13 Replies Latest reply on Jun 16, 2018 11:17 PM by jw0752

    Operation of simple Li-ion Battery Charge System


      I am testing out a cordless screw driver. When I first plugged it into the charger a red LED light was suppose to light but it didn't. After a couple hours I opened the unit up and checked for circuit problems,


      The SOT89 in the center right of the circuit board is a B772 NPN transistor and it controls the charge current. Below it, the small transistor is a 2N7002 ( an N Ch MOSFET) and it is the driver that controls the B772. The U1 which I haven't IDed is probably the charge controller. When I first opened the unit I had 5 volts on the emitter of the B772 and no volts on the Collector. The Li-ion battery was sitting at 0.6 volts. In the course of my testing I eventually put a small token charge into the battery and at this point the unit started charging as normal.


      My question is this: Did the extremely low charge level of the battery cause the charge circuit to default to a no charge situation or is there likely a secondary fault in the circuit that has just decided to temporarily heal itself? Any insights would be appreciated. I have minimal experience with this type of charging circuit.



        • Re: Operation of simple Li-ion Battery Charge System

          Hi John,

          For some reason the photo does not show for me...


          • Re: Operation of simple Li-ion Battery Charge System

            You should know by now that anything that is broken, always works when you take it to get fixed ...

            It wouldn't surprise me if the low voltage caused the charging to shut down.


            Lithium Ion cells shouldn't go that low, so don't be surprised if the battery performance is reduced either in capability or life.



            The best source of information is here.  http://batteryuniversity.com/


            Charging information http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries




            7 of 7 people found this helpful
            • Re: Operation of simple Li-ion Battery Charge System



              I am working on a Li-ion battery charger right now (and I have developed others in the past).  Most of the Li-ion chargers have a 'battery check' (injecting a small current into the battery and looking for the voltage at the battery terminal to stay below the charge termination voltage, i.e 4.2V) process that must pass before the device goes into the charging mode.  The next step is usually a preconditioning phase, where again a low current (% of normal charge current) is applied to the battery until the battery voltage rises to the proper level at which point the battery enters a constant current charge.


              The charger chip that I am using is a Microchip MCP73831/2.  This chip turns on the 'charging' LED as soon as the device enters either the preconditioning or charging stage.  Not all devices have the same logic, but if my charger failed the 'battery check' the device would sit with the LED off.


              Having used other charging chips, I have seen a fair number of chargers fail to charge batteries.  One of the reoccurring failure that I have seen has been due to a high impedance path (connectors, switches, wires, etc.) on the charger input voltage.  As the charger enters 'charging', the current rises quickly and the 'high input impedance' causes a large voltage drop, causing the charger to shutdown due to insufficient voltage.  What happens is the charger chip begins to oscillate and the charger never delivers charge to the battery (sometimes actually further draining the battery).


              As pointed out by mcb1, operating/allowing Li-ion batteries to drop to this low of a voltage is very bad for the life/performance of the cell.  Keep an eye on this drill and/or replace the battery or return it as damaged to the vendor.


              Good luck,


              10 of 10 people found this helpful
              • Re: Operation of simple Li-ion Battery Charge System

                I reuploaded the picture in case it still wasn't coming through.


                1 of 1 people found this helpful
                • Re: Operation of simple Li-ion Battery Charge System

                  Hi John,


                  I believe I've hit this issue a couple of times with new products too : (

                  One was a wireless keyboard, which was a model presumably in a warehouse for too long, and it refused to charge, and the replacement had the same issue. Another was a label printer, which was new but sold as faulty (at a lower price) because it wouldn't charge either. In that latter case, it had a removable battery pack inside (NiMH I think from memory).. so I removed it, carefully gave it a bit of charge, and it's been fine ever since.

                  4 of 4 people found this helpful