# how build a simple charger

#### dashhossein

Joined Jul 30, 2020
48
hello everyone , for educational goals we want to make a simple charging board which charge simple 3.7 V mobile batteries ...
i have some question for example what happened if i directly connect that battery to 5V 2A ? will charge correctly ? will battery damaged ?

what's the correct form to charge batteries ?

#### Irving

Joined Jan 30, 2016
2,067
if you connect a 3.7v lithium-ion battery to a 5v supply it will charge it too fast and could overheat & explode. Even if it doesn't you will severely reduce the life of the battery.

The battery needs to be charged at a constant current, typically 1/10 - 1/2 of its rated capacity, until it reaches 4.1v and then at a constant voltage of 4.1v until the charge current drops to a few mA, but generally no longer than a few (typically 5) hours.

This isn't easy to do 'simply' for educational purposes (I have previously designed one using about 20-25 discrete parts). There are cheap boards you can get from eBay/Amazon that use a dedicated charger IC, but that has little/no educational value.

Research Li-Ion charger IC such as TP4056 and read the data-sheet to see how they do it.

#### DickCappels

Joined Aug 21, 2008
7,704
This may be too simple but I use a lab power supply and a series resistor, set the voltage so that the current as measured by the voltage drop across the resistor is 1/10 the battery's amp-hour capacity. I have been doing this for years for Ni-cd, mercury, alkaline, and NimH batteries. Small ones, by the way.

To make a charger for regular use I suggest buying an integrated circuit designed to charge the kind of battery you want to charge. If an NiMH batter is charged with the wrong profile (current over time) it might catch fire in the future -they are pretty picky.

Here is an example of one such charger chip:
MCP73831/2 Data Sheet

#### dl324

Joined Mar 30, 2015
13,096
i have some question for example what happened if i directly connect that battery to 5V 2A ? will charge correctly ? will battery damaged ?
Li-ion batteries don't tolerate abuse well. Even when charged properly, they can do bad things.

Since this is homework, you must have done some research about these batteries.

#### crutschow

Joined Mar 14, 2008
27,718
This may be too simple but I use a lab power supply and a series resistor, set the voltage so that the current as measured by the voltage drop across the resistor is 1/10 the battery's amp-hour capacity. I have been doing this for years for Ni-cd, mercury, alkaline, and NimH batteries. Small ones, by the way.
The op wants to charge 3.7V batteries, which are likely Li-Ion.
I wouldn't recommend that technique for those type of batteries.

#### DickCappels

Joined Aug 21, 2008
7,704
I agree fully. Neither do I for batteries of any significant size.

#### MrAl

Joined Jun 17, 2014
8,475
This may be too simple but I use a lab power supply and a series resistor, set the voltage so that the current as measured by the voltage drop across the resistor is 1/10 the battery's amp-hour capacity. I have been doing this for years for Ni-cd, mercury, alkaline, and NimH batteries. Small ones, by the way.

To make a charger for regular use I suggest buying an integrated circuit designed to charge the kind of battery you want to charge. If an NiMH batter is charged with the wrong profile (current over time) it might catch fire in the future -they are pretty picky.

Here is an example of one such charger chip:
MCP73831/2 Data Sheet

View attachment 245056
You charged alkaline? How did that work out in the long run?

I have used a similar technique, but only for NiCd and NiMH. NiCd is quite tolerant for a time, but NiMH is less tolerant to overcharge.

Strange as this may sound, you can do the same with Li-ion but you have to have a power supply with good regulation and good voltage limit adjustment. In theory, you set the max voltage to 4.20v and the max current to maybe 700ma (most Li-ion 18650 will take this easily but adjust as per your actual size cell).
As the cell charges it will start out at max current but with less voltage (typically 3.5v or so) until the cell starts to become close to fully charged and then the current will cut back and the voltage will rise slowly from around 4.10v to 4.20v. When the cell gets down to around C/20 you can terminate the charge and you really should terminate at that point.
But i say "in theory" because a better setting for the voltage is around 4.15v for a typical normal cell. That gives some room for error in the voltage measurement and allows the cell to age slower.

Self discharge management is also a good idea for NiCd and NiMH. A regular charge current of maybe 100ma for short time twice a day is better than a small current like 10ma over the entire day. I got NiCd cells to last pretty long like this while keeping them topped off 24/7 for quick use in a portable drill.
A microcontroller works wonders for these applications.

#### DickCappels

Joined Aug 21, 2008
7,704
At the risk of taking this thread off track, yes, I recharged AAA alkaline cells for year. I had a pocket radio that I would take on my daily walks and I recharged the alkaline cells it used. The capacity was nothing like the capacity when they were new, but recharging with a simple current source set to a low current saved me from buying a lot of cells.