Vape PCB battery management

[mickmaz]

Hi!

I have this board taken out from a disposable vape. It has a 1100mAh Li-ion battery so I thought it would be nice to power some project with it. It also has the display to show a percentage of the battery charge and that capacitive touch sensor to tell the display to turn on.
I wasn't exactly sure how to wire it up, but I got about 3.8V between M+ and M- (which were connected to the electret microphone). So as a test I put an LED in between them and let it run for a few days. The problem is, I noticed that the LED started to fade, but the display still showed 59%, the same before I put on the LED. Though when I measured, the battery and M+/M- were down to 3V. I tried to charge it and after a few minutes, the value on the display changed to 60%.
I'm not sure what is happening here. The BMS should have cut the power I guess, though the display still flashes at 0% when I tried to draw it, so I think some parts are going around the BMS and only the heater connection is actually protected. I did identify the 5 pin IC as a charge controller though I'm not sure what is the 6 pin one but supposed to be also part of the BMS?
I wonder if anyone here has reused a board similar to this one.

[RoGeorge]

I don't know the model you have, but in general, for Li-ion batteries the under-voltage protection (when the BMS disconnects the battery) is somewhere at about 2.8V or so, but the circuit powered by the Li-ion cell should never discharge the battery that low on purpose.  2.8V is a protection for malfunction and extreme situations only.

During normal use the battery should never go that low.  There is not much energy left under 3.2 ... 3.5V, so there is no point by over discharging the battery.  Same with the over voltage protection, the BMS will disconnect the cell at about 4.3V or so, but you should never go over the specified 4.2V top charging voltage.

These limits might depend with the exact type of battery.  For example, there are Li-ion of 3.6V nominal, or 3.7V (most common these days), or 3.8V or even 3.85V nominal (for example Samsung phones, some Xiaomi too), which are charged up to 4.4V instead of only 4.1V (for 3.6V nominal type) or 4.2V (for the 3.7V nominal type).

Other Li based rechargeable are much lower nominal voltage than that (IIRC 3.2V or so), but they are not that common.  The vape and/or cigarette heaters I've opened they were all heaving 3.7V nominal and 4.2V top charging type of Li-ion.


About the battery level indicator, that might not be a coulomb meter.  Maybe they just count the number of puffs and consider an average energy taken with each puff, but I never looked into that, only speculating.

You can still reuse the vape battery without knowing its exact discharge state.  Just charge it more often, or make the load circuit to detect when the voltage goes lower than 3.5V or so.

[SamElectronics616]

What you're seeing is actually quite plausible for a disposable-vape PCB.

The main point is that I would not assume the 59–60% shown on the display represents the actual Li-ion state of charge. The percentage is most likely calculated from battery voltage and/or an internal fuel-gauge algorithm, and it may be calibrated for the original vape's load profile. With a different load connected to M+/M−, the displayed percentage can remain relatively unchanged even while the battery voltage is falling significantly.

The fact that you measured approximately 3.8 V initially and later about 3.0 V under load is important. A 1100 mAh Li-ion cell at ~3.0 V is already heavily discharged, depending on the cell and measurement conditions. The LED also isn't an ideal test load because the current can vary substantially as the battery voltage changes.

I also wouldn't assume that M+/M− is the protected battery output. In many disposable-vape designs, the battery protection/charging circuitry and the heater switching circuit are separate. The protection IC may monitor the cell and control MOSFETs, while the heater has its own high-current switching path. Other circuitry, including the MCU/display, can remain powered even after the main load has been disconnected.

The fact that the display still briefly flashes at 0% is actually consistent with the electronics remaining powered from the battery while the load path is disabled.

I'd recommend tracing the PCB before using it for another project:

1. Identify the battery + and − connections.
2. Identify the 5-pin charging IC and its surrounding components.
3. Identify the 6-pin IC and trace its connections to the battery and MOSFETs.
4. Determine whether M+/M− is directly connected to the battery, switched through a MOSFET, or generated by another circuit.
5. Identify the MOSFET(s) used for battery protection/load switching.
6. Measure the battery voltage directly at the cell terminals rather than relying on the display percentage.
7. Check the voltage at M+/M− both with no load and under a known resistive load.
8. Check whether the protection circuit disconnects the load when the cell reaches its undervoltage threshold.

I would also avoid continuing to discharge the Li-ion cell to around 3.0 V just to test the board. If the protection circuit isn't actually protecting M+/M−, you could over-discharge the cell.

If you can post clear photos of both sides of the PCB, especially the markings on the 5-pin and 6-pin ICs and the traces around the battery/M+/M− connections, it should be possible to work out the circuit and determine exactly what M+/M− is doing.