# Calculate battery life

1. Jul 5, 2013

2. Jul 5, 2013

### Staff: Mentor

Hi, arhanghel, welcome to Physics Forums.

This is a home work exercise, is it? Are you able to analyse the circuit and determine the currents through three resistors in particular? Does the photocell have strong light shining on it most of the time?

3. Jul 5, 2013

### arhanghel

@NascentOxygen: No is not a home work exercise, I'm trying to make a circuit that when the night comes to light an LED (I want to help my brother who is a biologist and needs something like this to do some tests in the open field).
The curent is I=9V/((R1||R2)||R4)) (i think so ).

The photocell because it will be in open field it's going to be exposed pretty much light.

P.S: If I have not posted in the correct thread, sorry. Can an moderator move this thread.

4. Jul 5, 2013

### CWatters

As it's not homework...

Got data on the LDR? I suppose in bright light the resistance might be as low as say 470R. In which case

In the daytime the current will be roughly..

Iday = (9-Vbe)/(R4+LDR) + (9-Vcesat)/R1

= (8.3)/(330+470) + (9-0.3)/1k
= 10mA + 9mA

Call it 20mA

At night the current will be roughly..

Assume LRD has very high resistance so negligible current.

Inight = (9-Vbe)/R1 + (9-Vled-Vsat)/R2

= 8.3/1k + (≈6V)/470
= 8.3mA + 12mA

Call it 20mA

So average consumption is around 20mA.

The data for that battery is here...
http://www.tme.eu/en/Document/bd5cb4300332df481401f8d3b7b55fd8/BAT-6F22_GP.pdf

It includes a graph showing how the voltage will fall with time when used with a 620 Ohm load. Now 620 Ohms would draw about 9/620 = 15mA which is less than your load.

I think you will be disappointed. It suggests that at 15mA the voltage will be down to around 6V in 20 hours.

5. Jul 5, 2013

### CWatters

PS: You would get longer battery life if you left the LED on all the time and simplified the circuit so you just had the LED and a 470R.

6. Jul 5, 2013

### arhanghel

@CWatters: Yes, I'm a little disappointed. Any idea how I could make the circuit work more with a 9V battery? Maybe another circuit ?

P.S: Data sheet for LDR: http://www.farnell.com/datasheets/612931.pdf

P.S2: Any idea if I can use a 18V battery or bigger whit the same circuit (maybe it will keep runing more time) ?

Last edited: Jul 5, 2013
7. Jul 5, 2013

### Staff: Mentor

As it stands, it is not a very efficient circuit for battery operation. What purpose will the LED serve? Quite likely it could be run much dimmer while still being just as visible. You want to light the LED for the entire 12 or so hours that the sky is dark? A blinking LED could use 90% less current, but may do the job equally well, maybe giving even better visibility.

8. Jul 6, 2013

### CWatters

I think that's the place to start.... What's the purpose of the LED? Is it to light up an area of ground with a given brightness or ??

You may have to show your brother what several different LEDs look like if he's not able to specify the brightness required in scientific terms. Find out what is the lowest level of illumination he can accept.

9. Jul 6, 2013

### CWatters

It would certainly be possible to modify the circuit easily to operate from something like a 12V car battery or a similar lead acid battery. For example a 20-30AH battery would probably last around 500-1000 hours before needing recharging. If you are reusing an old car battery you might not do quite that well.

There is an old saying that goes something like... "An engineer can do for $1 what any old fool can do for$10".... but sometimes it's just quicker and easier to spend \$10.

Are there any limits on the size/cost of the battery? Risk of theft?

10. Jul 7, 2013

### arhanghel

Yes I want to light the LED for the entire 12 or so hours that the sky is dark. But I can diversify the methods of making the tests, so I thought of a blinking LED (but as in the first case, it should start blinking when the night falls). Any idea how I can do the circuit without using a microcontroller?

The LED will be placed at 30cm from earth, to light up an area. The LED must light the area so you can see where it is (not to be complete darkness).
He want to use UV LEDs, something like this: http://www.superbrightleds.com/more...30-degree-viewing-angle-400-nm-40mw/630/1905/ or this http://www.adelaida.ro/led-5mm-ultraviolet-ossv53e1a-uv.html (the last link is form a site form my country, you can find the datasheet, if you click near "Descriere" and then click Descarcare (Download in english)).

