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bq24170 battery charging

Other Parts Discussed in Thread: BQ24170, CSD17313Q2, BQ24172

I am designing a Li poly charger circuitry using the BQ24170 chip. The circuit (schematic is as shown below).

I am connecting a bench power supply instead of an adaptor to monitor the current consumption. Also I am using a 2S li pol battery pack. I found that I had connected the CELL pin in the FB configuration which is wrong as I am using the BQ24170. So the CELL pin should be kept floating for 2S battery pack. Hence-forth I removed the R11 and R18. But it still does not charge. The chip datasheet states the following -

The following conditions have to be valid before charging is enabled: • ISET pin above 120mV • Device is not in Under-Voltage-Lock-Out (UVLO) mode (i.e. VAVCC > VUVLO) • Device is not in SLEEP mode (i.e. VAVCC > VSRN) • OVPSET voltage is between 0.5V and 1.6V to qualify the adapter • 1.5s delay is complete after initial power-up • REGN LDO and VREF LDO voltages are at correct levels • Thermal Shut down (TSHUT) is not valid • TS fault is not detected • ACFET turns on (See System Power Selector for details)

Two conditions afrom the above are not met in my design -

Pin OVPSET - gives me 2.48 where as it should be between 0.5 and 1.6. and ACFET should be turned on which is not.

ACFET is not turned on. This means that ACDRV pin voltage is not enough and therefore the MOSFET Q2 is not ON. I see that R165 and C62 should be connected between the Source and the GATE of the MOSFET which is not so.

Is there any other issue which is not spotted by me please. ANt pointers will be welcome. The MOSFET used are CSD17313Q2.

One of the application engineers state that-

" The bq24170 only supports 4.2V/cell, but you can use the bq24172 if you would like an adjustable cell voltage such as for Li-Po battery. "

Does it mean that once charging using the BQ24170 starts , it stops only when the voltage in each battery is 4.2V/cell ? And that if I use BQ24172 I can stop charging it when the voltage reaches 3.7V/batter, right ? BTW , I am using a 2S battery which is giving me 5.6V.

Regards,

Vinod Karuvat.

  • Vinod, 

     

    If you're measuring >1.6V on OVPSET the IC is triggered in to input over-voltage protection and will not turn on Q1 and Q2. The bias on this pin should be set by external resistors.  What voltage are you putting on VIN? It should be <17V.  A 12V input with a 18V OVP would be pretty typical (100k/22.kOhm) as shown in Figure 1 of the bq24172 datasheet. 

    Unless your battery is 4.2V/cell, you should use the bq24172 to have adjustable battery voltage. This will allow you to charge your battery to 3.7V/cell. 

    Please reference the bq24172 EVM for best practice schematic and layout: http://www.ti.com/lit/pdf/sluu462 

     

    Thanks,

    Rachel 

  • Hello Rachel

    What do u mean by 4.2V /cell. The Max voltage per cell?

    I made the cell on float and reduced the Vin. The batt is 2S and it charges the batt. But it stops when the total voltage is approx 7.4V total. That is 3.7V/cell. I guess that makes my cell pack 3.7V/cell. Right?

    Vinod

  • Vinod, 

    The bq24170 is designed to be used with batteries which charge up to 4.2V/cell. 

    Charge termination is governed by termination current limit, which is equal to 10% of the fast charge current limit as controlled on the ISET pin. 

    The voltage/cell of your battery is determined by the chemistry and should be available from the datasheet/supplier of your battery pack.  I would guess that yours is 3.7/v cell, so you need to use the bq24172 to charge your battery.  

     

    Thanks,

    Rachel 

  • Hello Rachel,

    Thank you. I am able to make it work in the 2 Cell mode as I mentioned. Now, I got a lipol (3S and total of abt 12.4). This is 4.2V/Cell. I placed it in a system and drained it to abt 7.4V.

    Then I connected the battery to the BQ24170. The input charging is supplied via a bench power supply. The circuit is as shown above with modifications done.The bench power supply gives 9V.

    Now, the Icharge should be 4Amps and the initial charging should be around 0.4 (10%). But when I rig it up the charging circuitry draws 0.0034Amsp and in bursts sometimes 3Amps2.7Amps,1.7Amps and such. How so.

    Kindly assist me please.

    Regards,

    Vinod.

  • Vinod, 

    Your Vin supply must be higher than the pack voltage in order to charge. Please raise your supply to 15V.

    Thanks,

    Rachel 

  • Hello Rachel,

    I dont get the point abt 3.7V and 4.2V. I was reading an article abt li pol (http://www.instructables.com/id/Lithium-Polymer-Etiquette/). It says that all li pol will charge upto 4.2V and 3.7V is considered a kind of middle of charging value. 

    So, technically speaking all li pol batteries will be a max of 4.2V and a standard value is 3.7V. This means that anyy li pol batteries will by default be able to charge upto 4.2V. So why would 3.7V be an issue here.

    Also, my circuit works fine and I was able to charge my 3S batteries a couple of times. I charged the battery and drained it and so on and so forth. The battery is a 3S Turnigy battery. The battery shows 11.1V capacity which means each battery is 3.7V(standard). The battery is http://www.hobbyking.com/hobbyking/store/__8932__Turnigy_2200mAh_3S_20C_Lipo_Pack.html

    Then suddenly it is not charging proper. It draws abt 0.29Amps while charging and after abt 30 mins or so it reaches abt 7.5V (from an initial of 5V). Then it stops charging. Why so. Could you kindly point me to some documents abt li pol batteries (any TI specific docs) pls.

    Hope to hear from you