looking for DC battery charging circuit

[KlausST]

Hi,

so can I use this circuit for charging these two 18650 batteries in series

it says "2-cell Li-Ion " what does it mean ?

Two = 2
18650 = LiIon cell
==> yes, you can use this circuit

also the input voltage is 13-20v , can I use 9-12v adapter?

Is the adapter voltage of 9...12V within the specified input voltage of the circuit13...20V?
==> No! You can't use this adapter with this circuit.

Klaus

 

[Audioguru]

Two 18650 cells in series is 2-cell Li-Ion.
The input to the charger circuit must be 13V to 20V so a 9V to 12V adapter voltage is probably too low. The two cells will make 8.4V when they are fully charged.

 

[aliyesami]

ok i will go with the last schematic I posted with an AC-DC adapter of 15v 1a to charge the batteries.
now my second question is how is it done , does the battery remains connected to the main circuit in my case the DC-DC buck or its isloated during charging and the adapter is supplying input to the DC-DC buck n also charging the battery?
and how to do all this electronically ?

 

[Audioguru]

A battery charger usually charges the battery and measures its charging current. Since it is measuring the current then another load (a buck converter powering a circuit) will mess it up.
While charging the battery you must switch the input of the buck converter away from the output of the charging battery and connect it to the output of the adapter that feeds the charger.

 

[aliyesami]

While charging the battery you must switch the input of the buck converter away from the output of the charging battery and connect it to the output of the adapter that feeds the charger.

so in the charging mode (when i plug in the adapter) the adapter will be feeding the charging circuit and the DC-buck .
so how do I isolate the battery from the DC buck ?
are there standard circuits for this ?
in this circuit for example the batteries are in parallel with the DC adapter and not isolated from the input to the circuit so not a good design?
**broken link removed**

 

[Audioguru]

The circuit you found has errors:
1) No Lithium cell has a charge voltage as low as 2.6V, they are 4.2V.
2) The circuit simply monitors the charging voltage which is done with Ni-Cad and Ni-MH cells. But a Lithium cell must be monitored for current and when the charging current drops below about 1/40th of its rating then the charge must be terminated. When the voltage of a charging Lithium cell reaches 4.2V then the cell is only about 70% of fully charged.

You need a battery charger IC that is designed for Lithium battery cells and some have an extra terminal to sense battery current while charging and supplying power to a load.

- - - Updated - - -

Did you notice that there is no Lithium rechargeable battery that has the size of an AA carbon-zinc, alkaline, Ni-Cad or Ni-MH cell? Guess why.
Therefore the article you posted of charging an AA Lithium cell is completely wrong.

 

[aliyesami]

ok ignore the circuit that i posted last , it was just from random person on web, but what about this schematic from Texas instrument , Klaus approved it I think earlier .
if this is the right circuit then can you or someone tell me how to isolate the battery during charging from the DC-buck.

thanks

 

[Audioguru]

The charger circuit from Texas Instruments is missing important safety features that are included in a Lithium battery charger IC:
1) Detection of a battery voltage that is too low and either refusal to charge (to prevent setting the battery on fire) or attempt charging at a reduced safe current.
2) An input for a thermal sensor to detect a battery that is getting too hot.
3) Balanced charging circuit to prevent one cell from becoming over-charged which might set nit on fire.

I think Maxim has good Lithium battery charger ICs.

 

[aliyesami]

The charger circuit from Texas Instruments is missing important safety features that are included in a Lithium battery charger IC:
1) Detection of a battery voltage that is too low and either refusal to charge (to prevent setting the battery on fire) or attempt charging at a reduced safe current.
2) An input for a thermal sensor to detect a battery that is getting too hot.
3) Balanced charging circuit to prevent one cell from becoming over-charged which might set nit on fire.

I think Maxim has good Lithium battery charger ICs.

hi audioguru can you recommend me one from Maxim?
how about this one from MAXIM ?
https://datasheets.maximintegrated.com/en/ds/MAX745.pdf


thanks

- - - Updated - - -

ok talked to Maxim engineers and I am giong to use their evaluation kit for this chip . this chip has all the features that audioguru mentioned.

 

[aliyesami]

I am designing the charging circuit using MAX745 chip and talked to the engineer there and he is asking me that I should know the "Charging current " of my batteries.
where do I get that? the 18650 batteries being sold in market are not listing any such thing

thanks

 

[Audioguru]

Li-Ion cells are not sold in a food "market". Some Chinese 18650 Li-Ion cells sold on ebay are fakes and will catch on fire or explode if charged at a normal current.
www.batteryuniversity.com says that a Li-Ion cell is usually charged at 0.5 to 1.0 of its mAh rating which might be 2600mAh so it would be charged at 130mA to 260mA. They say then it takes 2 to 3 hours.
https://www.youtube.com/watch?v=eOshOXcSkDA

 

[KlausST]

Hi,

usually charged at 0.5 to 1.0 of its mAh rating which might be 2600mAh so it would be charged at 130mA to 260mA

Shouldn't it be :
1300mA to 2600mA ?

Klaus

 

[Audioguru]

Hi,


Shouldn't it be :
1300mA to 2600mA ?

Klaus

Yep. My arithmateek was wrong.

 

[D.A.(Tony)Stewart]

20h charge or 1C is safe. C/20 or 1h charge demands ESR *I² power dissipation to be low so that T rise is < 20'C for safe operation. This is controlled by aging and quality of battery.

you can measure Vrise with current rise and compute ESR, keep under 1Watt for safe charging and monitor temp periodically.

 

[aliyesami]

this charging circuit is too much to bite I think so I will revert back to my original plan of just charging the battery outside the device.
also i found out that my circuit wont use more than 75mA peak . so if i use a 9v battery together with MAX748A buck do you see any issues with it ? i think i can get 900mAh from a 9V battery so it should last couple of hours?

the datasheet of MAX748 is below :
https://datasheets.maximintegrated.com/en/ds/MAX748A-MAX763A.pdf

 

[Audioguru]

The datasheet for an Energizer 9V alkaline battery shows its voltage dropping to 6V in about 1.7 hours when its load is 200mA and in one hour when the load is 300mA. It is rated for 500mAh when its load is 100mA (and its voltage has dropped to 4.8V).
I saw a woman getting burned by a 9V alkaline battery in her pocket when it got shorted by a coin. Then the battery was dead in a few seconds.

 

[Audioguru]

Two Lithium cells are fully charged at 8.4V. The old LM317 voltage regulator needs an input of at least 2 V more and the diode needs 0.7V so a 9V adapter will not work. When a circuit shows a 13V minimum input then that is what it is.