how to drop down the current of 18650 battery

Thread Starter

ifjdjrj

Joined Aug 17, 2020
4
hello
guys i want to modify a torch led light work with 3 AAA battery
to be rechargeable with an 18650
but the problem im facing when i connect the 3.7v 18650 battery
the led get so hot and start to melt (desoldering) ...
is there any way drop down maybe the current or i dont know
thank you
 

dl324

Joined Mar 30, 2015
12,209
Welcome to AAC!

If the battery compartment is long enough for an 18650 battery, it must have been designed to use one.

Are you certain it's the LEDs causing it to get hot? Do you have a spacer to center the button on the battery with the anode contact?

Is it a cheap flashlight that depends on the series resistance of the AAA batteries to limit current?
 

Thread Starter

ifjdjrj

Joined Aug 17, 2020
4
Welcome to AAC!

If the battery compartment is long enough for an 18650 battery, it must have been designed to use one.

Are you certain it's the LEDs causing it to get hot? Do you have a spacer to center the button on the battery with the anode contact?

Is it a cheap flashlight that depends on the series resistance of the AAA batteries to limit current?
thank you so much

well iv tried with an NICD bettery 4.2v or 4.7 i dnt really remaber exactly
it work perfect but the light its not so bright

iv tried a powerbank charging board connected to the 18650 battery to charge it and use the usb port as load for the led light but it get hotter
if i plug it to a 5v 2000mah wall charger it get hoter also .

the problem that NICD battery its kinda dead and doesn't keep charge for a long time
it discharge quickly
i hope that iv explain well
and sorry for my english
 

dl324

Joined Mar 30, 2015
12,209
The NiCD cells were probably weak and couldn't source enough current to cause a problem.

How much current is the flashlight drawing when it's getting hot? Can you experiment with a power supply and monitor how the current changes with voltage? Once you find a voltage that works, you can work on reducing the voltage from the 18650 battery.
 

Audioguru again

Joined Oct 21, 2019
2,353
The designer of your "torch" probably used Ohms Law to calculate the value of a suitable series resistor to reduce the CURRENT.
Ordinary throw away battery cells are 1.5V each so three produce 4.5V with no load. Their quality determines the max current and the resistor in the torch reduces the max current.
NiCAD cells are only 1.25V each so three produce 3.75V, then your torch is dim.
A 18650 is a Lithium 4.2V cell so the LED current is way too high and makes a lot of heat. Calculate the value of a suitable series current-limiting resistor.

Some cheap torches have no series resistor. They use the series resistance of a cheap battery to limit the current.
 

Thread Starter

ifjdjrj

Joined Aug 17, 2020
4
The designer of your "torch" probably used Ohms Law to calculate the value of a suitable series resistor to reduce the CURRENT.
Ordinary throw away battery cells are 1.5V each so three produce 4.5V with no load. Their quality determines the max current and the resistor in the torch reduces the max current.
NiCAD cells are only 1.25V each so three produce 3.75V, then your torch is dim.
A 18650 is a Lithium 4.2V cell so the LED current is way too high and makes a lot of heat. Calculate the value of a suitable series current-limiting resistor.

Some cheap torches have no series resistor. They use the series resistance of a cheap battery to limit the current.
this is some image of the led and circuit board
 

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