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

[Flyback]

Are NiCd batteries superior to NiMH batteries for emergency lighting unit use?..........


the following seems to suggest that it is...do you agree?

As discussed, i was looking into NiMH vs NiCd cells for our Emergency Lighting Unit......
Unfortunatley, NiMH is not looking suitable for Emergency use, because Emergency use requires the battery being charged up and then staying charged up by being continuously trickle charged.....NiMH cells cannot generally tolerate being trickle charged at a low rate without their capacity lifetime being too badly affected.
NiMH is smaller than NiCd, however, for a similar capacity (4000mAh) we would need a SAFT "VHT 7/5 Cs" NiMH cell, which is the same length as our current NiCd D cell (60mm). However, the NiMH "VHT 7/5 Cs" cell is slightly thinner at 22mm diamater, compared to our current NiCd's 33mm diameter.

NiMH , "VHT 7/5 Cs" Datasheet (SAFT):-
https://www.electro-tech-online.com...-061220e40a20-b014-4c8f-ae5f-cb6a5cd0298d.pdf


Its worth noting from the datasheet, that a "trickle" or "permanent" charge rate is not offered for this "VHT 7/5 Cs" NiMH cell, and this is likely to be because NiMH's cannot handle being trickle charged well.
However, SAFT do say that they are able to make custom built NiMH batteries for Emergency Use.
The only off-the-shelf SAFT NiMH cells whose datasheet offers a "trickle" or "permanent" charge rate , are those NiMH's whose cell size is 'D' and above. -(Though our current NiCd cell is a 'D' size).
The NiMH "VHT D" cell from SAFT offers a trickle charge rate, and four of these D cells would be needed for the Emergency light unit, instead of the current five NiCd 'D' cells being used. (-so the NiMH solution would mean saving the space of just one D cell (60mm high by 33mm diameter).

NiMH , "VHT D" Datasheet (SAFT):-
https://www.electro-tech-online.com...-0313e84b71c5-ca47-4790-81a0-88acff6808a3.pdf

Other reasons for using NiCd over NiMH for emergency use are:-
#Lower self discharge rate.
#Better recovery from deep discharge such as may occur in prolonged mains outage
#Cheaper
#NiCd can generally be used with cheaper chargers..The NiMH cells cannot be simply charged at double the trickle charge rate, and then trickle charged therafter....unless they have cell
temperature monitoring during the double-trickle charge rate. NiCd's do not need cell temperature monitoring during the C/10 (double the trickle rate) charge.
#NiCd has better high temperature performance than NiMH
#Individual cell monitoring, is less likely to be necessary for a NiCd battery pack, than a NiMH battery pack, because NiCd Is more robust than NiMH.
#NiCd cells can be C/10 charged at below freezing temperatures....NiMH cells cannot be, unless they are custom built to do this.
#Though NiMH cells do exist for emergency use, they are not that much smaller than similar capacity NiCd's, because the NiMH cells that are "emergency-capable" have to have thicker seals and
bodies so that they can handle the trickle charge....all in all, NiCd looks best, and is not that much bigger than "emergency capable" NiMH's.

Conclusion that NiCd is the way forward for Emergency Lighting Battery.

...is this all correct?

 

[MrAl]

Hi,

Yes NiCd's take the trickle charging better than NiMH's, but neither should really be charged continuously. The best way is to charge once a day for a short time period at the recommended normal charge rate which makes up for the self discharge.
Trickle charging both types works, but the two both will fail sooner than if charged once per day. The NiMH's will fail sooner, but supposedly the newer NiMH cells take it better than the older models.

But now there is also another alternative: low self discharge NiMH cells. These things can hold their charge many times longer than the normal NiMH cell, so you could charge maybe once every three months. They should last for a long time this way.

Another battery type often used for higher power emergency lighting is Lead Acid.

 

[schmitt trigger]

Only problem, is cadmium's toxicity, which may preclude its use in certain countries.

