Swapping out a NiMh for a LiPo?

[eccerr0r]

Re: Swapping out a NiMh for a LiPo?

THE ADDED BENEFIT that I am figuring on getting from having the diode inline, is losing .7 volts across the diode ... since the original battery is 3.7 volts and the LiIon is 4.2 volts, having the diode in place brings the voltage at the emergency light back to 3.5 which is a little lower than the original battery but it certainly won't damage anything.

Not that much of a benefit as when the li-ion drops to 3.7V nominal voltage you'll be at 3.0V after the diode... then what.

[stj]

Re: Swapping out a NiMh for a LiPo?

the voltdrop is dependant on the current actually.
that's why i cringe when people use diodes to link 5v and 3,3v logic or to drive leds!

[sofTest]

Re: Swapping out a NiMh for a LiPo?

About voltage drop, if the fixtures are to be used for anything where legal code requirements apply, you need to be sure that the new battery type maintain the voltage needed during discharge/usage time. NiCd maintain their full voltage during discharge until they are nearly depleted. Other batteries might have a different voltage drop curve, so unless the fixture is designed to take that in to account, you might find that the light cut out before the fixtures rated time, even with a new battery.

https://www.mpoweruk.com/performance.htm

[stj]

Re: Swapping out a NiMh for a LiPo?

there are more regulations than that - a visible charge/float led for example

[sofTest]

Re: Swapping out a NiMh for a LiPo?

^ If you are referring to legal regulations, in most of Europe, this modification would probably break the regulations regardless, unless it was approved by whoever issued the fixtures EN 60598-2-22 compliance papers. I'm a Technical Property Manager in my professional life. Since the OP is based in the US, I don't know.

[delaware74b]

Re: Swapping out a NiMh for a LiPo?

Typical fire/building codes in US require 90 minutes minimum run time on emergency lighting. Although 90 minutes seems like a long time, there have been times where the power was out for extended periods of time such as 4 hours for the utility to replace a failed 3-phase transformer.

[EasyGoing1]

Re: Swapping out a NiMh for a LiPo?

Are you continuing using them as emergency fixtures? If yes, I wouldn't bother changing battery technology. In my experience, NiCd does not have a lower life span than other, "newer" type batteries in this application. From my observations, NiCd might be somewhat better in this application.

That's interesting ...

Remember, emergency fixtures should be regularly run down to their hourly rated running time to test the battery capacity.

The only way to actually do that would be to cut power to the device and then let it run the battery down. And since this device is hard-wired into the building's electrical, then I would have to kill the fuse at the fuse box every so often then let the battery drain.

I'm not opposed to doing this, but the question is, HOW OFTEN should it be done? Every three months? Six? Every year?

[eccerr0r]

Re: Swapping out a NiMh for a LiPo?

That's true, emergency lighting does have charging requirements that a wall wart USB charger probably doesn't meet either...

I'd say the drain down should be done with all other building emergency preparedness takes place... however I'd say the drain down is not for draining down the battery but rather checking if the battery is still good - I'm not sure how you found out the battery needed to be replaced in the first place...

[EasyGoing1]

Re: Swapping out a NiMh for a LiPo?

Not that much of a benefit as when the li-ion drops to 3.7V nominal voltage you'll be at 3.0V after the diode... then what.

Good point ... but do you think the voltage of the LiIons will drop significantly after just running some LEDs for a few hours? That, of course, depends on the current draw on the batteries which I would have the measure and figure out.

However, there seems to be a change of thought here as sofTest posted that in his experience, it's best to leave the battery in these devices as they were built, then just refresh them by letting them drain ever so often...

... which gave me an idea ... I have solid state relays designed to work with 120V AC ... an Arduino with an RTC programmed to cut power to the device every so often at high noon until the battery drains might be fun project ...

[EasyGoing1]

Re: Swapping out a NiMh for a LiPo?

That's true, emergency lighting does have charging requirements that a wall wart USB charger probably doesn't meet either...

I'd say the drain down should be done with all other building emergency preparedness takes place... however I'd say the drain down is not for draining down the battery but rather checking if the battery is still good - I'm not sure how you found out the battery needed to be replaced in the first place...

OH I never said it needed to be replaced. This device is brand new, I just bought it and installed it last month. I'm thinking of the future as it sits there month after month, year after year ... eventually (in my thought processes) that NiMh battery will be useless as is the case with cordless phone handsets after only a few years.

[EasyGoing1]

Re: Swapping out a NiMh for a LiPo?

