Emergency lighting unit battery .....NiCd is better than NiMH?

BTW i'd like to see the data sheet on the NiCd's where they state the 5 year life spec, thanks for mentioning that.

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Here is the datasheet, its nicd, which have huge amounts of test data for emergency use, because theyve been used for it for years.
Nimh datasheets never give lifetime in years, -but always in number charge/discharge cycles, which is irrelevant for emergency use which will have few charge/discharge cycles over time.

The first thing the customer asks when they want an emergency lighting battery is "how long will the batteries last"?

With nimh datasheets, this question cannot be answered....with nicd datasheets , this question can be answered.

Its just not possible to sell a emergency light battery to an installation without being able to give them an idea of how long the battery will last.

With nicd, you can give them this data, with nimh , you cannot give them this data.....because "constantly topped up" nimh batteries dont have lifetime data........and low selff discharge nimh's are not likely to be emergency capable....and in any case, "low self discharge" still has a significant self discharge, its nowhere near zero.............of course, most , if not all nimh datasheets dont even state what the self discharge is, so its a non-runner

Hi again,

I really dont think you are being very open minded about this topic. You seem bent on using NiCd's regardless of what anyone else suggests. Part of this i think is because you seem to believe that the NiCd's will last a long time with constant trickle charge and i dont see that as being realistic. NiCd's dont last as long as we would like to think at constant trickle charging because that means they are constantly being overcharged. So the best solution is always one that does not charge continuously, regardless of what battery chemistry you choose, and that takes just a little bit more of a circuit one way or the other for longest life.

Also, low self discharge really does mean "low self discharge" which means after 6 months there is still significant charge remaining. I know this for a fact because i've tested some of the brands of low self discharge cells because i NEVER take the manufacturers specs and recommendations as fact when it comes to rechargeable cells.

You also have not commented on Lead Acid yet. Lead Acid being used in the bigger emergency lights. Maybe you work for a company that makes and/or sells NiCd's

Why don't you just build a charger that pumps .3C into the battery when its below 1.3v and tapers off to .05C at 1.43 volts?

I have to agree that the decision seems to have been made to use NiCd's despite good evidence not to go that route.

I don't recall any discussion of the failure mode known to be common to NiCd's, namely shorting out from whisker formation.

That failure mode can't be predicted easily in the situation of an emergency lighting system (NASA equipped its batteries specially). When it happens, it can be unexpected and lead to almost immediate loss of the battery. Here is a US government presentation on it : https://www.electro-tech-online.com/custompdfs/2013/04/2006-cyganowski-presentation.pdf

Note, whisker growth is from the cadmium. I have had such failures in various pieces of NiCd-powered equipment, but never with NiMH.

John

Why don't you just build a charger that pumps .3C into the battery when its below 1.3v and tapers off to .05C at 1.43 volts?

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because for nimh, those voltages are not set in stone, the required voltage points vary with temperature , and from cell to cell. We would end up overcharging nimh, which nimh doesnt like.

So the best solution is always one that does not charge continuously

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...here is a nimh cell which is specially built for emergency use.....
https://www.electro-tech-online.com...313e84b71c5-ca47-4790-81a0-88acff6808a3-2.pdf

......its nimh, and requires constant trickle charge as the datasheet says......so i cannot understand why it is being said that constant trickle charge is not good...becuase even this cell, which is probably the finest nimh-for-emergency cell in the world, demands a trickle charge.

...notice from the cell datasheet that the "fast" charge requires a "charge termination" method.......that means the charger will be expensive, -more expensive than a charger for nicd.

Note, whisker growth is from the cadmium.

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..thats actually very interesting to hear about......but our nicd battery distributor sells emergency nicd's to companies all over the world and doesnt have this problem.
At the end of the day, our customers dont care.......most are only fitting the emergency unit because the regulations say they have to have it.....they are not particularly concerned if one or two cells fail here and there.........they just want that tick in the box which says they have passed the regulations, and cheap nicd allows them to do that.

....they certainly dont want a cell charging solution which has dv/dt or dT/dt monitoring etc........ but rather just C/10 for 24 hrs , then C/20 therafter...nice and cheap.

"C/10 for 24 hrs , then C/20 therafter" cannot be done with nimh because the C/10 for 24 hours would overcharge the nimh and invalidate the lifetime garantee.

Hello again,


Where do you read that that cell 'demands' a trickle charge? What cell in the world 'demands' a trickle charge? Just because they state two different ways to charge doesnt mean that they are the only two methods that can be used.

But i have a feeling all this doesnt matter because you are looking for the simplest possible charger without looking to improve the designs so that the cells last as long as possible. You are ready to accept NiCd with the simple charger and accept the life you'll get from them. That's ok i guess. If you want longer life however, you'll have to stop trickle charging and go to daily charging, which would mean a bit more circuitry.

I got a full five years of service from my sub C NiCd's charging twice per day with half the charge needed to (approximately) replenish the charge lost daily due to self discharge.

Flyback said:

[regarding whisker growth] ..thats actually very interesting to hear about......but our nicd battery distributor sells emergency nicd's to companies all over the world and doesnt have this problem.

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Then he must know something NASA and the US DOE don't know. Ask him if he will sell you cells with a $1 million guarantee each that they will not self short.

In other words, ask him for his data.

Somewhere in this thread, you stated in effect that you wanted facts to debunk using NiCd's. Well, when we provide facts, you counter with anecdotal information like that above. Please counter the fact of cadmium whisker growth with documented facts from a normally reliable source, not some NiCd salesman.