 

[Flyback]

Yes NiCd's take the trickle charging better than NiMH's, but neither should really be charged continuously

...allright in theory, but they do need continuous charge in practice as the emergency incident could happen any time, so the batterys should be constantly topped up and ready.

The best way is to charge once a day for a short time period at the recommended normal charge rate which makes up for the self discharge

The problem is........if you do this, how do you know when to stop charging, that is, how do you know when the batteyr is full again?.......the answer is that you don't, because such "shallow" charging will not give you significant dV/dt or dT/dt changes for you to know that its charged up fully........therefore, you can only assure constantly topped up by trickle charging constantly.

If you dont trickle charge constantly , then your battery over time, (maybe weeks or months) will simply get flatter and flatter.

low self discharge NiMH cells

These are only ever available as custom builds , and are extremely expensive. Also, due to their special design, they are about 40% bigger in volume than "Normal" NiMH cells.

Only problem, is cadmium's toxicity, which may preclude its use in certain countries.

Now this is THE reason.......the toxicity of cadmium.
However, every country in the world allows NiCd batterys to be used for emergency lighting applications.

Then again, regarding toxicity, fluorescent tubes have a toxic chemical in them, (i have forgotten what it is) but look at the ceiling and notice that LED striplights have not replaced fluorescent ones yet.

In any case, ALL battery chemistries need to be disposed of in recycle centres, so having to do it for NiCd is not a problem particular to NiCd.

 

[MrAl]

...allright in theory, but they do need continuous charge in practice as the emergency incident could happen any time, so the batterys should be constantly topped up and ready.

Your reply does not consider the entire post. Either trickle charge or charge once per day. Emergency charging is no different there and gets much better with low self discharge cells.

[Charging once per day]
The problem is........if you do this, how do you know when to stop charging, that is, how do you know when the batteyr is full again?.......the answer is that you don't, because such "shallow" charging will not give you significant dV/dt or dT/dt changes for you to know that its charged up fully........therefore, you can only assure constantly topped up by trickle charging constantly.

Not true. You find out what the self discharge is, calculate the required daily charge, go a little higher it's ok.

If you dont trickle charge constantly , then your battery over time, (maybe weeks or months) will simply get flatter and flatter.

You dont have to trickle, you can charge daily.

[topic of Low Self Discharge cells]
These are only ever available as custom builds , and are extremely expensive. Also, due to their special design, they are about 40% bigger in volume than "Normal" NiMH cells.

Im actually not sure what you mean by "custom builds". These are available all over the web for very reasonable prices. They fit in everything a regular cell fits in because they are the same size.

 

[Flyback]

Either trickle charge or charge once per day

for emergency lighting, you cannot charge nimh once per day, an nimh typically self discharges by 20% in the first 24 hrs after being charged up full.

You find out what the self discharge is, calculate the required daily charge, go a little higher it's ok.

..sorry but self discharge varies very widely with temperature and cell age...you cannot just put in a "Bit more", as you wont know what is a "bit more", and will end up overcharging the nimh, thus reducing its capacity lifetime.

Another point is, at low charge rates, the charging effiency is unknown for nimh, so therefore you wouldnt know how much charge to put back in to replensish a given amount of charge lost to self discharge.

Cadmium is a heavy metal and is banned the world over...except for emergency lighting use........this tells us that nicd is obviously massively superior to nimh for emergency lighting use.

As you know, Disposing of nicd is not a problem, because all batteries, regardless of chemistry, need organised disposal.....so the toxicity argument is nulled somewhat also.

Its Nicd all the way for emergency use, you agree now?

 

[MrAl]

Hello again Flyback,

You appear maybe to be working from old data. The older NiMH were not as good as the more recent models. I think this is what makes the difference. I dont disagree that NiCd's are good for emergency lighting, but i also believe that the low self discharge NiMH cells would be good too as they would not have to be charged very often in order to have a decent level of charge remaining when the time comes to use it. Surely you cant think that we are not able to charge once every three months and maybe even use one of the more comment end of charge termination methods like -dv/dt.