The size doesn't matter, but the cost does matter. I do not want to spend a lot.

The circuit should work about 5 days without having to check it. It would be perfect if it works 15 days.

I'm trying to do a replica of this: http://we.tl/Ed97spLW62 [Broken] (it's a link from where you can download the PDF).

In the PDF they say that they used: "6-V lantern alkaline battery and a circuit for electronic control of a 1.8-W miniature blue fluorescent tube." and keep the circuit up for 15 day. From my point of view is impossible. The only explanation is that when the collected the samples(once every 2-3 days) they changed the battery.

If I would use a car battery how I should do the circuit. I've never worked with anything like that.

Thanks

Last edited by a moderator: May 6, 2017
11. Jul 7, 2013

### CWatters

If you want to use these with a 12V car battery...

http://www.superbrightleds.com/more...30-degree-viewing-angle-400-nm-40mw/630/1905/

They are rated at 30mA continuous. The forward voltage drop is typically 3.5V so you could just put two in series with a 180R resistor. eg so it's on all the time. The current draw would be around..

(12 - 7)/180 = 28mA

There are lots of 12V Scooter, motorbike and car batteries come in a range of capacities. See if you can find one in the range 10-20AH. You might find a car breakers yard prepared to give you a few old batteries to experiment with for free.

In theory a new 20AH should power it for a 20/28E-3 = 700 hours = 29 days.

However lead acid batteries don't like being fully discharged so I would aim to recharge it say once a week.

12. Jul 7, 2013

### arhanghel

13. Jul 7, 2013

### Staff: Mentor

Still not clear what you are needing the light for. Are you wanting it to be a beacon that can be seen from some distance, or are you needing it to illminate an area of the ground so that the ground can be photographed?

Would a garden solar light be bright enough? Those switch on automatically during darkness.

If a blinking LED is acceptable, how many times per minute would you like it to blink? How many seconds would you like it to stay on every time it blinks on?

14. Jul 7, 2013

### arhanghel

The LED need to light up an area of ground, I don't want to be seen form some distance. Just to light up an area. The led will be placed 30cm from ground.

A garden solar light maybe it will be ok (i have not had the chance to see so many. In order to get an opinion about a garden solar light). But I want to have the satisfaction that I did that thing, being a student I want to learn :D.

And in case of a blinking LED: blink once every 10 sec, and 5sec stay on.

Another option is to use three different color LEDs to alternate, to light up every 5 seconds something like this:
1. LED1 on - LED2 off -LED3 off;
2. 5 sec
3. LED1 off - LED2 on - LED3 off;
4. 5 sec
5. LED1 off - LED2 off - LED3 on;
6. 5 sec
7. Start again whit 1.

I hope you understand what I mean.

Thank you very much for willing to help me.

15. Jul 7, 2013

### Staff: Mentor

The LMC555 is a versatile low power timer IC that will allow you to build an efficient LED blinking light. Do a Google search for its data sheet. You will need batteries giving a voltage of at least 4.5 volts to light your white LED unless you borrow an IC out of a garden solar light --- that's a source of a clever IC that allows you to drive a white LED off a lower voltage, even just 1V. The lowest operating voltage of the LMC555 is 1.5V, so the whole circuit could* be run off a single D cell . You can use the LDR to halt the timer and stop its daytime blinking. Higher current through the LED will deliver brighter light, but use up your battery sooner.

It would be easy to alternate between two colours, but quite a bit more complicated to do so for three colours.

Last edited: Jul 7, 2013
16. Jul 8, 2013

### arhanghel

How do I set to flash once every 10 seconds and stay on 5 seconds?

And if I want to use more LEDs, I need to use and to program a microcontroller?

17. Jul 8, 2013

### Staff: Mentor

The datasheet will tell you that. Together with a capacitor, use one resistor to set the HIGH duration and another to set the duration of LOW.

You could use the 555 to clock a decade counter IC and then decode the outputs to obtain sequential HIGHs to drive 3 to 10 LEDS.