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The system is battery powered. When the battery is low, the user can connect the AC/DC adaptor to the board, then the DC input can charge the battery through the charger circuit.

I don't have the problem in charger circuit design because there are a lot of examples on the internet. The issue is I don't know how to combine the battery charge and discharge circuit. I mean when the battery is charging, the current flows into it and vice verse. How can I control the current direction around the battery?

Edit: One of the solution to control the current direction according to the different scenario is Power Path. Take the BQ2403x of TI charge controller for example.

For the battery based handheld device, when the battery is low, the users connect AC adapter to the board. The FET Q1 and Q2 are both on. The current flow on Q2 is running into battery. enter image description here

When the AC adapter is removed and the battery is not depleted, the battery powered the system. The FET Q1 is off and Q2 is on. In this case, the current flow on Q2 is running out of battery. enter image description here

In a word, with the help of power path management, the battery controller can control the current running into or out of battery as required.

Unfortunately, I haven't found other solution without power path that can implement the same function. I would be very appreciated if anyone can share me a link with the different solution.

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  • \$\begingroup\$ Don't you think that this is a problem that has been solved many times over? There are plenty of devices that can be charged and used at the same time, and the chips are designed to handle that scenario. Take a closer look at those datasheets. \$\endgroup\$ – Dave Tweed Oct 8 '18 at 4:00
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    \$\begingroup\$ What happens if you try to search google for [design of battery-powered systems]? Surely a few links on the top of 192,000,000 hits will have the information you need, like this one ww1.microchip.com/downloads/en/AppNotes/39610d.pdf or this one ti.com/lit/ml/slyp088/slyp088.pdf to start... \$\endgroup\$ – Ale..chenski Oct 8 '18 at 4:15
  • \$\begingroup\$ It's not quite clear what information you are missing. Maybe the arrangement is simpler than you think. Maybe this old thread can shed some light. \$\endgroup\$ – Nick Alexeev Oct 8 '18 at 5:06
  • \$\begingroup\$ I've found the key feature I'm after is DPPM(dynamic power path management) which can charge and discharge the battery according to the DC capability. The trick to control the current direction is the use of MOSFET's bidirection feature. \$\endgroup\$ – Ross Oct 8 '18 at 5:23
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    \$\begingroup\$ @NickAlexeev, I didn't mean disconnected physically. I meant that the charger has to stop charging upon certain current threshold, and if the load is always connected to battery (without power path), this condition will never occur (if system load is above that threshold), resulting in battery overcharge and subsequent destruction. Or external charger has to be disconnected. \$\endgroup\$ – Ale..chenski Oct 8 '18 at 19:58
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  • Monitor the current going into (or out of the battery).
  • Monitor the current coming from the battery charger.
  • Monitor the battery voltage.
  • Pump current into the battery according to how that battery wants to be charged, without overloading your charger (i.e., a LiPo will get a CC/CV charge, except when the load is pulling more than the charger is supplying).
  • Figure out how to do intelligent charge termination when the load is changing (that's the hard part).

You don't need to route power around -- just pump current onto the battery rail from the charger, and drive your device from the battery rail. Unless the charger is mostly limited by the battery capability and not it's own driving power, the charge time will be longer when your device is on. For a LiPo cell, charge termination has more to do with how the charge current tapers off at full voltage -- as long as the charger is always pushing enough current so that the battery is charging continually at this point, charge termination should be a calculable thing.

Particularly if the charge is guaranteed to always go into the battery, there's chips that can do this. It's been too long since I've looked into it for me to quote part numbers, but ADI and TI both have nifty stuff. Just go shopping.

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  • \$\begingroup\$ You are proposing to control the charger current by measuring a differential between input and output currents. Aside the questionable stability of such control system, the battery will be exposed to charging voltage all the time (if external power is permanently on), which will lead to overcharge for Li-Ion batteries. This schema is wrong, that't why industry went with PowerPath method. \$\endgroup\$ – Ale..chenski Oct 9 '18 at 15:50
  • \$\begingroup\$ @Ale..chenski: Well, that's certainly what you read into what I said. "Figure out how to do intelligent charge termination" certainly implies that I want the OP to terminate charging. And I did not propose measuring the difference between input and output currents -- I propose measuring the battery current, because thats what matters to the battery, and measuring the charger current, because that's what matters to the charger. As far as I remember, the last time I used a charge management chip (from TI) that's exactly what the chip did. The customer hasn't complained yet. \$\endgroup\$ – TimWescott Oct 9 '18 at 17:21
  • \$\begingroup\$ A Li-ion battery MUST be disconnected from voltage source upon end-of-charge, for the reason explained above. No matter how well you measure the current, there must be a switch, this is a fundamental necessity.TI chips do have that switch, if you examine their architecture (and theory of operation) of BQxxxxx products more carefully. \$\endgroup\$ – Ale..chenski Oct 9 '18 at 20:00

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