The newer batteries can take overcharge better than the older models. Not as good as NiCd's but better than they used to be. The low self discharge can hold a charge for a year before reaching around 75 percent of capacity. So we have better stuff to work with these days then we did say 10 years ago.

So although i do agree that NiCd's are good for emergency lighting, there are other ways too. Lead acid, Li-ion, it just depends what you are willing to do to get that lighting working well

I should also mention that i design almost all of my own battery chargers so it doesnt hurt me to make one a little more complex than we might find at the store. For example, i made one with a timer that would come on once per day and replenish the cells with the estimated self discharge lost that day. It's not perfect but it still works.

Funny though i never got around to building my own emergency light even though we have electrical problems around this area. I ended up buying one at Home Depot
And here's something that should make you happy: i think it has NiCd's inside

My ideal emergency light would have lead acid though, because that is the most economical for lighting that has to stay on for a long time. We get some bad power outages around here so i need long term lighting too. For now i connect when needed but it could be all automatic.

 

[Flyback]

Thanks

Surely you cant think that we are not able to charge once every three months and maybe even use one of the more common end of charge termination methods like -dv/dt.

For dv/dt charge termination you need your charge rate to be at least C/3.........and that means charger complexity, as opposed to a typical emergency lighting charger which charges for 24 hours at C/10, then trickles constantly at around C/20.

You speak of "low self discharge" nimh's, but the self discharge is still bad, and worse than it is for nicd. You cannot simply leave the nimh not charging for three months inbetween fast charges......in emegergency lighting you need to have the maximum battery capacity available at all times......there could be a fire or whatever.

Lithium has major problems for emergency lighting.........what if a long mains outage occurs?......so long that the lithium self discharges below 2V/cell....in that case that lithium cell would be unusable therafter.......lithium cells usually come sold with protection circuitry because of such things, -in other words, the prtoection circuit most often won't even allow the lithium to charge or discharge if the voltage goes below 2v/cell.
Also, lithium has appalling cold weather charge capability (and poor hot weather capability)......at low temperatures, (above zero deg c), lithium cannot even be charged or discharged.
Lithium is known to cause fires if not treated right....and so lithium is virtually never used in emergency lighting applications.

Most emergency lighting installations dont have an expert like yourself to mind over the batteries......so therefore they have to use the safest possible batteries that will work under all conditions.........

 

[schmitt trigger]

"You speak of "low self discharge" nimh's, but the self discharge is still bad, and worse than it is for nicd"

Wholeheartedly disagree. The newer low discharge NiMh are orders of magnitude better than any NiCd.

Just yesterday, I was able to take photos with a camera whose batteries had not been charged since November of last year. I know, because of the date on the previous pictures.
The batteries are low self-discharge NiMh.

I used to use NiCd or conventional NiMh previously, and in a similar circumstance, they would be totally flat.

 

[Flyback]

those nimh's are for camera use.....its very different to emergency use...where robustness and constant topping charge is a must.

 

[jpanhalt]

Agree with schmitt trigger. I switched to Sanyo eneloop NiMH more then 3 years ago. From last flight in October to first flight in May, they stay charged to about 90% or more.

I have limited experience with the other brands. I have used a battery from Walmart that was made in Japan, and it worked well too, but I have not used them in critical applications.

John

@Flyback
Don't know what you mean by "robustness." Certainly a flight pack in a fuel-powered model puts stress on a battery. As for topping off, if they last 6 months and retain >90%, why bother trickle charging them? I read somewhere about a bumping charging and will try to find it when I get home. Right now, I am mobile and can't do that. Maybe "Battery University?"

 

[schmitt trigger]

those nimh's are for camera use.....its very different to emergency use...where robustness and constant topping charge is a must.

I understand that.
I agree that NiCd have some advantages over NiMh in emergency lighting, but I'm only talking about the self-discharge.

