Hoover FM216LI cordless vacuum refuses to charge

[flat-earther]

Thanks sam_sam_sam,
The vac is already assembled, but I saw before that the cells are Samsung ICR18650-22P.
I already completed full charge after replacing the resistor and I monitored the battery voltage during the first charge and it stopped before 25.2V I think the last reading I got before it stopped charging was 25.16V then the voltage settled a bit lower, then I vacuumed for probably over 30mins and it was still going and had 1 out of 3 LED left when I turned it off, then I completed another full charge, so at the moment I think it's good.

[sam_sam_sam]

First of all you need to know if the battery pack cells are good or not please show a picture of the battery cells so I can see what type you have
But basically you need to know the exact voltage of each battery cell under a load

Use the same load ( incandescent light bulb preferably) on each battery cell if the battery cells are more than 0.250 volts ( run the test for at least five minutes at a minimum ) different then you might be having an issue with the BMS protection system circuit locking up and refusing to turn on properly

Now if your battery pack cells are unbalanced then you might have a bad case of end of life syndrome issues and there is no cure for it other than replacing the battery cells all of them reason being that the other battery cells might also be close to end of life syndrome but are not to the point of self discharging

There is a test that will verify if this is the case fully charge each battery cell to it's recommended voltage and then let the battery pack sit disconnected from the device for 30 days and recheck the battery voltage on each battery cells and the voltage should not have dropped more 0.500 from where you left it but let the battery pack sit for 3 hours before you take the voltage reading so the battery voltage becomes stable for accurate reading

How these BMS protection devices work is that if the voltage is different more than a certain amount it locks up the charging circuit if the voltage goes beyond 4.2 volts depending on the type of battery cell used it could be 3.8 volts or a little lower than that but in general if any battery cell reaches this voltage limit it turns off the charging current circuit or if a battery cell is below 3.volts or lower depending on BMS protection parameters to protect the battery cells from damage and from it self

[flat-earther]

Finally got a new resistor soldered. It's very small so was difficult, I stopped trying to make it better aligned since it is practically attached.


I verified with multimeter that it's connected to the traces.

After that I plugged in the charger and the 3 LEDs turned on, I think it blinked slightly different than before the resistor, but immediately all 3 LEDs stayed solid which is as if charging is complete, just like before the resistor.
I was disappointed, I thought there still was a problem...
I replugged the charger many times and same thing.
I turned on the motor and it spun up and immediately had only 1 LED light and blinking as if battery is running out then motor stopped, although I previously manually charged it to 24V and before replacing the resistor the motor would stay on and with 3 LEDs showing full charge.

I repeated plugging charger in and out, and trying to spin the motor (it didn't try to spin again unless I plugged the charger in and out, it's probably related to a feature that once voltage cut off has been reached you can't turn the vacuum on unless charged has been plugged in and out first (but in this case the controller for some reason falsely thought battery was empty)), and then all of a sudden it started charging again!
Maybe the controller needed time or many tries until it cleared a fault and was happy again? IDK... Anyway now it charges so all good.
Thanks CapLeaker for advice.

[flat-earther]

Alright, I have an 0603 resistor kit on the way.

[CapLeaker]

Now replace that 68k resistor and make sure the traces are good around that area.

[flat-earther]

D1 diode soldered back.

Inductor repaired. I bent the copper wire down and soldered it to the pad.

[flat-earther]

I removed the diode D1, in diode mode it measures 0.575V one way, OL other way.
In resistance mode 68kOhm one way, OL other.

[CapLeaker]

Take it out or lift one leg and check that diode again. Diodes should read to 99% one way but not the other, unless it is a bidirectional TVS diode.

[flat-earther]

I tested all ceramic caps, didn't find any shorted. Ceramic cap C9 near motor wires is about 1 Ohm but the motor wires are soldered there so I think that's just the motor resistance.

All power disconnected, I measured in diode mode diode D1, 0.663V one way, 0.571V the other way. Is that okay?

[CapLeaker]

I’d say replace that resistor that’s broken make sure the traces around there are going where supposed to be going, check across all the MLCC that there is no short, then try charging again.

[CapLeaker]

That’s o.k.

[flat-earther]

All power disconnected, measuring Z5 in diode mode, OL one way, 0.725V the other way.

[CapLeaker]

Check that gate drive circuit again with that z5 etc in diode mode, no power.
that zener diode is a 15V zener. So it should be clamping down any excess voltage to 15V.

[flat-earther]

Disconnected all power from board, D13 in diode mode is open line one way and 0.217V the other way.

BTW while the inductor is essentially disconnected is there anything I should test in this state before reconnecting it?

[CapLeaker]

The inductor should measure a short. Disconnect everything from the board and check D13 in diode mode. Should have something like 0.3V drop one way and an open the other way.. hmm dunno… maybe the HT66F018 went south. That would suck, as it is a programmable chip.

[flat-earther]

Well removing that glue was extremely difficult. I used a 25W iron with sharp tip to pick it, then tweezers, and I destroyed the inductor by accidentally pulling the copper wire along with the glue with tweezers and the wire snapped.
I will need to try and resolder that wire back but it's so short I'm not confident in it, if that doesn't work I may unwind it one rotation to make it longer. I hope I didn't short the winding by scratching it with the tweezers.

From one side of the inductor to the end of the broken wire it measures 0.3 Ohms, is that still okay?

[flat-earther]

Just to experiment, I have set an external power supply to 26V because that way I can see current draw on the power supply.
When I plug that power supply to the vacuum I see a very brief about 1A current draw and then it instantly drops to zero.

[flat-earther]

BTW I have manually charged the cells with an external charger so they wouldn't be empty. Right now the battery is at 24V.

How do you want me to check D13?

[CapLeaker]

Carefully get rid of that white beigeish goo on the inductor and suroundings. All of it. You may use heat and 99% IPA to pick / peel it off...

[flat-earther]

With the charger plugged in, black probe on ground, measuring voltage at input of the charging mosfet, I see it's constantly jumping between 27.5 and up to even 27.88V:

At the mosfet's output the voltage is constantly jumping between 7.6 and 8V:


At the mosfet's gate it's 27.2-27.6V:
Is such high voltage at the gate normal?