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I have a LiIon Battery Charger IC which will draw around 50µA when not needed, see circuit 1. This is a lot of current for no work in my opinion. Now I search for a solution how I can disconnect the charger if not needed. There is one open drain status pin from the charger I can use for it (active low).

What I came up with is in circuit 2. If the Charger is loading the battery the #CHG pin is pulled down and opens the PMOS (controlled by the charger control inside the charger). If the charger is not charging the charger control is giving out a undefined voltage as I understood.

So my Question is: Can this solution do the job of disconnection the charger if not needed and connect it if needed?

Circuit 1: enter image description here

Circuit 2: enter image description here

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    \$\begingroup\$ Which specific battery charger is it? A datasheet link would help. By the logic presented in the question, if the charger control goes active, the #CHG pin pulls low, removing power from the charger (?), thus shutting down the charger - so what keeps the #CHG pin low once the charger is shut down? \$\endgroup\$
    – Anindo Ghosh
    Jan 31, 2014 at 12:09
  • \$\begingroup\$ It is a WPC charger from TI: ti.com/lit/ds/symlink/bq51050b.pdf (I do not use the AC adapter/USB option).If #CHG is pulled down the PMOS is open, so the battery can charge. But if the charger has no energy charger control would be undefined I think. So maybe I can just pull up #CHG? \$\endgroup\$
    – moritz
    Jan 31, 2014 at 13:47
  • \$\begingroup\$ I just saw that they updated the datasheet. So it is just 12µA typical. But still, can I switch it off? \$\endgroup\$
    – moritz
    Jan 31, 2014 at 13:53
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    \$\begingroup\$ For 12 uA, it really isn't worth switching off, for most practical purposes: Even a 10 mAh rated tiny hearing aid battery would last roughly 35 days of continuous discharge at this current. Most likely the rest of the circuitry would be a bigger drain on the battery, especially the battery self discharge itself. \$\endgroup\$
    – Anindo Ghosh
    Jan 31, 2014 at 14:02
  • \$\begingroup\$ Can you live with a schottky diode in series with the charge path to the battery? \$\endgroup\$
    – Andy aka
    Jan 31, 2014 at 15:56

1 Answer 1

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Can this solution do the job of disconnection the charger if not needed and connect it if needed?

It has 'problems'.
All readily available MOSFETS have a reverse polarity "body diode" from drain to source. In the case of a P Channel FET connected as shown, the body diode will conduct when Vdrain is more positive than Vsource. Alas, this is the situation that occurs in your circuit when the charger has power removed.

One solution is to use series connected opposite polarity MOSFETs with the sources connected and the gates connected. This arrangement is often used in LiIon cell protection circuits.

10 uA drain is 8765 hours/year x 10 uA = 88 mAh/year
= ~ 1.7 mAh/week

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