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Battery life
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Battery life

by Tuom Larsen on Wed Aug 18, 2010 8:02 pm

Newbie here....
Please, say I have 4.5V battery (3x 1.5V type "D" in series battery, rated 18000 mAh each) and a LED.
I would like the LED to shine as long as possible -- what do I have to do?
In other words, I guess this has something to do with Ohm's law, which I, unfortunatelly, totally don't get:
- how do I find out if I need a resistor, and its resistance?
- all I'm interested is battery life, will the resistor make the battery life shorter than without it?
- is it possible to use "zenner" diode for the same thing? (preventing LED from burning) What is the difference?
- if I another 3x such batteries, connect them again in series and the resulting packs (2 pieces) in parallel (3x + 3x), is the LED going to shine two times longer?
- why do I need the resistor in the first place, can I just use single 1.5V battery for the same effect?
- what are the factors which determine the duration the LED is going to shine? I know there are voltage and current limiters, are the prolong the life?
- what I just don't get is, if a LED consumes say 1mA, will it consume from the battery just 1mA every hour, so it shines 18000 hours? Or will consume more and waste it say for heat?
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Re: Battery life

by madworm_de on Wed Aug 18, 2010 9:00 pm

Ohm's Law itself is quite simple. Using it properly needs some thought.

(I): R = V/I = constant

The "constant" part is only true for 'ohmic' resistors, and an LED is definitely not among those. Actually 'ohmic' resistors are an idealization. Some resistors (materials) show 'ohmic' behaviour in certain temperature ranges (or not), in certain voltage ranges (or not). Wikipedia will tell you more about that. Have a look at kirchhoff laws, which deal with the voltages and currents in electric circuits.

Short form: a) in any closed loop in a circuit the sum of all voltages is _zero_
b) at any given point in a circuit the sum of all currents is _zero_

Going back to the LED + resistor example:

a): --> the battery voltage is split between the LED and the resistor according to (I)

If you want a certain current to flow (say 1mA) and you know the 'forward voltage' V_f of the LED (datasheet), you can calculate the resistor according to (I): R = V' / I. Mind that here V' = V_batt - V_f.

b): this states that no current can get lost. If you feed current into a junction it gets split up. If you feed it into a single 'wire' it is the same everywhere along it.


--------------


- how do I find out if I need a resistor, and its resistance?


There is no 'IF' - rule of thumb: you always need one.

- all I'm interested is battery life, will the resistor make the battery life shorter than without it?


Some energy is wasted in the resistor. P = V * I . V is the voltage drop across the resistor, not the battery voltage.

- is it possible to use "Zener" diode for the same thing? (preventing LED from burning) What is the difference?


Forget about that.

- if I another 3x such batteries, connect them again in series and the resulting packs (2 pieces) in parallel (3x + 3x), is the LED going to shine two times longer?


Yes.

- why do I need the resistor in the first place, can I just use single 1.5V battery for the same effect?


Remember R = V/I = constant. For LEDs it is not true. If you google for typical V-I graphs of LEDs, you'll find that once you've passed the forward voltage, the LED will effectively act as a short (sort of, not really). Therefore you need a resistor. If you power an "1.5V LED" with _exactly_ 1.5V it may survive (small LEDs will, high power LEDs will die too as they get hot and start to allow even more current to pass through them), go to 1.6V or 1.7V and you may kill it. The current grows exponentially with voltage, and it gets hotter and hotter. See above for the calculation of R using Ohm's Law.

- what are the factors which determine the duration the LED is going to shine? I know there are voltage and current limiters, are the prolong the life?


(II): Conservation of Engery. E = Power * time.
(III): Power = Voltage * Current.

- what I just don't get is, if a LED consumes say 1mA, will it consume from the battery just 1mA every hour, so it shines 18000 hours? Or will consume more and waste it say for heat?


For an "1.5V LED"

Power = 1mA * 1.5V = 1.5mW

If you use a 4.5V battery pack, you'll need a 3kOhm resistor as well. It consumes 3V * 1.0mA = 3.0mW and turns that into heat.
Total power consumption: 4.5V * 1.0mA = 4.5mW - the sum of LED + resistor consumption ( using b) and (III) ).

An approximation of the energy content of the battery is Capacity * Voltage. It assumes that the battery voltage stays constant, which is not quite the case, but you know that. Here it's just 18000mAh * 4.5V = 81000 mWh (current*time*voltage).
So in theory the LED should shine for 18000 hours. ( time = energy / power ).

As in reality the battery voltage drops, you won't get that of course. The resistor has been calculated to supply the LED with 1mA only if the supply voltage is 4.5V. As soon as the batteries get empty that is not the case anymore and the LED is fed with less than 1.5V, it gets dimmer and dimmer and shut off. Some energy will be left in the batteries.

The only sensible way to drain the batteries completely is by using a DC/DC voltage converter such as the mintyboost or similar circuits. Some other chips (e.g. LT1618) can be used to build a switching (energy efficient) constant current driver for LEDs that use up all of the energy in the batteries, but that gets complicated and costly. Maybe not worth for just a simple LED.

