Swapping out a NiMh for a LiPo?

[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.

[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?

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.

[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?

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 ...

[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?

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?

[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.

[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.

[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?

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?

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!

[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.

[sofTest]

Re: Swapping out a NiMh for a LiPo?

... Other thoughts?

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.

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

[stj]

Re: Swapping out a NiMh for a LiPo?

so if you do that,
2 things to watch.

1: keep the usb socket isolated from any metal casing - or your ground will bypass the battery protection.
2: dont overload the battery protection circuit with your load.
see the pdf's

[EasyGoing1]

Re: Swapping out a NiMh for a LiPo?

If you don't have protection circuitry on the Li-ion then get protection circuitry. And if you can tweak the protection circuitry to stop charging at 4.2 volts (or less), all the better. Other than being a bit inefficient when the cell is fully charged, it should work just fine.

OK, check out this diagram on that charging board I referred to:



Notice that the battery is protected from whatever it's powering by the charge circuit being in the middle...

So what I was thinking of doing was something like this:



So that the charger circuit basically becomes the battery that is connected to the emergency light, with the diode preventing any charge current from being back-fed into the output of the charger ... but that charger is designed to monitor the battery at all times and prevent it from being over-discharged (by cutting power to whatever the charger is powering and then continuing to charge the battery) and it prevents the battery from being overcharged because it actually does the charging...

That circuit by the way ... is meant to be used as a portable power source for whatever ... and when you have 5 volts connected to it, then it won't even let the battery supply the power to the outputs ... it feeds the output directly from its 5 volt / USB input until the input power is cut then it lets the battery take over supplying voltage to the outputs.

It's a pretty cool circuit.

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.

[EasyGoing1]

Re: Swapping out a NiMh for a LiPo?

So does your 'new' lithium pack have protection circuitry?
It's probably more work to incorporate a built-for-Li-ion charging circuit into an existing design that's tightly coupled with other circuitry.

So this is where I struggle - though I wholeheartedly agree with this statement, this particular circuit is about as stupid simple as it gets ... we have:

1) A circuit that connects to standard home AC electricity.
2) The circuit converts AC to DC (though it's still kinda noisy, I checked).
3) The circuit has two primary functions:
a) Keep the battery charged while its AC source is hot.
b) When the AC source goes cold, switch on the LED lights and power them from the battery.

I mean ... THAT'S ALL IT DOES... and all I wanna do is replace said battery with batteries that are better suited to constantly being charged without developing memory, and would also provide a longer run time for the lights when the power goes out. Without question, the NiMh battery pack MIGHT provide the 2 hours of run time they claim TODAY ... but next year? That number will be noticeably smaller and it will continue to get smaller the more it develops a memory.

SO, All I wanna do is replace that battery pack with better batteries, but the problem being, that the connection where the battery connects to the circuit, will be trying to charge the batteries all the time ... and I was hoping that a simple diode properly placed, would block that charge current from getting to the batteries, while I then provide my own method of charging the batteries.

But so far, No one has told me if my diode theory for blocking charge current would actually work. I suppose I could just test it ... what's the worse that could happen?

If you don't have protection circuitry on the Li-ion then get protection circuitry. And if you can tweak the protection circuitry to stop charging at 4.2 volts (or less), all the better. Other than being a bit inefficient when the cell is fully charged, it should work just fine.

These little gizmo's should do the job...

[eccerr0r]

Re: Swapping out a NiMh for a LiPo?

So does your 'new' lithium pack have protection circuitry?
It's probably more work to incorporate a built-for-Li-ion charging circuit into an existing design that's tightly coupled with other circuitry.

If you don't have protection circuitry on the Li-ion then get protection circuitry. And if you can tweak the protection circuitry to stop charging at 4.2 volts (or less), all the better. Other than being a bit inefficient when the cell is fully charged, it should work just fine.

[EasyGoing1]

Re: Swapping out a NiMh for a LiPo?

- Agent24 - While the NiCd may handle trickle charging better than NiMH, they do need to be deep-cycled on occasion or they develop memory issues.

This is exactly why I wanted to replace it with either LiPo or Lion

[eccerr0r]

Re: Swapping out a NiMh for a LiPo?

Uh...a bit of confusion here.
The original battery is NiCd. Apparently OP wants to swap for Li-Ion.
Likely the current charger trickle charges already which is a no-no for Li-Ion.

Still the same outcome - likely a re-design is needed.
If your li-ion battery pack has overcharge protection, it likely will work just fine in its place but likely you'll find its cycle life severely degraded or possibly destroyed. Something that will limit charge voltage to shunt off the trickle charge, whether it be another auxiliary protection circuit or redesign will be needed.