Need help with Power Adaptor voltage

Thread Starter

somdutttah

Joined Jan 18, 2024
9
I have a 12v rated adaptor but the output voltage it is giving is 13.7v which is very high to charge a 12.6v 3s lithium battery pack. So need help to understand this circuit and how to make this circuit produce 12.6v
 

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LowQCab

Joined Nov 6, 2012
4,075
You need to add an accurate LDO ( Low-Drop-Out ), Voltage-Regulator.
Then You need to determine the final Charging-Voltage that You will need based on
the type of usage the Battery will be subjected to,
and the type of Battery Life-Expectancy You want to aim for,
and the speed at which the Battery receives a Full-Charge.

Lithium Cells can not be Charged directly by an unregulated Power-Supply,
they can easily catch on FIRE.
.
.
.
 

Ya’akov

Joined Jan 27, 2019
9,169
I have a 12v rated adaptor but the output voltage it is giving is 13.7v which is very high to charge a 12.6v 3s lithium battery pack. So need help to understand this circuit and how to make this circuit produce 12.6v
Welcome to AAC.

I strongly advise against trying to modify that power supply. It is a dangerous design with no anti-tracking slots to isolate the high voltage section from the low voltage section. It also has extremely minimal protection against problems with the mains. I wouldn't use it for anything.

You are best off with a purpose built Lithium charger. Lithium cells need at least a CC/CV (Constant Current/Constant Voltage) strategy and some kind of protection against over-charge and over-discharge to be safe. A power supply without a charger circuit is a very good way to start a Lithium battery fire due to thermal runaway.

Please rethink this and do it the right way—faulty batteries and chargers have burned down many homes and businesses.
 

Pyrex

Joined Feb 16, 2022
267
hi,
you can modify a little voltage feedback circuit. As i see, there's the optocoupler close to the transformer. In most cases, feedback is built on the optocoupler and 431' regulator, so only one resistor to be changed
 

Dodgydave

Joined Jun 22, 2012
11,303
I have a 12v rated adaptor but the output voltage it is giving is 13.7v which is very high to charge a 12.6v 3s lithium battery pack. So need help tohe understand this circuit and how to make thiif s circuit produce 12.6v
The output will be regulated by the Zener diode Z1 on the right side ( next to the optocoupler) with a resistor, it's a poor type of psu design, you could put a diode in series with the output to drop 0.7V to see if the voltage drops, otherwise changing the Zener diode to lower it
 

Thread Starter

somdutttah

Joined Jan 18, 2024
9
hi,
you can modify a little voltage feedback circuit. As i see, there's the optocoupler close to the transformer. In most cases, feedback is built on the optocoupler and 431' regulator, so only one resistor to be changed
Can you please help me with this. Can adding a potentiometer in place of the resistor be a good idea?
 

Dodgydave

Joined Jun 22, 2012
11,303
Put your voltmeter across the Zener diode (in the picture) and see what voltage it is, its probably a 13/14V type, the if you can get a 12V one it should lower the output voltage.

IMG_20240119_101505.jpg
 

MisterBill2

Joined Jan 23, 2018
18,538
13.7 volts is a reasonable no load output voltage for a 12 volt POWER ADAPTER. If a device is not identified as a battery charger then IT IS NOT A BATTERY CHARGER, and is not suitable for use as a charger for a lithium battery without some additional circuitry. Others have already advised about that.
In an emergency A battery can have greater voltage applied for short time intervals with cooling off time in between. But there is a risk involved and the consequences can be violent, such as exploding the battery being charged.

Moderator edited: changed 132.7 to 13.7 volts
 

Pyrex

Joined Feb 16, 2022
267
I already changed the zener diode (12v), but it didn't had any effect.
The output voltage was tested with load , or without load?
As i see, it's a simplified adapter. Do not expect very stable output voltage, imho. Add a resistor at the output and check difference at the output with load and with no load. Say, 0% of rated current and 50% of rated current
 

Thread Starter

somdutttah

Joined Jan 18, 2024
9
The output voltage was tested with load , or without load?
As i see, it's a simplified adapter. Do not expect very stable output voltage, imho. Add a resistor at the output and check difference at the output with load and with no load. Say, 0% of rated current and 50% of rated current
Without load it's 13.7 volt, with load it's 13.5 volt. Added 47 ohm resistor with the output (positive output wire). With load it came to 13.3 volt, the voltage is not fluctuating with or without load. The amps provided with load is 1.97
 

Dodgydave

Joined Jun 22, 2012
11,303
Here's the output circuit, the Zener diode sets the output voltage to trigger the optocoupler.

NOTE, this is Not for Lithium Batteries, it's a constant voltage output.
IMG_20240119_181948.jpg
 
Last edited:

Audioguru again

Joined Oct 21, 2019
6,696
Even if the adapter produced exactly 12.6V at a reasonable charging current, the adapter is not a balanced 3 lithium-Ion cells charger. Bal;ancing senses the voltage of each cell and prevents the weakest cell from charging first to a voltage too high that causes it to catch on fore or explode. But the battery might have a protection circuit inside it to prevent over-charging.
 
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