Will voltage across transformer drop when charge a lithium ion battery?

Thread Starter

MrsssSu

Joined Sep 28, 2021
200
If I have a 4W transformer that has high inductance, thick windings, (4V ,1 A) and converted to DC via a rectifier, in order to charge a lithium ion battery (low internal resistance) ,
will the output will still be slightly less than 4V or will it be very low (close to 0 which makes it impossible to charge a battery) because the battery load has so little internal resistance?
Has anyone tried that before? :)
 

Ya’akov

Joined Jan 27, 2019
5,998
If the transformer is really 4V you will not be able to charge Lithium cells with it. Even without the voltage drop of the bridge rectifier (≥1.2V) the 4V isn’t enough To charge the cell to the typical 4.2V required, add in the drop to 2.8V and it’s just not going to work.
 

BobTPH

Joined Jun 5, 2013
5,227
You cannot simply connect the DC voltage to the battery unless you want to start a fire.

You don’t know enough to make a Li Ion charger safely. And why would you, when you can buy one cheaper than you can nake it?

Bob
 

Ya’akov

Joined Jan 27, 2019
5,998
You cannot simply connect the DC voltage to the battery unless you want to start a fire.

You don’t know enough to make a Li Ion charger safely. And why would you, when you can buy one cheaper than you can nake it?

Bob
I didn‘t even get to the charge controller problem because the power source is a non-starter, but as you point out, it’s not just a matter of hooking up a power supply and letting it go.
 

BobTPH

Joined Jun 5, 2013
5,227
If the transformer is really 4V you will not be able to charge Lithium cells with it. Even without the voltage drop of the bridge rectifier (≥1.2V) the 4V isn’t enough To charge the cell to the typical 4.2V required, add in the drop to 2.8V and it’s just not going to work.
Using Schottky diodes you can probably get enough voltage to charge at something well below 1A.

Edit: Peak voltage of 4x1.414 = 5.65V minus 1V for the bridge.

Bob
 

Ya’akov

Joined Jan 27, 2019
5,998
Using Schottky diodes you can probably get enough voltage to charge at something well below 1A.

Bob
If the transformer is actually 4V it’s not enough without any other losses. If it is 5V, yes, Shottkys would rescue it, very barely and it would still be undercharging the cell.
 

MisterBill2

Joined Jan 23, 2018
12,399
Use a voltage double rectifier circuit and the a current regulator to deliver only the recommended charging current. And limit the charge time.
 
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