL Diver]

Arnaud once bubbled...
I'm reading PADI's manual for night diving and, in the light section, they mention that using Nihm batteries in a dive light is dangerous because of the hydrogen that's produced.

What version of the manual are you reading? I've just reread the chapter on Night Diving twice in the PADI Adventures In Diving manual (2001 second edition) and can't find the reference you cite. The only reference I see about rechargeables is that they hold their voltage until they need charging, so the ligth doesn't dim as the batteries weaken like disposables do.

Marc :jester:

[Genesis]

ALL consumer NiMH cells/batteries are sealed.

The chemical reaction inside the battery produces free Hydrogen when charged or discharged. In normal use this is absorbed and re-combined inside the battery; thus, the electrolyte is not lost. This electrolyte balance is critical for the cell's capacity and proper operation.

These "sealed" cells have an overpressure valve in them. In the event of gross overcharge or overdischarge (e.g. a short circuit inside the light) these overpressure valves can release as the gas is produced faster than the internal catalytic reaction can recombine it. Once they do the cell in question is destroyed, as the electrolyte balance is altered, the seal never completely and properly closes, and you need to replace that cell.

The bottom line is that in normal use there is no issue. If your dive light suffers a short internally underwater there is a non-zero risk of a hydrogen release. This is usually handled by a hydrogen catalyst in the battery compartment somewhere that is designed to be consumed as it absorbs the H2.

If that catalyst is overwhelmed by a catastrophic failure (or you simply don't replace it after a problem and its no longer there) it is POSSIBLE to get enough H2 in the battery compartment where it could ignite. This would be bad, but it is a very low risk, and that same risk exists for ANY of the common chemical cell (e.g. "battery") chemistries in use today (alkaline, NiCad, NiMH, Li-Ion, etc)

PADI is blowing smoke on this one (what else is new).

NiMH cells are nice because:

1. They have a flatter discharge curve than some other technologies; similar to NiCad cells (that is, the cell voltage is flat from full charge to nearly-full-discharge)

2. They tend to have an extremely high energy density (amp-hour rating) compared to other technologies.

3. They are not subject to the "memory effect."

4. They have EXTREMELY low internal resistance. This allows extremely large amounts of current to be demanded of them when required for short periods of time, which makes them excellent for use in things like cameras, strobes, etc.

They do, however, have some disadvantages:

1. Their internal self-discharge rate is relatively high. If you leave them unused but charged for a month, a third to half of their capacity may be gone! Thus you do need to pay attention to them; they are a poor choice for "standby" use where they are stuck in a drawer or piece of equipment and then called on at some future date.

2. Their internal resistance, being very low, makes them potentially dangerous if short-circuited. The extreme current levels they can produce means that they are quite capable of getting dangerously hot if shorted, and can "vent", releasing electrolyte and gas in that condition.

3. If discharged all the way flat to the point that a reverse-charge potential is impressed on them they are usually irrevocably damaged. It is therefore important not to allow that to happen and to turn off any equipment IMMEDIATELY at the first sign of voltage droop (e.g. dimming of a light, etc)

4. They tend to be expensive.

[Arnaud]

Thanks everyone for your replies. I just noticed that the manuel Sport Chalet sold me is from 1994! Apparently, the first version... Anyway, I think the answer to my question is that the hydrogen is emitted when the batteries are charging, not when they're used. The guy who wrote the manual back then probably didn't know much. It seems that Nihm is definitely the way to go.

[Iguana Don]

On most dive lights there is a small round plug somewhere on the light. All Pelican lights have one. It's a fail safe device in case of a hydrogen build up.

[Master Chief]

Thats not true, it may be some one talking about something they no nothing about. I've used Sartec HID lights for years and have never had a hydrogen problem, plus if there was a possible problem they would state it in the manuals and they would be liable. on the safe side when charging your batteris pull them out of the canister.

Master Chief

[WetDane]

most all the consensus here is right on the mark -

I built canister lights and have found that the NiMH are great as they hold the charge long. Any battery is ging to offgas during charging and discharge, you can't get around it - this is the reason you don't see a lot of sealed battery comparments...

when selecting cells, one always have to look at a number of things - for instance, not all NiMH cells are equal, you can use a high AmpHr Panasonic cell, vs, a lower AmpHr powerstream cell, but the PS may have a more even discharge voltage, meaning that in the long term, your light will burn longer before the voltage drops below what the bulb needs (this applies to HIDs - not halogen).

Anyway - there are no dangers from explosion of a NiMH battery in a scuba canister - there are other applications where you may have issues, but this is not one of them.

HTH,

Dane