# Building a battery recharger

I got a battery management board.
According to the specs the charging
Voltage is 12.6 to 13 volts. What if it
Is a little more say 13.7 volts. Will it
Still recharge the 18650 lithium ion
Cells at 13.7 volts.

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Borek
Mentor
Not only it will charge the battery, it will also fry it in the process, if there is no current cut-off.

Wrichik Basu
Not only it will charge the battery, it will also fry it in the process, if there is no current cut-off.
Yes. You must always limit the current. If the battery is empty you should limit the charging current, because the current will get too large even with the charging voltage per cell below 4.2V. The battery will get too hot and likely burn.
Once the voltage is at 4.2 V, it should be held steady at this voltage until the charging current is at 10% of the maximum charging current, and then the charger should switch off.
You should really measure both the voltage and the current during the whole charging cycle and make sure it never goes out of spec.
If the output voltage of the charging circuit is 13.7 V without a battery connected, this might not mean much however.

An 18650 cell is 3.7 V. Trying to charge cells in series is a bad idea. If one of them is charged somewhat more, the total voltage might be within spec, but the voltage across the cell with the most charge on it might be too high. A balance circuit can be used to adjust the charges and keep the voltages across all 3 cells at max 4.2 V. This will get complicated.

18650 battery cells also often have a built-in protection circuit. In that case you will likely get incompetely charged batteries once the protection kicks in.

It was described as a protection board
With balance function. Does that mean
It has built-in protection circuit? Does
That mean it can limit current?

berkeman
Mentor
It was described as a protection board
With balance function. Does that mean
It has built-in protection circuit? Does
That mean it can limit current?
Can you post links to the datasheets for the board and the battery? We obviously need to have more information in order to try to help.

Also, what is your background in electronics? What is the application for the battery?

The model of bms board is hx-3s-25a.
I have not studied electronics at any
I have a breadboard and practice making electronic circuits. I tinker
With electronic components such
As laptop parts. I have some experience programming microcontrollers. I am trying to learn
How to recharge 18650 lithium ion cells. Possibly for future diy projects.
I am an electronics hobbyist.

I bought this bms board at banggoon.
I don't think banggoon provided much
Information. To look up datasheets
I just type hx-3s-25a and check whatever website I find.

It has ports labeled p+ and p- and no
Ports labeled c+ and c-. That means
It has only discharge ports and no
Recharge ports.

I have never used bms boards before.
This all very new to me. This is the first
Time I have used a bms board.

CWatters
Homework Helper
Gold Member
The model of bms board is hx-3s-25a.
It looks like there are many clones of that board all with slightly different model numbers but all similar in appearance. All appear designed to be added to a 3S battery pack made from 3 cells in series (and several strings in parallel) to make a pack of 11.1V nominal. They appear to provide both over and under voltage protection.

One web site said you need to feed it 12.6 to 13v which suggests to me that the board isn't a charger, just a protection device. If you feed it more than 13V it might just disconnect the battery so it isn't charged.

You need to feed it a voltage in range and current controlled. The current will depend on how many strings you have in parallel.

What current in amps should you use
To recharge a 18650 lithium ion cell? I
Know the voltage is 4.2 volts. But what
Current in amps should I use to recharge 1
18650 lithium ion cell.

berkeman
Mentor
Thread closed for Moderation because of safety concerns...

berkeman
Mentor
What current in amps should you use
To recharge a 18650 lithium ion cell? I
Know the voltage is 4.2 volts. But what
Current in amps should I use to recharge 1
18650 lithium ion cell.
Thread will remain closed due to safety concerns. @David lopez -- please find a local Mentor who is good at electronics and already understands the safety issues involved in battery chargers (especially for Lithium Ion batteries). Mistakes made in charging batteries can cause fires and injuries or worse. Please be safe.

dlgoff and anorlunda