# Low voltage warning for NiMH



## dsvilko (Dec 23, 2008)

I am in a process of building two 2xCree R2 WG lights with the DX 3256 drivers powered by two 4xAA NiMH battery packs (one for each light). As I am planing on using a high quality eneloop batteries, even with almost completely depleted batteries the LEDs should still be quite bright. If I am not mistaken, the NiMH battery packs don't like being completely depleted as there is a chance of destroying the batteries through the reverse polarity.

Don't know if it will help but I made a tiny low voltage warning 'circuit' that lights the red leds when the voltage drops below 4.25V. The circuit is from https://homepages.paradise.net.nz/bhabbott/lvw.html with R3=6k8.
This is what it looks like:








What do you think, is it necessary?


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## kuksul08 (Oct 8, 2006)

What driver? 3258 is http://www.dealextreme.com/details.dx/sku.3258

Seems like a cool idea to me :thumbsup:


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## dsvilko (Dec 23, 2008)

Sorry, the driver is http://www.dealextreme.com/details.dx/sku.3256. Does anyone know can this driver completely drain four NiMH batteries before I notice a significant drop in the light intensity?


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## tobymack (Nov 12, 2008)

thats great, I was just about to start looking at battery warning lights and haven't come across active zeners before.

now that made out of surface mount components would make it *really* small...


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## tobymack (Nov 12, 2008)

btw ,to answer the "is it necessary" question, I would say yes.
I don't know about the driver you mention but I ran my 4xAA pack down to about 3.3V before I realised my Q5 wasn't very bright anymore.

if nothing else it can give you some warning your light is about to go.


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## VaughnA (Jan 21, 2004)

dsvilko said:


> Sorry, the driver is http://www.dealextreme.com/details.dx/sku.3256. Does anyone know can this driver completely drain four NiMH batteries before I notice a significant drop in the light intensity?


I use the 3256 with 4 AA's, you'll notice it long before the batteries go dead. I'd think that the circuit would be unneccessary.


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## Jim Z in VT (Sep 9, 2007)

That's a clever little circuit. Thanks for posting it. How much current do you suppose it uses? Might think about adding it to my battery packs if it's not a significant drain.

I'm kind of ignorant when it comes to electronics. When you say "R3=6k8", what does that mean? 6800 ohms?

How would one calculate the value of R3 for other battery packs?

Thanks.

JZ


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## dsvilko (Dec 23, 2008)

@JZ:
It seems I have fried a milliamperemeter part of my multimeter so I can't measure the actual current but it should be in the 1mA range. You correctly guessed that 6k8 is 6800ohm. As explained on the linked page the diode will stop conducting current (the leds will turn on) when the voltage drop across the R3 falls below 2.5V. R3 can be calculated from:
_R3_/(_R2_+_R3_)=2.5/_Vmin_
where _Vmin_ is the voltage at which you want the warning leds to turn on. I think that in place of two red leds you can put one white or blue led to make the circuit even smaller.

@VaughnA:
I guess that's a good news that I will notice a drop in intensity though I must say that I don't understand why should there be a drop. According to Sanyo Eneloop discharge data the cells should fall below 1V maybe a minute before they are completely drained. At 4V the driver should, according to a forum post on DX, still deliver over 0.8A to the leds. Did you do any current measurements with this driver? Can you confirm that it's really as good as the above forum thread would imply (above 0.85A Iout)? Any other advice on using this driver? I am still waiting for my parts to arrive 

Did anyone here actually destroy a NiMH battery by over-discharging or do I worry too much?


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## ocean breathes salty (Oct 1, 2006)

What a nifty little cicuit... I will be sure to copy this one some time soon  bookmarking the thread for future reference 

Most of the dead Nimh cells i have read about seem to have happened after prolonged storage where the self discharge has completely drained the battery.


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## TOMMO (Dec 12, 2008)

I have problems with my Nimh batteries sometime when my boy leaves a toy switched on and totally drains it.
When i put them in the charger they come up with an error light (might be my crap charger) so what i do is get a 9v battery and stick it in parallel with the AA Nimh to give it
a jump start (about 10 secs normally) and when i put them back in the charger they all seem to charge ok after that.
I had thrown a couple away before i tried that,glad i tried it now!
Tommo.


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## Kraygus (Jul 9, 2008)

I bought one of these only yesterday!

http://www.jaycar.com.au/productVie...d2=&pageNumber=&priceMin=&priceMax=&SUBCATID=

Can't complain about the price though.


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## dsvilko (Dec 23, 2008)

Well, $10+S&H is a bit more than $1 that I have paid for the parts  For $10 you can buy another LED and driver at DX


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## Kraygus (Jul 9, 2008)

They're at my local store, but I know.....I should have used this emoicon after "yesterday" :madman:


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## VaughnA (Jan 21, 2004)

dsvilko said:


> @VaughnA:
> I guess that's a good news that I will notice a drop in intensity though I must say that I don't understand why should there be a drop. According to Sanyo Eneloop discharge data the cells should fall below 1V maybe a minute before they are completely drained. At 4V the driver should, according to a forum post on DX, still deliver over 0.8A to the leds. Did you do any current measurements with this driver? Can you confirm that it's really as good as the above forum thread would imply (above 0.85A Iout)? Any other advice on using this driver? I am still waiting for my parts to arrive
> 
> Did anyone here actually destroy a NiMH battery by over-discharging or do I worry too much?


Well, I'm not using eneloops, just normal NiMH batteries. But the only time I've ever had a dead cell was from letting the batteries sit over the summer without recharging. I run a dual SSC-P4 for about 1-1.5 hours on 4AA's with the 3256, they noticably dim at around 1 hour or so, I swap packs and keep on going.