If you don't use a resistor at all, the LED will burn.

Please, say I have 4.5V battery (3x 1.5V type "D" in series battery, rated 18000 mAh each) and a LED.
I would like the LED to shine as long as possible -- what do I have to do?


Well. If you know how much current you need to get an acceptable brightness, calculate the resistor for that value.
How to get the current value? Experimentation using a potentiometer and reading the datasheet, so you don't burn it.
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Re: Battery life

by Tuom Larsen on Thu Aug 19, 2010 5:07 am

First, thank you very much for the reply!

Second, ahhhh, still blank! I apologize, I just don't get it!

Few more questions:

- because of what does a LED burn? Because of current or voltage? Or both?

- what troubles me that you said I'll definitely need a resistor but that burns power, too, which is not good for battery life. Is there another way, without the use of a resistor?

- and most importantly, once again, why there needs to be a resistor at all? Let's say I have a LED with whatever specification. I guess this "forward voltage" is important... I can get any battery voltage I like, either by connecting batteries in series or simply getting the appropriate voltage from some source. So I thought I can get the precise voltage I need. What I don't understand is how does a LED "gets" 1mA current? (1mA shine exactly how I want, say). Why exactly 1mA? Does it depend on the type of LED? If I don't put a resistor in the circuit, how much current will it consume? Too much, because a LED has low resistance?
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Re: Battery life

by richms on Thu Aug 19, 2010 5:39 am

For higher power use, there are constant current switchmode power supplies that will waste less power than a resistor.

Problem is for small loads like a single LED they are very inefficient, so would be as bad or worse than a resistor.

Look at the joule thief designs and parallel all the batteries. That will keep on going down to really low voltages and step the voltage up, but you would have to do some tests on the runtime each way since batteries are not normally specced on their performance below what the manufacturers call "flat"

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Re: Battery life

by madworm_de on Thu Aug 19, 2010 6:19 am

- because of what does a LED burn? Because of current or voltage? Or both?


It burns because of excessive heat. Without a resistor (or driving electronics) it consumes too much power.

Power = Current * Voltage(measured across the LED)

- what troubles me that you said I'll definitely need a resistor but that burns power, too, which is not good for battery life. Is there another way, without the use of a resistor?


Not using a resistor requires other methods of keeping the current at bay. Switch mode drivers (google buck/boost regulator) typically use inductors for that purpose. Depending on their inductance they only allow for a certain increase of current over time. The regulator switches on/off accordingly. On: current rises to near max value or slightly above. Off: current decreases again. Do it fast enough and you get an average current that suits the LED.

- and most importantly, once again, why there needs to be a resistor at all? Let's say I have a LED with whatever specification. I guess this "forward voltage" is important... I can get any battery voltage I like, either by connecting batteries in series or simply getting the appropriate voltage from some source. So I thought I can get the precise voltage I need.


Getting the 'right' voltage is kind of like trying to balance an egg on the tip. If you do it precisely, it will stay upright. If any disturbance occurs it tips over. The current going through LEDs is very sensitive to the applied voltage, 0.1V more can double the current going through it. You can't get that right by removing or adding batteries.

Also setting a voltage does not regulate anything. If the LED changes only slightly due to getting warmer, it's forward voltage changes (decreases) and the formerly correct voltage is now utterly wrong.

What I don't understand is how does a LED "gets" 1mA current? (1mA shine exactly how I want, say). Why exactly 1mA? Does it depend on the type of LED? If I don't put a resistor in the circuit, how much current will it consume? Too much, because a LED has low resistance?


Why 1mA? That was just a convenient example. The datasheet will tell you how much current the LED can withstand. Using a resistor is just a cheap way to make sure the current will never go above that value.

http://en.wikipedia.org/wiki/File:Diode-IV-Curve.svg

Using a resistor you're limiting the current. You calculate it using Ohm's law by taking the forward voltage (named Vd in the picture above) into account. If you go slightly above Vd, you'll see that the current is skyrocketing = instant incineration.
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Re: Battery life

by Entropy on Thu Aug 19, 2010 11:58 am

Tuom Larsen wrote:Newbie here....
Please, say I have 4.5V battery (3x 1.5V type "D" in series battery, rated 18000 mAh each) and a LED.
I would like the LED to shine as long as possible -- what do I have to do?
In other words, I guess this has something to do with Ohm's law, which I, unfortunatelly, totally don't get:
- how do I find out if I need a resistor, and its resistance?
- all I'm interested is battery life, will the resistor make the battery life shorter than without it?
- is it possible to use "zenner" diode for the same thing? (preventing LED from burning) What is the difference?
- if I another 3x such batteries, connect them again in series and the resulting packs (2 pieces) in parallel (3x + 3x), is the LED going to shine two times longer?
- why do I need the resistor in the first place, can I just use single 1.5V battery for the same effect?
- what are the factors which determine the duration the LED is going to shine? I know there are voltage and current limiters, are the prolong the life?
- what I just don't get is, if a LED consumes say 1mA, will it consume from the battery just 1mA every hour, so it shines 18000 hours? Or will consume more and waste it say for heat?

