I have three "power-off" lights that are dead because the batteries have died. They take two AA batteries which I want to replace. Which type would be the best - Lithium, Nickel-Cadmium, or Nickel Metal Hydride? They would be plugged in and constantly charging until the power goes off.
---MIKE---
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The type of battery depends on what the charger is designed to charge. You cannot just stick any battery into it and expect it to work -- especially lithium, as they can catch fire if charged wrongly. Each type of battery has a different working and charging voltage requirement. Replace the batteries with the same type the lights came with.
My experience is that Ni-Cd's are virtually worthless (don't hold enough charge). I have no experience with lithium ones. I'd recommend the NiMH's
HTH,
EJ > I have three "power-off" lights that are dead because the batteries have
Even if the charger is not designed for NiMH'S? Each type has its own charging requirements. A few chargers will charge multiple types, however, it is highly unlikely the charger for these special lights will accept multiple types of batteries. Putting the wrong type in will likely cause a fire.
Either NiCD or NiMH will be fine, since the things are always plugged in, get the highest capacity cells you can find. Charge time and self discharge rate are not really important. Any reputable brand should be fine.
Lithium AAs are disposable, never attempt to charge one. Lithium rechargeable cells are completely different electrically from NiCD and NiMH, the latter two being virtually identical. Li cells are more than double the voltage of the other chemistries, and require intelligent charging circuits to avoid damage or fire.
These lights use a simple "dumb charger", that is the charge current is limited by a resistor. NiMH and NiCD are close enough electrically that they are interchangeable in most situations. They will undoubtedly withstand a higher charge rate than the light provides, but that isn't an issue. Batteries with a higher mAh rating will take correspondingly longer to charge, but with something that is plugged in all the time that isn't really an issue.
It will not cause a fire.
These lights use a simple "dumb charger", that is the charge current is limited by a resistor. NiMH and NiCD are close enough electrically that they are interchangeable in most situations. They will undoubtedly withstand a higher charge rate than the light provides, but that isn't an issue. Batteries with a higher mAh rating will take correspondingly longer to charge, but with something that is plugged in all the time that won't matter.
It will not cause a fire.
Ernie Willson wrote in news:494666b2$0$5536$ snipped-for-privacy@news.newshosting.com:
if you examine Digi-Key's catalog,there are NiCd cells specifically made for continous charging.
NiMHs require monitored charging.
Monitored charging is better for any battery, but not required. My cheap cordless phones all continuously charge the batteries, and they came with NiMH AAAs. The key is to charge at a low enough current, if you fast charge either continuously they will vent and fail early, but a gentle trickle charge is fine. These cheap emergency lights don't come with anything special, they come with the cheapest Chinese NiCD cells available and virtually anything you find on the shelves will be an improvement.
---MIKE---
RTFM and follow. Really. My first suggestion would be NiMH, as I have good luck with them on my cameras and they save a lot on disposable alkalines. But each, alkaline, NiCd, and NiMH have different operating characteristics related to slow draws, the curve when they pass a certain use point, and just things individual to each. I'm sure that the directions may clearly state that you could use any kind, or specifically if there is some type you should NOT use. I'd trust the manual more than anyone here.
Even me.
Especially me.
Steve
Huh? NiCd and NiMH identical?
I don't think so, Tim.
Steve
On the contrary, you should do a bit more research, or maybe just read my post more closely, as my statement of charge time not being important reflects the specific condition of trickle or float charging. This isn't really the best forum for this though, might wanna pop on over to one of the EE groups, or candlepower forums, lots of discussion with a lot of very knowledgeable people, both professional engineers and hobbyists/enthusiasts.
Much debate has raged over the years on the topic of NiCD memory effects, and to this day it remains controversial as to whether the memory effect exists. My own experimentation suggests that the perceived memory effect is due to poor quality chargers that severely overcharge cells in the specified time unless they are discharged first, leading to a dramatic reduction in lifespan. With any charger, there is a trade off of charge time vs cell life and people like things to be fast. Most inexpensive chargers have no onboard intelligence, they are simply a constant current source and rely on the user to manually terminate charging. If you want to charge a battery in a short amount of time with one of those, you had better discharge it first so you know where you start out, and then charge for a specific amount of time to prevent overcharge. Better chargers are timer controlled and will charge at a constant current of a few hundred mA in order to charge the batteries relatively quickly, then revert to a 5-10mA trickle charge to maintain them at a fully charged state, these will also overcharge if the batteries are partially charged when you start out. The best chargers are microprocessor controlled and monitor one of two things, cell voltage vs time, or cell temperature rise to detect a fully charged state and drop to a low current trickle charge. They are nice because they are capable of individually charging multiple cells to full capacity rather than charging groups in series and assuming all started out the same.