If you look at many of the graphs of the Current Vs Voltage of the 3256 you'll see that it isn't real tightly regulated. That's why current still drops as the battery weakens. I'm waiting on a 7.4v battery for my next project and I'm a bit worried that the current may higher than I want with that battery. I may end up tweaking the resistor values to drop the current to around 800mA.


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## [email protected] (Mar 28, 2007)

Take a look at voltage monitor chips, sometimes called supervisor chips. They are used to reset microcontrollers when the input voltage goes too low.

These are typically 3 pin devices. One pin goes to the positive side of your power supply (+ terminal of battery in this case). One pin goes to the negative side. The last pin floats (sinks no current) when the input voltage is high enough. When the voltage gets too low, this pin will sink a few milliamps of current. You could use this to turn on an LED with its cathode connected to this pin and its anode connected thru a 1K resistor to the plus side of the battery. Note: make the resistor larger if using a battery larger than 4.8 volts.

There is one supervisor made by Microchip that should work: TC54VC4202EZB. It will trip about 4.2 volts. You used to be able to get samples for free from them, but they now have a $7.50 handling charge in the US. Still, for that you can order 5 of TC54VC4302EZB (trips at 4.3 volts) and 5 of the TC54VC4202EZB.

Or if you need to order something from digikey, look at: S-80840CNY-B-G. It's a supervisor made by sony.

The above examples use TO92 3 pin packages, look like a small 3 pin transistor and are easy to solder to. If you go looking for surface mount parts you'll find lots more.

The advantages of voltage monitor chips is they draw almost no current (microamps not milliamps), they are usually pretty accurate, and typically easier to build into a circuit (3 parts vs 6).

Mark


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## dsvilko (Dec 23, 2008)

My circuit is still much cheaper and also does not use any exotic elements so in all probability you can buy the parts locally. There is really no need for the circuit to be constantly powered. I'll connect it so it will be powered only when the light is turned on. Additional drain of 1-2mA makes no difference when the LED needs 0.75-1.5A.


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## [email protected] (Mar 28, 2007)

I'm not so sure about the cost being cheaper once you include the precision resistors, 2 LEDs vs 1, etc. The price is about a wash as far as I see.

And if you live live in the boonies like me there is no local source of parts, except for what I have in my junkbox  

The thing I like about your circuit is it is adjustable for different voltages.

Mark


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## tobymack (Nov 12, 2008)

[email protected] said:


> The thing I like about your circuit is it is adjustable for different voltages.
> 
> Mark


I was going to say the same thing but you can probably add a potential divider on the front of the voltage monitor chip. Only trouble is the divider would be on the supply to the chip as well so you might have problems. I suspect its active draw isn't much though in open collector mode.

I think at the end of the day either cct would be fine, it probably depends on how easy it is to get the bits locally.


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## dsvilko (Dec 23, 2008)

I just did a quick calculation and the parts for one circuit have cost me about 50 cent. For that price and if you have enough space (maybe go with the SMD components) you could go nuts and make multiple parallel circuits with different trigger voltages and so provide a more detailed voltage indication (three circuits would be a good compromise between precision and complexity). 
But it is true that it all depends on what is available in your shop.


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## Jim Z in VT (Sep 9, 2007)

I don't know if I could come close to that price unless I was buying the parts in bulk. But I may be able to scavenge the resistors and leds from some old dead computer power supplies I saved. The TL431 will have to be ordered though....my only local electronics source is Radio Shack and they don't list it on their website. 

JZ


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## Jim Z in VT (Sep 9, 2007)

*Found one!*

Two actually  I was looking over those dead computer power supplies I have, and saw a bunch of things that looked like the one in your photo. Got out the lighted magnifier, and sure enough, two of them are labeled TL431 :thumbsup: Just have to work out my R3 value and I'll give this a try.

Why would a single blue or white led work in place of the 2 red ones? Different voltage requirements?

Thanks again dsvilco.

JZ


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## dsvilko (Dec 23, 2008)

It's nice to have a bunch of junk around the house  The TL431 in the 'on' state has a voltage drop of 2V and we want the leds not to light in that state. As one red led has a voltage drop of around 2V or less it would light. Green and specially blue and white leds have a Vf higher than 2V so they should be ok.


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## neilt (Apr 7, 2007)

dsvilko said:


> I just did a quick calculation and the parts for one circuit have cost me about 50 cent. For that price and if you have enough space (maybe go with the SMD components) you could go nuts and make multiple parallel circuits with different trigger voltages and so provide a more detailed voltage indication (three circuits would be a good compromise between precision and complexity).
> But it is true that it all depends on what is available in your shop.


Out of interest, here's the inverse circuit that turns on the LED above a trigger voltage: http://www.reuk.co.uk/TL431-Battery-Voltage-Monitor.htm
Might be useful as part of a more comprehensive setup.


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## Jim Z in VT (Sep 9, 2007)

Neil: nice....when the led goes _out_, it's time to get _out_ of the woods!



dsvilko said:


> It's nice to have a bunch of junk around the house  The TL431 in the 'on' state has a voltage drop of 2V and we want the leds not to light in that state. As one red led has a voltage drop of around 2V or less it would light. Green and specially blue and white leds have a Vf higher than 2V so they should be ok.


Would this change for a higher voltage battery (i.e. 14.4v) or does the circuit keep the voltage being fed to the leds in the right range? ( again, excuse my electronic illiteracy  )

JZ


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## dsvilko (Dec 23, 2008)

You may have to adjust the R1 resistance to keep the current below 20mA.
EDIT: 2.2kOhm is OK for 14.7V


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## Jim Z in VT (Sep 9, 2007)

Yeah, that's what I meant. Thanks. I currently don't have any battery packs higher than 14.4v.

JZ


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