charging a Battery and using it at the same time

[abuthemagician]

I have a project (Wii Wireless sensor bar) that i am working on for a friend. I plan to power 4 infrared LEDs with a 3.7v 810mAh lithium Ion cell phone battery charged by a dismantled charger made for that phone (you can charge the phone or a spare battery). What i want to do is use the device (4 powered LEDs) whether it is plugged in or not. I have a feeling that there will be two different voltages depending on if it's plugged in or not. What is the simplest way to get a constant voltage to the LEDs?

[audioguru]

LEDs do not operate from voltage, they operate from current. Lightbulbs operate from voltage.

Each LED needs to have a current-limiting resistor in series with it. If the minimum battery voltage was higher then two LEDs could be in series and in series with a current-limiting resistor.

Post the range of forward voltage and max continuous current spec's for the LEDs and we will show how to calculate the resistor value.

The charger circuit might get confused by the current for the LEDs then over-charge the battery. Some lithium batteries catch on fire or explode when they are over-charged. Other lithium batteries have a thermal sensor or complwete protection circuit inside.

 

[abuthemagician]

Sorry i should have put more detail in, but my brain doesn't work before noon. Anyway, I will be using a 3.6v 700mAh battery with 4 1.2v 30mA infrared LEDs. In a calculator I found online it says a 2x2 parallel array will need 2 47 ohm resistors and will run at 60mA. What i wanted to know was how to regulate the current whether on the battery or while it was charging.

Also, how would i calculate how much run time i will have on the battery? I have higher capacity batteries I may change to in the future.

[audioguru]

Hi Abu,
LEDs don't have a set voltage. They have a range of voltages. It might be 1.0V for one and it might be 1.4v for another. If these two are connected in parallel then the lowest voltage one will take all the current and burn out, then the other one will burn out. So LEDs should never be connected in parallel.

If you connect two of your LEDs in series and in series with a 56 ohm resistor then they will both have the same current and will not burn out. The current will be 28.6mA if both LEDs are 1.0v, 21.4mA if both LEDs are 1.2V and 14.3mA if both LEDs are 1.4V.
The LEDs will dim as the battery runs down. The average current is 21.4mA so a 700mAh battery will last about 16.4 hours with two strings of two LEDs in series.

We don't know what is in your battery and don't know the circuit of your charger. The current should be limited to an amount recommended by the battery manufacturer. Then the voltage must also be limited. The LEDs mess up the current. Maxim have some battery charger ICs that charge and power a load at the same time.