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USB NiMH Battery Charger

Sidhyesh Ramugade62853
Prodigy 130 points
Other Parts Discussed in Thread: BQ2002

Dear All,

                    We are developing Battery Charger Circuit for our Hand Held Product. We are using NiMH Battery comprising of 3 Cells in series (1.2V per Cell). The capacity of the Battery is 800mAh. USB is to be used for charging this battery. We want to charge this Battery to maximum voltage of 4.5V (1.5V per cell). Can anyone suggest suitable charger IC for the same?

 

Thanking You,

 

Regards,

Sidhyesh

 

  • 0
    TI__Guru**** 150910 points

    Searching our battery charger forum with 'USB Nimh' yields this thread which is extremely related: http://e2e.ti.com/support/power_management/battery_management/f/179/t/148221.aspx

    We also have these 2 design notes that explain a little bit about nickel charging:

    http://e2e.ti.com/support/power_management/battery_management/w/design_notes/nickel-cell-requires-a-constant-current-charge-for-full-duration-of-charge.aspx

    http://e2e.ti.com/support/power_management/battery_management/w/design_notes/nickel-cell-charging-voltage-required.aspx

    As you should see from the above discussions, it is very difficult to charge 3 series nickel cells from a USB port due to not having enough headroom in your charging voltage and not being able to generate a high enough charging current.  You should either switch to a 2-cell pack or lithium chemistry that are easier to charge.

    The bq2002 would be the simplest charger, running off a boosted input voltage.

  • 0 in reply to Chris Glaser
    TI__Mastermind 31120 points

    Optional Opinion:

    Use a LDO set to 4.15V, for three series cells, and a series 4 ohm resistor on the input of the LDO.

    Reasons:

    Its cheap.....It works fine, where the other method is problematic with less than 5.8V on the input (3x1.6V (min. drive per cell) + 1V overhead = 5.8V)

    There is not enough power in the USB port to charge the cells at "fast charge" to create enough heat to get any termination detection., so I think you would be disapointed in the bq fast chargers. 

    Nickel cells typically terminate at ~1.45V/cell and relax to 1.35 to 1.4V with no load.....thus the theory that if an LDO was applied to a cell the current would go to zero around 1.4V, thus this method would not be a safety issue (4.15V/3 cells = 1.38V/cell). 

    The 4 ohm resistor would limit the amount of current that could be pulled from the USB port.

    The customer should work with the battery manufacturer to understand how this slow charge effects capacity and health of the cells.

    All suggestions and opinions implemented should be verified by customer and the customer assumes all risks and responsibility for their design.

  • 0 in reply to Charles Mauney
    Prodigy 30 points

    Dear Charles,

                                Thank You for your valuable suggestion & information. But i have some doubt regarding the above method:

           A series resistor of 4 ohm at the input of LDO would cause a drop of 2V (4ohm X 0.5A) across itself. That gives 3V (5-2V) at the input of the LDO. So can you explain how can i get 4.15V at the output of the LDO?

     

    Thanking You,


    Regards,

    Sidhyesh

  • 0 in reply to Charles Mauney
    Prodigy 30 points

    Dear Charles,

                                     I tried a charger circuit using BQ2002 the schematic of which is attached with this mail. Can you provide inputs for the circuit?

    2804.NiMH.pdf

     Thanking You,


    Regards,

    Sidhyesh

  • 0 in reply to Sidhyesh Ramugade
    TI__Mastermind 31120 points

    You are right.  The 4 ohms is to protect against pulling too much current with the cells are discharged (5V-3V)/4ohms = 0.5A.

    When the batteries are about 25% full, they will be at ~4V, so (5-4)/4 = 0.25A.

    I admit that we do not have a good solution for a 5V input since it really takes (3*1.6 +1V) ~6V for a linear or buck charger to charge 3 series nickel cells.  Thus this would require a buckboost charger, which we do not have.

    The optional 5V USB solution is one of those back up solutions where if you forgot your adaptor to charge the battery properly, then you can get enough power via the USB port to operate with less performance.