With your battery configuration, you need a resistor. The only cases where one does not need a resistor are with small batteries that have high enough internal resistance to perform that function. (This is why LED Throwies don't use a resistor - button cells have such high internal resistance that no additional resistor is needed.)
D cells, however, have quite low internal resistance.

The resistor will waste some power, but as others have said, in your particular configuration, a switching regulator won't be any more efficient.

A zener diode will not help you in any way.

As others have said, doing a 3S2P configuration will give you at least twice the battery life of a 3S1P configuration. (See the above comment about internal resistance, although when driving a typical 5mm LED from D cells, the contribution of the internal resistance of the cells will be insignificant.)

You need to choose the resistor such that when dropping the difference between Vbat and Vf, the resistor will conduct no more than the LEDs rated current. For most 5mm LEDs, this is 20 mA. If you increase the resistor value, the current will be less. It sounds like your primary goal is battery life, so targeting 5 mA or less may be a good idea for at least three reasons:
1) Human vision is logarithmic, and so an LED at 1/4 brightness will not actually appear to be that much dimmer. This is one of the reasons LEDs have become so dominant in flashlights - they dim gracefully
2) LEDs become more efficient as their current is reduced below their maximum ratings
3) LED lifetime increases as you reduce the current

You absolutely need some form of current limiting external to the LED. LEDs have very nonlinear I/V curves. For example, an LED with a Vf of 3.4 volts at 20 mA will likely conduct less than 1 mA at 3.3 volts, and will conduct a few hundred milliamps at 3.5 volts. LED Vf varies from LED to LED within a batch, and also varies over temperature. As the LED warms up, Vf drops. This leads to a condition known as "thermal runaway" - Vf drops a little -> current goes up -> LED heats up more -> Vf drops more -> current goes up more -> etc until the thing melts.
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Re: Battery life

by Tuom Larsen on Thu Aug 19, 2010 1:33 pm

Ok, ok! I get slowly get it!

Thank you all, people!
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Re: Battery life

by Tuom Larsen on Thu Aug 19, 2010 4:31 pm

One last thing: while the LED shines, after half a year, how do I find out what is the remaining capacity, i.e. how much time is left before it stops to shine? I mean, how do I measure it?
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Re: Battery life

by Entropy on Fri Aug 20, 2010 9:46 am

You're getting into the arena of battery management, and capacity estimation makes my head explode. (Some batteries are easier to measure SOC in than others, the most accurate methods involve logging the current in/out of the battery, but in your case such an approach will likely use more current than a low-current LED.

Also, batteries do self-discharge.

Last, since LEDs dim so gracefully, when the battery is nearly dead an LED-based light will often enter a sort of "moon mode" where brightness is very low (but still potentially usable), and this state can last a VERY long time (battery nearly dead BUT not heavily loaded any more.)

If you're checking on the unit after 6 months, just replace the batteries.

Also, what sort of situation are you using this in? Maybe a solar-powered device is your best bet. (Like those solar garden lamps.)
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Re: Battery life

by Tuom Larsen on Sat Aug 28, 2010 5:03 am

Ehm, what I meant was literally how to simply measure battery life. I'm total noob -- should I buy some "multimeter", turn its dial to "Amper", connect it to "+" and "-" of battery and measure how many miliampers there are left?

On related note, how do I find out how much power my circuit draw?

I mean, how do I find out, empirically, given a battery and a circuit, how long is going that thing to run?
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Re: Battery life

by richms on Sat Aug 28, 2010 5:43 am

Tuom Larsen wrote:Ehm, what I meant was literally how to simply measure battery life. I'm total noob -- should I buy some "multimeter", turn its dial to "Amper", connect it to "+" and "-" of battery and measure how many miliampers there are left?

On related note, how do I find out how much power my circuit draw?

I mean, how do I find out, empirically, given a battery and a circuit, how long is going that thing to run?


short circuit current is a viable test for very small alkaline cells, but will melt your meter or blow its fuse on anything AA sized and rechargeable.

You can measure your devices current, and look at the datasheet for the batteries you are using and see what they list the capacity as, and do division, but that is assuming that the manufacturer decides it is flat at the same point you decide it is flat. Some devices like wall clocks will keep working for ages past manufacturers rated low voltage, others like remote controls become flakey well above what it listed in the datasheet.

Really, all you can do is test it and see what voltage your thing will work down until, and see how many mAh the cells have in them to get them to that state, but at really low currents you can expect self discharge to be more of an issue.

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Re: Battery life

by dchriz01 on Wed Sep 08, 2010 3:33 am

You know, all you can do is to test it. See what voltage your thing will work down. You can see how many mAh that cells have in them.
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