• State
  • Locked Locked
  • Replies 8 replies
  • Subscribers 63 subscribers
  • Views 2127 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ25504 design for charging 3.7 V Li-ion battery

Zheng Ge
Prodigy 50 points
Other Parts Discussed in Thread: BQ25504

I want to use BQ25504 to charge 3.7 V Li-ion battery but I have no idea about how to determine the design requirement for my cells. In solar application example, the requirement is  VBAT_OV = 3.15 V, VBAT_UV = 2.20 V, VBAT_OK = 2.44 V, VBAT_OK_HYST = 2.80 V. Can anyone tell me how to select right voltage for my design?

  • 0
    TI__Mastermind 20570 points

    Please refer to datasheet page 11 to 12 on explaination of what VBAT_UV, VBAT_OV, and VBAT_OK  threshold means. Figure 11 shows the operation of the IC when VSTOR reach each of the threshold. Then please refer to your battery datasheet to find out the corresponding value.

  • 0 in reply to Jing Zou
    Prodigy 50 points

    Thank you Jing for reply! I think I understand the meaning of those terms but still not quite sure how to select right value based on battery. 

    I found a Li ion battery from digi-key, EFL700A39 (here is a link of its datasheet: www.st.com/.../CD00270103.pdf)

    And some voltage showed in datasheet are: Vnormal: 3.9 V,  Vop: 3.0-4.2 V. So based on these voltage value,  I guess VBAT_UV is 3.0V, VBAT_OV is 4.2 V, and VBAT_OK is 3.9 V. Am I correct on this? But what is the value of VBAT_OK_HYST?

    Since I don't have any background in electrical engineering, my questions could be too basic but I really appreciate if you can help me figure it out. Thank you again!

  • 0 in reply to Zheng Ge
    TI__Mastermind 20570 points

    When the voltage on VSTOR rises above VBAT_UV + internal VBAT_UV_HYST,the FET between VBAT and VSTOR will turn on shorting  VBAT to VSTOR.   When voltage on VSTOR fall below VBAT_UV, the FET between VBAT and VSTOR will turn off, VBAT will be disconnected to VSTOR. Since you dont want to overly discharger your battery,  in your case,  you can set VBAT_UV to be slightly higher than 3V. 

    VBAT_OV should be where your battery is going to be charged to. You are correct, you can set VBAT_OV to 4.2V.

    BAT_OK is digital output signal for your MCU or host  to indicate your battery voltage is at an acceptable leve. When the voltage rise above VBAT_UV_HYST threshold, the VBAT_OK output will be pulled high. When the battery voltage fall below VBAT_OK threshold, the VBAT_OK output will be low. This will depend on your application need.

  • 0 in reply to Jing Zou
    Prodigy 50 points
    Thank you so much! I think I fully understand how it works.
    Here is another question, can I get this EVM customized by TI so that I can get right R1 to R10 for my application?
  • 0 in reply to Zheng Ge
    TI__Mastermind 20570 points

    Here is the link to order the BQ25504EVM.

    https://store.ti.com/BQ25504EVM-674-BQ25504-Battery-Management-Evaluation-Board-Ultra-Low-Power-Boost-Converter-IC-Energy-Harvester-P2689.aspx

    Note that solder residue causing parasitic resistors in the 1M range which is much lower than the 10M+ resistor dividers. This will cause regulation issues. Therefore, when changing resistors make sure the solder flux is cleaned thoroghly. Soldering the resistors on it's narrow side (tubestone) will also help reduce the flux residue.

  • 0 in reply to Jing Zou
    Prodigy 50 points

    Thank you jing, I 've ordered one. Thanks for your help!

  • 0 in reply to Jing Zou
    Prodigy 50 points

    Hi Jing

    I did some test with BQ 25504 to charge two serial-connected capacitors (5 F, 2.7 V each) and the curves are shown in the attachment. The overvoltage was set at 5 V and the undervoltage was set at 3.8 V.

    For the blue and purple curves, MPPT did not work until the voltage of capacitor reached 1.6 V. I checked input voltage that it was close to OCV before output voltage reached 1.6 V. I'm wondering why it happened like this?

    Best,

    Zheng

    Picture1.tif

  • 0 in reply to Zheng Ge
    TI__Mastermind 20570 points

    Which voltage are you refering as "voltage of capacitor"?

    The MPPT loop is not active in the cold start. Once the IC gets out of cold start, for the firs open circuit voltage sampling, the IC only gives 64 ms for the input to settle to its VOC. From the second MPPT cycle and onwords, the IC wil allow 256 ms for the input to settle to its open circuit voltage.

    When doing measurements on high impedance source and system, it is important to set the meter output impedance to Hi-Z.