Looking for parts advice - using led to show battery strength

Thread Starter

LVF

Joined Sep 2, 2009
3
Hi,
I'm doing a small project, and could use techy some input.
I'm putting 3 - 9volt batteries together in series to get 27 volts.
I'd like to know when the battery power is getting low.
So I want to use an LED light. I want it to come on with the project switch if the batteries have at least 12v of power or up to 18 v would be ok too. But if it is less than that, I want the LED to stay off so I know to change the batteries.

Could someone help me select parts? Radio shack it the easiest local source.

Thanks!
 

Audioguru

Joined Dec 20, 2007
11,251
If your three 9V batteries in series have a voltage of only 12V then they are dead and will barely light an LED. Each battery is only 4V but one might be zero volts and the other two might still have some life left.
 

SgtWookie

Joined Jul 17, 2007
22,201
9v batteries have a very limited current output, even when new.

Lighting up an LED will require a good bit of the battery power.

You would be better off to use more capable batteries to begin with, and replace/recharge them as necessary.
 

Thread Starter

LVF

Joined Sep 2, 2009
3
Thanks to all of you for replying.
I appreciate your comments.

I think you're all saying that my objective of using an led to monitor the battery strength of 3 series 9volts is not productive.

The project is set up to need output of 18 to 27 volts.
I want a way to be alerted when the voltage drops below 18 volts.

Is there a better way to obtain such an alert than using an led?
Would an led use up too much of my battery power just being lit while in use? The project is designed to run for 5 to 20 min at a time.

Thanks much for your input!
 

jj_alukkas

Joined Jan 8, 2009
753
You can build the circuit, but it would be more ideal to check the battery level only when desired. Like only on power on, or a seperate microswitch which temporarily runs the circuit to check if battery is ok and goes off when you release it. It would save a lot of your battery.

What we all meant is that a 9V battery is only the size of 2 AA batteries, so since its giving 9V, its current is only a few ma and that for a few hrs. An LED alone draws 20mA nominal. So along with the circuit, you will lose around 30mA for this circuit if you keep it connected continuously.
 

Thread Starter

LVF

Joined Sep 2, 2009
3
Thank you for your reply!

I had not even thought of using a separate switch.

Let me repeat that to see if I understand correctly.

I'm using a toggle switch to turn the project on.

You're suggesting that I put a small push button micro switch in the line after the switch and connected to an led.

Then by turning on the toggle, then pushing the micro the led would light if the battery pack was putting out more than 12volts?

I like that idea.

Would I need any other parts - or just the led and the micro switch?

Do I need a specific type of led?
A 12 volt led needs 12 volts to light up but won't blow if it gets 27v?

Any other comments would also be appreciated.

THANKS!
 

CDRIVE

Joined Jul 1, 2008
2,219
There is no need to operate a LED at 20mA. Most LEDs will give a usable indication with just a few mA. For most LEDs, 20mA is not nominal. It's closer to maximum.
 

Audioguru

Joined Dec 20, 2007
11,251
If you power an LED and its current-limiting resistor from 27V then 25V is wasted by heating the resistor instead of lighting the LED.

A 9V battery has AAAA cells inside that have a very small capacity.
Look at the datasheet of a 9V alkaline battery to see how quickly it goes dead.
Its voltage begins dropping as soon as it has a load.
Here is a graph of its voltage when its load is only 27mA:
 

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CDRIVE

Joined Jul 1, 2008
2,219
I guess someone should have asked this by now, including me. What are you building that needs 27V and how much current does it draw?
 

Bernard

Joined Aug 7, 2008
5,228
Maybe this will fit your need. On power-up if battery is 18V or greater, the green LED blinks about 2X /sec., for 5 sec.total circuit drawing 5 mA. then shuts down drawing about 1 μA.or less. Total drain about 1/10 mA hr.Est rechargable battery battery [NiCd]@120mA hr.All caps are μF. The 130k & 157k resistors are really a 250k pot.representing a trip point of 18V. A pot from 50k to 1M would allow adjusting any low battery set. desired. All was tested on bread-board except for P-CH MOSFET, which I did not have. Just ask if you wish any further explination.
 

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Bernard

Joined Aug 7, 2008
5,228
I found a P ch MOSFET, surface mount, FDS9435A, by Fairchild,-30V[close],-5.3A, G-S +_ 25V. Added a V divider on input so as not to exceed G rating.Q1 & Q 4 are optional if a reducod output after 5 sec. falling to o after a few more sec.is not objectionable. Output of Q2 is pin 1.
 

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