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Is there a simple way to detect when a lithium-ion battery charging process is complete?

I have a basic system as shown here:

enter image description here

The load represents my application (including its own voltage regulation), and the voltage regulator symbol represents a cheap off-the-shelf Lithium-ion battery management system that handles tapering off the voltage and current as the battery becomes charged. Unfortunately, the BMS doesn't have pinouts to detect charging status.

What would be the simplest way to alter this circuit so I could get a high signal when the battery is fully charged? My application includes a microprocessor with some ADC pins. Could I measure voltage before or after the BMS to tell when charging is complete? I know that Lipo recharging includes separate constant voltage and constant current stages, so this might not be 100% accurate, but I just need a rough estimate.

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    \$\begingroup\$ Look at the datasheet for your lipo. It will tell you the "fully charged current" in the CV phase (usually 1% of the battery's C rating). Measure the voltage across a current-sense resistor and go from there. \$\endgroup\$
    – Bryan B
    Commented Dec 7, 2016 at 18:52
  • \$\begingroup\$ What BMS are you using? Maybe it can tell you, but there's just not a convenient connection to it. \$\endgroup\$
    – JRE
    Commented Dec 7, 2016 at 20:25
  • \$\begingroup\$ @insta, this is not what the battery datasheet says, it is what the charger does (or to which level it is configured to, if any). \$\endgroup\$
    – Ale..chenski
    Commented Dec 8, 2016 at 6:39
  • \$\begingroup\$ @AliChen: nkon.nl/sk/k/ncr18650ga.pdf -- the graphs say a 67ma cutoff on the charging. It's in different spots in the datasheet, but the battery datasheet is ultimately where that information should come from for a given system. \$\endgroup\$
    – Bryan B
    Commented Dec 8, 2016 at 16:11
  • \$\begingroup\$ @insta: the system is not "given" yet, OP tries to design it. Battery and charger are two separate entities. When they will be designed together, and the charger will be configured to charge a particular battery, then yes, the cut-off current should be set to whatever the particular cell says. So far this topic has severe misunderstanding what the charger does, and what the protection does, and who is supposed to report the end of charge. \$\endgroup\$
    – Ale..chenski
    Commented Dec 8, 2016 at 16:25

2 Answers 2

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Charging is usually terminated based on a charging profile and some termination current. So if the charging voltage is say 4.35V, the cell will charge at a current based on the state of discharge. When the current drops below a threshold, charging is terminated. Termination current and charge voltage are often programmable in a given charging system.

Here's an example from the TI bq25895 single cell charger:

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  • \$\begingroup\$ Is there any harm it interpreting full voltage as fully charged, and possibly disconnecting the charger then? Is the gradual dropoff in current necessary to maintain battery life? \$\endgroup\$
    – Cerin
    Commented Dec 8, 2016 at 1:39
  • \$\begingroup\$ The drop-off is the second half of charging process, approximately. There is no "full voltage" as well. The "full voltage" is a parameter specified by battery manufacturer to achieve guaranteed number of recharge-discharge cycles and acheve advertized capacity. It could be anywhere from 3.8V to 4.2V, and is not a criteria of end of charge. \$\endgroup\$
    – Ale..chenski
    Commented Dec 8, 2016 at 2:59
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There is no "simple way" to accomplish your task. Since you have a multi-cell battery pack, the charge must be "balanced", and any durable charge process must account for this. You are essentially asking how to make a "smart battery". If you ever disassembled any dead notebook battery pack, you would find individual wiring to cells with several current sensors, power gates, and a dedicated microprocessor to control all of this, with a communication channel to report the battery status to main consumer - notebook. This is a special area of industry, and engineering solutions are not very shared with public.

EDIT: Ok, I checked your other questions, and I see the problem. Your design does not seem to have any charger at all. The protection device "BMS" does not do any proper charging job.

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    \$\begingroup\$ You're overthinking the problem. I'm not trying to engineer something as complicated as a laptop battery, measure temperature, count Cololmbs, etc. The BMS handles balancing the charge of the cells. I'm essentially asking how to determine when the BMS cuts-off the battery from the external supply. That must effect the overall voltage of the circuit in a way that is measurable. An ADC pin connected to the positive battery terminal should read lower when charging is complete, since the BMS has disconnected it from the higher external supply? \$\endgroup\$
    – Cerin
    Commented Dec 7, 2016 at 18:15
  • \$\begingroup\$ @Cerin, if your (unspecified) BMS does the job for charging and balancing (which is not shown on your schematics), but does not report the status of charging process, then you pick a wrong BMS. \$\endgroup\$
    – Ale..chenski
    Commented Dec 8, 2016 at 3:05
  • \$\begingroup\$ @Cerin, actually, every Li-Ion battery charger cuts off (disconnects from source) when charging current drops below 3% of the battery charging current (or whatever the datasheet for your selected BMS says). All you need is simply to measure the current, not the voltage. Would this be an overthinking too? :-) \$\endgroup\$
    – Ale..chenski
    Commented Dec 8, 2016 at 6:32
  • \$\begingroup\$ A current sense resistor is a good idea. Also, about your edit, applying a constant voltage constant current source to a BMS is a common way to recharge a Lithium-ion battery. I measured the voltage and current of mine, and I confirmed the curves you'd expect from a proper charger. \$\endgroup\$
    – Cerin
    Commented Dec 9, 2016 at 13:09
  • \$\begingroup\$ @Cerin: so, you do have an external normal Li-Ion-compliant charger. I guess now you just need to know when your robot is OK to take off and leave the charging station when full charge is done, right? Then why don't you simply say so? \$\endgroup\$
    – Ale..chenski
    Commented Dec 9, 2016 at 18:32

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