LM317 Reverse current during Battery charging

[kay_niv]

I'm currently doing a project where I need to check the battery protection circuit of a LiPo .
My current setup is a 12v supply from which by making use of LM317,Arduino(i vary the Vadj) to get a voltage range of 2V ~ 5V output.

when i connect the charger it would produce 4.2V at the 3.7V (output of the LM317).
I'm not clear as to how LM317 handles reverse current and how much it can handle ?

My aim is to check the outputs Discharging,Charging over the different voltage range ( 2V ~ 5V).
How do i protect my test setup so that varying the bat_voltage(LM317 outputs) does not affect/burn the setup when i connect the charger.

Thanks

 

[Audioguru]

The datasheet for the LM317 says that its absolute maximum allowed input-output voltage differential is +40V, -0.3V. Then you should never allow the output voltage to be more than 0.3V higher than its input voltage.
They say, "When an output capacitor is connected to a regulator and the input is shorted, the output capacitor will discharge into the output of the regulator. ....this discharge path is through a large junction that is able to sustain 15A surge with no problem."
They say when a high value capacitor is added to the output then a diode should pass its very high discharge current to avoid damaging the LM317.

Why are you feeding a voltage to the output of the Li-Po battery cell that is higher than its charged voltage? You charge a Li-Po cell from a limited current then the cell's voltage slowly rises to a limit of 4.20V. The protection circuit does not protect the cell from feeding a voltage backwards to its output. The protection circuit limits the charging current then limits the maximum charging voltage to 4.20V, similar to a Li-Po charger circuit.

Don't you know that a Li-Po cell voltage averages 3.7V? Its actual voltages are 4.20V when it is fully charged, then as it discharges its voltage passes through its average of 3.7V, then continuing a discharge its voltage slowly drops to 3.2V when it should have its load disconnected. When a cell that is 3.2V is charged, then the charger feeds it a limited current and its voltage slowly rises. You never charge it from 4.2V at many Amps.

A Chinese company sells a photo of a protection circuit to Li-Po battery manufacturers. The photo is identical to an actual protection circuit, it is discussed and shown on You Tube. The "protected" battery cell you bought might have the fake protection circuit. Is that what you are testing for?

 

[kay_niv]

Actually I do not connect the battery. I'm currently testing my custom board and have a automated tester which produces 2 voltages one for battery voltage input to the board and other for Charge circuitry on the board. I need to test the behaviour of the DO(Discharge Output) and CO(Charge Output) of the battery protection IC. To check their behaviour i vary the bat_input voltage and check the DO and CO pins for ON/OFF.

 

[Audioguru]

I assume that DO (Discharge output) is the protection circuit limiting the maximum output current from a battery cell.
I have no idea what CO (Charge Output) is, it might be the protection circuit limiting the maximum charging current or the limiting the maximum charging voltage. When a Li-Po protection circuit limits the current or voltage then it does it linearly, nothing switches off.

 

[kay_niv]

the battery protection circuit i'm using is S-8241
**broken link removed**.
I need to test if those outputs(DO/CO) are properly turned on and off as i vary the battery input voltage.the DO/CO are the inputs to mosfets controlling the ground/bat_neg connection thereby controlling charging/discharging on the board.

 

[Audioguru]

The Seiko IC does not correct a wrong voltage or current from a battery charger. If it detects a wrong voltage or a wrong current then gives a digital output which should turn off the charger circuit when it detects an error.