So this is interesting ... According to this document, LiIon batteries have a typical life span of only 3 years or around 500 charge cycles, whichever comes first. And according to this document, NiMH has a typical life span of around 5 years. This is the exact opposite of what I was assuming since LiIons are more robust than NiMh ... but I suppose that's mainly in their ability to deliver raw power. And the fact that LiIon doesn't develop a memory.

[stj]

Re: Swapping out a NiMh for a LiPo?

emergency lighting should have a hidden or key activated switch on the power for routine testing.

[EasyGoing1]

Re: Swapping out a NiMh for a LiPo?

emergency lighting should have a hidden or key activated switch on the power for routine testing.

Yes, it has a button that when pressed, the lights come on, but as soon as you let go, it goes right back to normal operating mode where the lights are off and not using the battery.

[eccerr0r]

Re: Swapping out a NiMh for a LiPo?

hmm... have a very similar problem here too. Have a cordless phone I'm still using and its NiCd cell is dead. I was running on a single 26650 cell (since it was the only one that I had prot on) and it worked fine until I needed to charge it...

Looking at my old cell phone batteries I may end up re-purpose a cell that has protection, though dealing with charging will be another issue. Probably will just charge it with an external li-ion charger...

Ghetto mod?

[EasyGoing1]

Re: Swapping out a NiMh for a LiPo?

hmm... have a very similar problem here too. Have a cordless phone I'm still using and its NiCd cell is dead. I was running on a single 26650 cell (since it was the only one that I had prot on) and it worked fine until I needed to charge it...

Looking at my old cell phone batteries I may end up re-purpose a cell that has protection, though dealing with charging will be another issue. Probably will just charge it with an external li-ion charger...

Ghetto mod?

How do you deal with the incompatible battery size? Duct tape?

[eccerr0r]

Re: Swapping out a NiMh for a LiPo?

Paper clips, shoe string, and of course, duct tape!
(though a 3d printer might be involved...)

[eccerr0r]

Re: Swapping out a NiMh for a LiPo?

And let's not forget the zip ties! This looks a bit more hilarious than I thought it would before I put it together:

Hey you, (some person I hate), pick up my phone please?

Oh yeah, it does work and it's just a straight wire, using the protection circuit of the pack. I've not figured out a solution yet to overcharging but for now I'm just going to make sure I take it off the charger when it's fully charged. I left the connections open so I can easily measure the voltage and add the overcharge prot circuit...technically a ~4V 3W zener would do the job.

[sofTest]

Re: Swapping out a NiMh for a LiPo?

The duty cycle for an emergency light is way different from a cordless phone. Typically there are three discharges these batteries experience; self discharge, testing and loss of mains power.

How often you discharge/test the emergency lights, should be set by assessing the consequences of a fixture failure between the tests (risk management), not to optimize battery life. Typically somewhere between quarterly and monthly is normal.

The code here is that emergency lights shall be connected to the local general light circuit, so a loss of power to general lighting, the emergency lights switches to battery power. As you said, if connected directly to the same general light circuit, it would be somewhat cumbersome to test them. Some type of fixtures are easy to unhook from the socket so one can just unhook and lay them to the side and time them. As stj says, if planned before install, it is better to branch of a separate cable from the general lighting where one could install a test switch (not generally available), and perhaps use a relay for a little more elegant design. Otherwise, as you says, you would be relegated to trip the fuse. Some types of fixtures are self testing; they run down their batteries once a month, and flashes a diode if there is a failure.

The three last complete building emergency light refits I've managed, we installed automatic central monitoring and testing. The local building technician will get a warning with a fixture address after each test if a fixture fail, without running around in the building. His job besides this, would be to check the fixtures for if anybody has physically damaged them.

[ChaosLegionnaire]

Re: Swapping out a NiMh for a LiPo?

Hey you, (some person I hate), pick up my phone please?

i wouldnt pick that up or use it in public if i were u. it looks more like a remotely triggered ied. the fbi will swoop in suddenly and kick ur ass. dont use it or flash it around if u have a paranoid neighbor!! lol!

[eccerr0r]

Re: Swapping out a NiMh for a LiPo?

yet all of us ghetto modders know exactly what it is and why it looks the way it does

I wonder what I can do to make it look a little less menacing, I suppose hiding as much of the wire as possible by itself would make it look more presentable - yet it does nothing if it really was a questionable item...

BTW I charged it with the original charger meant for NiCd up to about 4V and stopped, it charged!