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bq2000 charging issues

Eric Lew
Prodigy 20 points
Other Parts Discussed in Thread: BQ2000

Our team is designing a digital hearing aid for a school project that will be rechargeable via a micro USB cable. The BQ2000 battery charger IC is being used to implement this but we are coming across some battery charging issues. This chip is located in the bottom right section of the attached schematic. I made the connections by following the examples found in the datasheet. The +5 V is the USB power supply.

We are using a single cell NiMH battery which has a battery voltage of 1.2 V. The intended charge time is 90 minutes. After taking some measurements of the battery voltage when connected to the circuit, we found out that the battery voltage did not increase at all. Interestingly, the battery voltage did increase when it was disconnected from the circuit possibly due to the battery's chemistry? I am not sure why it does that.

The other part of the schematic is the boost converter that will boost the voltage from 1.2 V to 1.8 V to power the digital signal controller on the other circuit.

Does this schematic show a correct configuration for battery charging? I was unsure of how the MOD pin should have been hooked up.

Thanks!

Battery Charger Circuit Picture.pdf
  • 0
    Prodigy 80 points

    Eric,

    Did you solve your problem? I have designed a similar circuit and could offer my thoughts if still needed.

     

    Regards,

     

    Phil

  • 0 in reply to Phil Bromley
    Prodigy 20 points

    Phil,

    Unfortunately we didn't solve the problem. I believe the issue is with the way I created the circuit for the modulation (MOD) pin. I didn't fully understand that pin and the datasheet didn't provide enough information on that. The project has been due but I welcome any thoughts on why the circuitry might've not worked.

  • 0 in reply to Eric Lew
    Prodigy 80 points

    Eric,

    I had one or two problems getting my circuit to work so I have read the datasheet and application notes quite thoroughly. These are my thoughts and may not be entirely correct (although I think that they are...).

    The main problem is the input to the BAT pin. You don't need a potential divider there, as you only have one cell, so get rid of the 200K resistor RB2. As it is drawn, there will be less than 1 volt on the BAT pin, which will put the BQ2000 into trickle charge mode - it thinks that the battery is totally discharged.

    The MOD drive looks OK to me. The MOD pin switches on to provide charging current, and is switched off every cycle by the voltage building up on the SNS pin. When the SNS pin gets to 50mV the MOD drive switches off. You might want to put in the other transistor shown on the datasheet (Q2 on P10) as it turns off your Q2 faster and thus makes the circuit more efficient.

    Do you need the load to be on when the battery is charging? You could tie the LED pin of U1 to the EN pin of U2, with a pullup resistor, perhaps. This is because if your load is varying it makes the end of charge detection more problematic. The end of charge is determined by measuring the peak voltage - when the cell voltage starts to fall at full charge. Any variation in load current can confuse the detection circuit.

    You show two cells in parallel - this can be dicey if they are not absolutely matched. I am sure that you checked with the supplier that they can be charged in parallel. Some manufacturers don't recommend it.

    Hope this is of use.

     

    Regards,

     

    Phil

  • 0 in reply to Phil Bromley
    TI__Mastermind 31120 points

    For cells in parallel, as long as they are the same chemistry, it does not matter if they are the same capacity.....as long as they are relatively the same voltage when connected together.  It is like adding capacitors together in parallel....is not a safety issue as long as none of the ratings are exceeded.