John

I used many Ni-Cads (American ones, not Chinese) many years ago before Ni-MH were sold and ALL of them became shorted with whiskers inside. I burned out the whiskers with a very high charging current from a big capacitor but they soon came back.
Since then I have used Ni-MH cells in the same applications for years and NONE developed whiskers.

NIMH does not require constant trickle charge.

and why does OP need them to be fully charged? nicd and nimh do not have problems with alpha lead sulphate turning into beta lead sulphate turning into gamma lead sulphate over a 2 day to 2 month long timeframe.

again, why not just turn off the charger at as low a voltage as 1.3 volts? this is 80-90% full. it will sit there and you should be able to float it at that voltage for 5 years with the cell only demanding the self discharge current to maintain that level.

Hee, hee.
The datasheet for the old Chinese Ni-Cad cell says it should be TICKLE-charged, not trickle-charged.
How many of us tickle batteries?

Hello audioguru,


Ha ha, that must be the Chinese to English translation side effects
Lucky for us they provide us with some humor even if not intentional.

I've had the same experience with the NiCd's. I've burnt out what we are calling 'whiskers' in this thread with a high current pulse too, from a capacitor just like you did. So you could say i gave them a BIG tickle
It worked very well but like yours my cells developed the same problem not long after that again so i gave up on those.

Personaly i wouldnt use either in that aplication. For me it was a small sealed lead acid battery, & a fixed voltage charger at 13.8V. Quite cheep & has been working for 5 yrs in a home security system ive built.

All this communication is from 2013. Can any of you update it ? Should we use NiCd or NiMh for emergency light?

My electricity in Canada is reliable so I cannot remember when it ever failed and I have never needed an emergency light.

Ni-Cad chemicals are toxic and fail soon so Ni-MH batteries have replaced them. Newer Ni-MH batteries hold a charge much longer than Ni-Cads.
A Ni-MH battery has a much higher capacity than a Ni-Cad even when they are the same size.

Of course a proper charger circuit (using a modern IC designed for Ni-MH) should be used.

Since the battery cell is a huge D size then the lights might be old fashioned high power incandescent instead of low power LEDs.

FastDude said:

All this communication is from 2013. Can any of you update it ? Should we use NiCd or NiMh for emergency light?

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NiCds have a memory effect, so not good if not used.

I replaced the NiCd's in cordless phones with no issues, just 3x of a capacity increase. Really decent life (up to 7 years) if used intermittantly.
Started using the phones to at least 1/2 discharged every day. That battery lasted about 6 months.

SLA (Sealed Lead Acid) seems to be the choice for UPS systems and alarm systems. You just need regular replacement about very 5 years,

audioguru said:

My electricity in Canada is reliable so I cannot remember when it ever failed and I have never needed an emergency light.

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Have you ever lived in the real world? - emergency lights are mostly to help with escaping if there's a fire, which kills the power - and are a legal requirement for many buildings (particularly public ones).

As for the NiCd/NiMh discussion, as I recall NiCd is the preferred option - I think because memory effect isn't a concern, as the cells are never discharged, unless they are called into operation. NiCd perform better than NiMh when keep on permanent trickle charge.

There should also be a regular replacement schedule for fitting new batteries, these usually come in an assembled pack, and plug in - although this obviously requires getting the exact same one from the manufacturer. You can buy standard packs though, and swap the connector over.
The advent a number of years ago of LED emergency lighting has really improved things, allowing much smaller and more efficient ones.

FastDude said:

All this communication is from 2013. Can any of you update it ? Should we use NiCd or NiMh for emergency light?

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Hi,

I have to agree with Nigel 100 percent.
NiMH need a very specific type of charger to charge correctly and get long life. NiCd are more tolerant of long charge times as long as the current is not too high.
In an application like an emergency light, it is hard to determine when exactly an NiMH cell needs to be charged and trickle charge is not a good option unless you dont mind replacing the batteries every 6 months maybe.
Now it may be possible to long-pulse the trickle charge of an NiMH. That is, trickle charge for 2 hours per day but no more or something like that. That may work but i've only done this with NiCd cells. It works very well for those cells. All it takes is a small microcontroller and a few other small parts.

Nope, I do not visit public buildings at night. If my home catches on fire then I can exit at 3 doors and many windows.
Modern (for at least 5 years) Eneloop type of Ni-MH batteries made by many companies today hold a charge for one year. Ni-Cads don't. A modern charger can be set tor a recommended low trickle charge so that the battery life is 5 years.

audioguru said:

Nope, I do not visit public buildings at night. If my home catches on fire then I can exit at 3 doors and many windows.

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Presumably you visit public buildings during the day?, are all areas fully lit by outside windows?, would those windows help you to see through smoke in case of a fire?, do those windows somehow magically point out where the fire exits are?.

You've certainly been within short distances of a GREAT many emergency lights powered by NiCd batteries.

Modern (for at least 5 years) Eneloop type of Ni-MH batteries made by many companies today hold a charge for one year. Ni-Cads don't.

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They don't need to hold a charge for a year, as they are on permanent trickle charge - something NiMh won't tolerate.

NiMh certainly have many advantages over NiCd, but emergency lighting is one area where they are much inferior - and is probably the only area where NiCd are still used?.

Energizer and a Japanese Ni-MH battery manufacturer say that the modern Ni-MH cells last for 5 years with the recommended low trickle charge that most charger ICs can provide, or last for 5 Years without a trickle charge when they are charged each year.

I did some work with portable microphones that were powered by Ni-Cads before Ni-MH was invented. Most of the Ni-Cads became shorted within a couple of years.

They built a sealed bank that had no windows so they used emergency battery-powered lighting. All the doors had electric locks. But they needed to program the battery-powered computer to open the doors during a fire alarm. So the bandits set off a fire alarm, went in and stole lots of money.