 

[Flyback]

The problem with self discharge is that most cell datasheets (either nimh or nicd ) dont even say what is the self discharge.
eg

https://www.electro-tech-online.com...313e84b71c5-ca47-4790-81a0-88acff6808a3-1.pdf

Another problem with emergency use is what happens if the mains is repeatedly going off and on...its rare but it may happen......most emergency units are prgrammed to go into a C/10, 24hr charge every time they power up, so with repeated mains ON/OFF, the emergency unit is basically going to be permanently in C/10 charge , and will never reach its C/20 trickle charge rate.....

...with this kind of abuse, you really dont want nimh's...you want the most robust type of battery....i.e., nicd

I admit that this problem can be circumnavigated with a nimh solution, though it is very very expensive, and most customers do not want to spend a lot of money on emergency lighting.....most customers are only having emergency lighting because the building regulations say they need it....they really dont want to be spending big bucks on it...........we all want to be lit up whilst we exit a building if the mains fails, but most customers feel that 15 minutes of emergency light is ample for them to evacuate the building......however, the regulations usually quote 1, 2 or 3 hours of emergency light, which annoys customers, because it means more cost for them...........so customers really want a cheap and cheerful solution for emergency lighting systems....theyre already paying through the nose for it.....so they want cheap batterys that last a long time with cheap chargers.

the emergency lighting market is a "grudge" market, people are buying emergency lighting because they have to due to regulations...they dont want it, and most feel that having to pay for a system which gives three hours of emergency light is excessive....as i said, mst costomers want just 10 to 15 minutes of emergency light so they can grab the bag and get out of the building.

Ive heard of people wanting to design highly computerised and expensive emergency lighting systems.....the market really does not want to know....emergency lighting market and aesthetic lighting market are massively different.

 

[MrAl]

Hi again,


Yes good point about the power going on and off a few times before it goes off completely or stays on again. But most of the time this doesnt happen.

There are simple ways around this, for example -dv/dt sensing, but a cheap low end microcontroller would work wonders here. Run the microcontroller in low power mode waking up every 10 seconds, and each time incrementing a counter. That counter would represent real time. If the charge cycle had run sooner than 1 day or so, dont charge again. Im sure we could come up with a better algorithm though without too much trouble. Would be an interesting project
And the low self discharge cells would mean we would not have to charge much.
Remember also that just because the power goes down doesnt mean we dont have any power...we have the backup battery to use to keep the memory alive and the long term timer still ticking.

Yes we are getting a bit more complex here now, but the choice you are suggesting is to trickle charge the NiCd's indefinitely and that isnt a perfect solution either. It's just simpler.

I mainly need the light so that when the power goes out i can find my regular flashlights. Last time the power went out it was completely dark outside and so inside was dark too and it took me a few minutes to feel around for my normal light. Once i find it no problem, but the emergency light helps so im not totally in the dark to find that first flashlight.
But i can also easily envision a larger light with a bigger LED or LED's, and have it stay on for a while longer. I think the one i have now has about 5 hours normal light run time, but a couple days light would be nice too.

You know what else is nice about using LEDs for the light is that we can always dim the LEDs if the battery power gets too low. That will conserve power while allowing at least some light to remain on. Cant do that with bulbs because they draw more current (with respect to voltage) as the voltage goes lower and they put out very little light at reduced voltages.

 

[languer]

So based on all the conversation the title of the Topic is incorrect. It should've been "Emergency lighting unit battery .....NiCd is better than NiMH" not "Emergency lighting unit battery .....NiCd is better than NiMH?". After all, you are not taking any inputs, you're just plainly stating your point of view (with no room for anything else).

 

[Flyback]

you are not taking any inputs, you're just plainly stating your point of view

Actually i am looking for somebody to come forward and totally debunk me and spray me over the wall.

