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LM3658: Initial charge current, pre-qualification mode, very high

Reg Webb
Prodigy 65 points
Part Number: LM3658

We are using NP50 type batteries where we have set the charge current to be around 150mA.  When most batteries are connected the initial charge current is around 50mA but sometimes it is around 360mA.  When this occurs the LM3658 will run for a few seconds then stop charging with both stat lines off.  When using a generic charger the battery gets charged and will, most times, after this work normally on the LM3658.

Can someone please explain what is going on and how we can get the LM3658 to charge the battery in this condition.

Thanks

Reg Webb

  • 0
    TI__Guru* 75656 points
    Hello
    The above behavior is odd. When battery voltage is below 3.0V the pre-charge current should be about 50mA.
    With charge current set to 150mA (R-ISET-16.6k) and input at CHG_IN unit should not go to 360mA. Current will be limited to 150mA.

    Could you send a schematic for review?
    The battery that are triggering the 360mA behavior, what is the cell votlage?
  • 0 in reply to Bill Johns
    Prodigy 65 points

    Thankyou Bill for the reply.  We did quite a lot more work on this.  When we started using this chip with a smaller battery about two years ago all was fine.  It is only since using a different larger battery that we have run into this issue.  We are using a 6k8 resistor to get the current to 150mA.  What we did notice is that the battery is very rapidly coming up to the 3v step and going out of the initial pre-qual mode, it is then that the current goes up too high.  We made a change to find out what will happen if we set the current higher to about 530mA with a 4k7 resistor.  After doing this the cct seems to be behaving correctly and the battery is charging normally though we have a number of cycles to go through to check it properly.  Making that change may well have solved the problem.

  • 0 in reply to Reg Webb
    Prodigy 65 points
    Looks like I made a mistake here which nobody here caught. Seeing your 16k6 above, did not notice the one and saw as 6k6 which would be near our 6k8. The 6k8 re-calculated would give 320mA which is what we had.
    Sorry for the mistake here.
  • 0 in reply to Reg Webb
    TI__Guru* 75656 points

    Thanks for the update.