Li-Ion battery charging requirements

[Phoibus]

Hello,

I am working on a project to design a charger for Li-ion batteries 7.2V 2000mAh. I am not a battery expert, so any help is welcome

I studied some tutorials regarding charging the Li-ion batteries. How should I determine the charging requirements? The original charger outputs Vout=10.5-12.5V @ Iout=700-800ma. Is it ok to charge a 7.2V battery with such a high voltage?

Thank you in advancve!

 

[FvM]

Unless the battery has a built-in electronic that cuts off charging, it should be charged constant I/constant V. Usual charge voltage is 4.2V per cell, refer to battery datasheet.

12.5V sounds pretty much like a 3-cell battery (10.8V) charger.

 

[Phoibus]

The battery I want to charge is NNTN4497 from Motorola. Unfortunately I don't have the battery datasheet and the manufacturer seems rather secretive... I don't find the charging requirements.

I found the charger specifications:

https://www.amazon.com/Charger-Motorola-NNTN4497-Li-ion-BATTERIES/dp/B016E50CWU

Are these charge specifications correct, or should I rather design the charger for 8.4V?

I designed the charger with LT1510 and I am very found of the charger controller LTC1731 (unfortunately the latter is not very easy to find).

 

[Audioguru]

I found the charger specifications:

"www.amazon.com/Charger"

Are these charge specifications correct, or should I rather design the charger for 8.4V?

When Amazon lists the size and weight for the detailed specifications of a complicated electronic device then you know that they are very very stupid about electronic products.
Motorola and most copycat batteries also list no technical details about the battery. I would be afraid that it is not a normal Lithium Cobalt or Lithium Manganese type that is fully charged at 4.2V per cell but it might be a newer Lithium Iron type that is fully charged at 3.6V and might blow up or catch on fire if charged to 4.2V per cell.

 

[FvM]

A typical charge specification for a 7.2V 2.4Ah battery:

Recommended charge rate is 466mA to 8.4V in a temperature range of 0°C to 45°C. Hold at 8.4V until current declines to 50mA. Maximum charge
rate is 1.63A @ 23°C±2°C.

The battery has built-in under- and overvoltage protection, but no charging controller that would allow to operate it with a higher charging voltage than about 8.4V.

Some "intelligent" processor controlled chargers are able to detect the battery voltage and would be able to charge a 2-cell Li-Ion battery with correct charge end voltage. I'm not sure about the linked charger, but the said "10.5 to 12.5V" sounds inappropriate.

 

[Phoibus]

it might be a newer Lithium Iron type that is fully charged at 3.6V and might blow up or catch on fire if charged to 4.2V per cell.

So your suggestion would be to design the charger for 7.2V?

- - - Updated - - -

Some "intelligent" processor controlled chargers are able to detect the battery voltage and would be able to charge a 2-cell Li-Ion battery with correct charge end voltage. I'm not sure about the linked charger, but the said "10.5 to 12.5V" sounds inappropriate.

Indeed, the original charger has an integrated circuit 084P03-543 from NEC. I couldn't find its datasheet, and neither do the suppliers have it available. Considering that it is a 30-pin IC I believe it is some kind of micro-controller. Then again,, since I don't know what this IC is, I can't figure out how the original charger works.

I've attached the circuit schematic available from linear.com, developed around the LT1510, and the corresponding Spice simulation results. I have modified the resistance values for a 7.2V battery and 800mA charging current. Is it ok that I get a 1.7A peak current in the initial transient?

Another question, is it ok to assume that the middle pin of the motorola battery stands for the thermistor?

 

[Audioguru]

There is a lot of guessing about this battery.
If the Lithium battery is the ordinary kind that is fully charged at 8.4V then it is nearly dead when charged to only 7.2V.

The schematic in the datasheet and yours show a resistor in series with the capacitor at pin Vc but the text in the datasheet says the capacitor is connected directly to ground (no resistor) for soft start.

The function of the center pin is unknown but you can guess what it is for.

 

[FvM]

Most 3-pin Li-Ion batteries have either a thermistor connected to the center pin or the pin is unused. You can identify a thermistor with a multimeter.

 

[Phoibus]

Thank you for your replies!

There is a lot of guessing about this battery.

I was afraid of so much guessing...

Most 3-pin Li-Ion batteries have either a thermistor connected to the center pin or the pin is unused. You can identify a thermistor with a multimeter.

Thanks for the tip! I will do this.

What I still don't understand is if the battery is labeled 7.2V and 2000mAh, is it OK to charge it at 8.4V as written by FvM earlier? And is the 800mA charge current OK with the 1.7A transient peak? Or if i do this it goes kaboom.

 

[navbp]

It is recommended to charge the battery at 0.3C in constant current mode initially and then in constant voltage mode, as mentioned by FvM.
In your case, 1C = 2000mA. So, 0.3C = 600mA.

Charging the battery at a rate of 0.3C means that it takes approx. 3 hrs to charge the battery.
And Its safe and battery wouldnt get heated.
Li Ion has limited temperature range. and heating of battery would affect its service life.
Charging at higher rate will affect the life of the battery.

I think, when Charging at 8.4V input, you dont have control over the charging current, unless you use an extra circuitry for the same.
"Intelligent" charging controllers charge the battery at constant current initially (80% of the charge stored), Monitor the cell voltages and when it reaches its rated voltage, switch to constant voltage mode to charge it completely.