I take this desperate action because for four months i have been desperately communicating with battery manufacturers of nimh and nicd, trying to get them to give me details of cells which can give at least 70% capacity after 5 years, with a charging regime that needs a current(s) of less than C/10.
I wish to use a cheap charger, and no dv/dt or dT/dt as its more cost. However, i will accept a bit more cost if the nimh solution is less than 40% of the size of the nicd size.

I am finding that battery manufacturers give very little detail in there cell datasheets, and this prevents one from making a good decision, -it means that one is forced to go to a specialist battery solutions provider and pay them a fortune for their services.....services which we cannot do for ourselves because we simply dont have the information in the datasheets of the cells.

But the one shining thing that i do know is that a cheap nicd solution is on offer.....and it needs no dv/dt senseing, and no dT/dt sensing....and the charge current need never be more than C/10....and we can garantee the 70% after 5 yr capacity lifetime....and we can assure always being topped up full as constant trickle is allowed.

So for us, its nicd all the way...............i have this niggling suspicion that there is a nimh solution, however, the nimh cell datasheets simply lack the information that is needed for us to design a relevant charger and make meaningful lifetime garantees to our customers.

So its nicd, unless somebody can provide the nimh cell datasheet of an emergency capable nimh cell, with charging regimen, capacity garantees etc.

I doubt ayone can do this...its inside info...the battery manufacturer's application engineers are moonlighting as engineers at battery solutions providers, so the last thing they want to do is to provide info to customers directly ........they want your money from the battery solutiomns providers......who hold the lock and key to all the info.........info that the likes of me and thee can but speculate about...as we have done

I can find lots of nicd cells that give charging regimen for 5 year capacity lifetimes...........i can find not one single nimh cell datasheet that gives lifetime in years.....for nimh they give it in charge/discharge cycles...which is irrelevant to emergency lighting batteries......where is this info?...does it exist?...........well put it this way, i can pay a battery solutions provider a small fortune and theyll do me a system, but i dont have that money.

So i must stick with the one and only nicd.

 

[MrAl]

Hi,

Once you get up to the five year mark you're asking a lot from the cells. If they live at all count yourself lucky. I would suggest creating the build such that the user can replace the batteries after a couple years.
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.

So you could ask the question, "how long do i expect my newly purchased emergency light to function before there is a problem with the cells". I would answer, about a year. That's because i know that manufacturers dont care about anything after a year, certainly not batteries. Then if the question, "what to do after a year or so". I would answer, replace the batteries or rebuild the entire unit to work better AND replace the batteries



I totally understand your desire to create an emergency light with a simple as possible circuit. I've thought about this myself because of the local area electric problems. Because of that i offer another solution which is also cheap and simple. That is, Li-ion to the rescue

Li-ion can be charged very simply with an LM317 based charger and a wall wart, and a few resistors and maybe a small transistor. And the charging doesnt matter if the power goes on and off, it will charge yes but only for a short time at a low level current because the charge algorithm is based on the voltage of the cell, which is easily measured. The LM317 and a tiny transistor take care of the charging so you never have to worry about it overcharging or undercharging.
So the circuit looks like a standard LM317 voltage regulator set to output 4.15 volts, and the transistor and sense resistor sense the current so the current never goes over the required max charge for the cell, or better yet lower than that like one half of that.
With this circuit, when the power come on the cell charges up to 4.15 and as it reaches near that voltage the current goes lower and lower. If you dont want it charging forever at tiny tiny currents then a simple cut out circuit can be used to turn it off completely.

I guess you dont like Lead Acid then as you seem to want small size too?

 

[audioguru]

Modern Energizer Ni-MH cells hold a charge for one year and are made for them in Japan (by Sanyo?) so they might be Eneloop cells that are re-labelled.

 

[MrAl]

Hello audioguru,


That's interesting, are they labeled as low self discharge?

 

[audioguru]

I have a new package of Energizer AA Ni-MH cells. It says, "Holds power up to one year. Made in Japan for Energizer".
They come pre-charged.

The Energizer 9V Ni-MH battery is made for them in Germany.

Energizer makes their alkaline cells in the USA.