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best design option for a power bank using bq2005

tolu collins
Prodigy 110 points
Other Parts Discussed in Thread: BQ2005, BQ2040, BQ34Z110I'm designing a small power bank that uses AA NiMH batteries, and I have the following design questions. 1. What is the impact on the charging capacity of my power bank if I use two sets of two NiMH batteries in series, and then parallel the output? 2. If I have 4 NiMH batteries in series, can I charge these from a USB source using the bq2005? 3. What dc/DC converter will best serve to provide the 5v output from the batteries, either 2,3 or 4 batteries in series. 4. If the bq2005 is not the best chip for this design please recommend a another. Thanks.
  • 0
    TI__Genius 13700 points

    Hello Tolu,

    1. The capacity of your pack is dependent on a number of factors between the ampere-hour capacity and the battery regulation voltage. One of the challenges of stacking your cells is series to charge is that you will need a multi-cell charger and your gauging if you plan on having any will need to be protected. The result of charging cells in series is that you are able to charge at a higher voltage with a lower current which could improve the charging efficiency.

    2. You could charge 4 cells in series assuming that the maximum cell voltage of all of the cells stacked in series is at or preferably below 95% of the 5V according to Pg. 9 of the datasheet. However NiMH usually have a VMCV of about 1.5V or so making 4 in series 6V. With that in mind unless you boost the 5V input to some higher voltage you will not be able to charge them in series.

    3. In the case of 1, 2, or 3 cells you will need a boost converter at all times to generate 5V from the batteries. The case of 4 cells in series you will need a buck-boost to buck the voltage down when the batteries are mostly charged and the boost it up once the batteries have discharged enough to have a string voltage of 5V or below.

    4.I would think that the BQ2005 could work well if you stick with a 2s2p or a 1s4p configuration and have a boost converter take the voltage up to 5V.

  • 0 in reply to Ryan May
    Prodigy 110 points

    Hello Ryan,

    thanks for your reply.


    I addtion to the above:

    1. How do i implement Charge level indicator using the BQ2005 based design, something like a 4-Level LED indicator
    2. How best do I implement an over-discharge protection circuit for the NiMh battery pack.

    Thanks.

  • 0 in reply to tolu collins
    TI__Genius 13700 points

    Hello Tolu,

    The best way to implement a 4 LED charge level indicator would be to incorporate a gas gauge into your design.


    Here is a link to a narrow down listing of NiMH gauges that should have integrated LED charge level indicators.

    http://www.ti.com/lsds/ti/power-management/battery-fuel-gauge-products.page#!p1152=Single%20Cell&!p308=LCD;None&!p351=Impedance%20Track&p338=NiMH

    A device like the BQ2040 has a 4 segment LED charge level driver. It also has under voltage due to over discharge protection.

  • 0 in reply to Ryan May
    TI__Genius 13700 points

    Hello Tolu,

    You could also look at the BQ34Z110. This would cover all of the gauging that you are interested in accomplishing and has alarm functions that would indicate BATLOW to warn the rest of the system. You would need to incorporate protection FETs to be triggered from this alarm signal.