Cheap items such as the emergency lights in question have very simple chargers, they are a constant current source with no feedback, they charge at a very low current which is essentially a trickle charge and they do so constantly. The trade off here is long charge time, usually close to 24 hours, but that doesn't matter since power outages usually have more than that much time between them, as well as a somewhat shorter cell life, given the resulting low cost of the item, this is deemed acceptable. On a related note, years ago I got tired of the backup battery in my digital clocks always being dead whenever I needed it so I did a simple modification and installed a 1K resistor across the isolating diode for the backup battery resulting in the power supply in the clock providing a steady 3-5mA of current to a NiCD "9V" (really
7.2V) battery. This has been working great, and more than 6 years later the original batteries in the four clocks I modified are still working great despite being trickle charged steadily for those years.In a nutshell, at high enough current level to charge a battery quickly, 180-500mA or more being typical, charge time is very important and once the cell reaches full charge that energy will go into heating the cell rather than the chemical reaction that stores energy. If you keep that up, the electrolyte will vent and permanent damage will occur. On the other hand, if you charge a cell at a low current of say 5-10mA, you can charge it indefinitely without damage.
I stand by my claims that NiMH and NiCD are virtually identical from an electrical standpoint and the differences only become important when you're trying to charge them quickly or power loads that draw a very high current. NiMH cells have a higher capacity generally speaking, while NiCD cells have a lower internal resistance and are thus able to dump that charge more rapidly. They both have a nominal cell voltage of
1.2V, they both have a recommended standard charge current of C/10, where C is the mAh rating of the cell, and they both need to be charged by a current limited source with an OCV higher than the nominal cell voltage, and both can be trickle or float charged indefinitely at C/100 or so. This is based on my education, research, and years of personal experience. If you insist that this is incorrect, please cite sources.
No one has mentioned the memory effect of NiCd batteries. They should be pretty well discharged before recharging because if you don't, they develop a memory, and will not fully charge. NiMH do not, and you can charge them even if you have only used them a little. It is better to discharge them more to get a better recharge on them, but they do not have nearly the memory effect of a NiCd.
James, you need to have your facts straight before stating things as Gospel. Charge time not important? BWAHAAAAAAAAAAAA! That's rich.
Steve
Talk about overly dramatic. These aren't built in emergency lighting in a commercial building, but rather cheap plug in power outage lights that you can buy in drugstores. They are not lifesaving devices any more so than a dollar store flashlight tucked in the nightstand. Manual? They might have had some instructions for use on the back of the package, if you're lucky, but none of mine ever said anything about the battery, it wasn't even meant to be user replaceable and weren't even name brand, just cheap generic AA NiCDs. I've had a couple similar lights, they seemed like a good idea and were handy at times, but hardly fail safe, and half the time they turned out to be broken by the time they were needed. Now I just keep flashlights stashed in convenient locations.
Okay, will you pay for damages if it causes a fire? When you personally sign responsibility for liability for any damages incurred, then I'd listen to you, maybe.
Please explain how it will cause a fire.
If you don't want to listen to me that's fine, my advice is free. If liability is a concern, don't replace the battery at all, throw away the light and buy a new one.
---MIKE---
You are betting your life and the lives of others on these lights functioning. Don't accept any half baked ideas you might get here. Read and follow the manual.
In my life, there has been many an item that specifically said what battery to use and which not to use. I have ignored or just not read these statements. They weren't things, though, that were life-saving devices, so my poor reading skills didn't make a very big difference.
This can make the difference in life and death. Be sure.
PERIOD.
Steve
Everything James Sweet has written on the battery question is 100% correct, for my 2 Cents opinion, and I have worked with Nicads since back in the late 1950's when we at Bell Labs were experimenting with them for backup power for electronic switching systems for the Bell System in place of the lead-acid batteries then in use, because of the hazards of using the lead-acid cells. Some of those early nicads actually lasted for 25 years, but of course they were treated carefully as far as charging current and they used much higher quality chemistry than the cheap cells you generally buy these days.,
I always tell people, if they want to know about backup battery power, talk to the guys who work on the backup batteries for the phone company. I don't think anyone has more knowhow when it comes to the care and feeding of lead acid cells.
TDD
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