XL6009 Boost-Converter overheating trying to charge a 2S LiPo

[Forsaken]

Hi there!

I'm trying to use a XL6009-based boost converter (HW432) to charge a 2S LiPo. Input voltage is 5V, Output voltage is 8.4V fed into a BMS. When testing the circuit it does seem to work, but the boost converter gets way too hot way too quickly. (The XL6009 IC reaches 80°C in under a minute)
It is drawing 5V @ ~1.5A / ~7.5W from the 5V input, but only around 100mA at the output, which is the best hint I have to what is going wrong right now.

For measuring I have a mid-range multimeter, an IR thermometer and an old 20Mhz Oscilliscope available.

The best guesses I came up with:
 - The BMS tries to draw too much current overloading the boost circuit. If so, how and where would I limit the current (before or after the boost circuit)? Since I am using a regular usb port as input, I assumed that would be the limiting factor before the boost converter.
 - There is current flowing back from the LiPo cells into the boost converter, messing with the circuit. I tried to put a diode in place to rule that out, with no change in behavior.

Any ideas / solutions / help would be greatly appreciated.

Best regards
Jens aka Forsaken

PS: After a lot of digging I found multiple tests of the module concluding the module should be able to handle around 15W before requiring heatsinking/cooling, so that should not be the issue. e.g.:

[tunk]

What's your BMS? - many (most?) BMSs only handle over/under-voltage and over-current,
and not CC/CV/cut-off charging. I don't think it's recommended, but you could try to add
a suitable resistor in series to limit current. And you could possibly have damaged the
boost converter by initially drawing to much current.

[Forsaken]

What's your BMS? - many (most?) BMSs only handle over/under-voltage and over-current,
and not CC/CV/cut-off charging. I don't think it's recommended, but you could try to add
a suitable resistor in series to limit current. And you could possibly have damaged the
boost converter by initially drawing to much current.

Thanks for the swift reply.
It's a ICQUANZX 2S BMS. ( https://www.amazon.de/gp/product/B07VT9M1Q7/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1 )
I tried putting a 1Ohm resistor in series at the input, which about halfed the current going in with the same results (still ~100mA on output, overheating boost converter plus the resistor getting hot). Trying the same at the output side (between the boost circuit and the BMS) did not change the flowing current at all (still ~100mA).

[tunk]

Does the boost converter output 8.4V when you disconnect the BMS+battery?

[Forsaken]

Does the boost converter output 8.4V when you disconnect the BMS+battery?

Yes

[tooki]

I tried one of those little boost boards shown in the second video. Couldn’t get a power amp to work right, kept cutting out during peaks. After running the numbers, I calculated that with the component values they used, the maximum current it could output when boosting 5V to 12V was just 150mA — the 1A or whatever they claim basically refers to the maximum input current.

Now, in theory that should work for you, but with the variability in Chinese modules...

What I don’t understand is why you’d attempt charging using a boost converter module instead of a charging module, which will do the job properly and not burn down your house. Lithium ion charging is something you do NOT want to mess up.

[Forsaken]

I tried one of those little boost boards shown in the second video. Couldn’t get a power amp to work right, kept cutting out during peaks. After running the numbers, I calculated that with the component values they used, the maximum current it could output when boosting 5V to 12V was just 150mA — the 1A or whatever they claim basically refers to the maximum input current.

Now, in theory that should work for you, but with the variability in Chinese modules...

The quoted (and more) videos do use the exact board I do and seem to work fine with 10-15W output. The Datasheet claims 3A max input current, which I indeed did not trust. That is why I did search for actual test results. I did not find any evidence of such a module overheating/failing below 10W.
Beside that the massive mismatch in input/output current and extremely quick overheating do seem odd to me for a "simple overload".

What I don’t understand is why you’d attempt charging using a boost converter module instead of a charging module, (...)

I want to be able to charge the battery from variable input sources. I did not find any module except 5V/USB-Chargers and ones like the one I did order.
Edit: Also I'd like to exclude the boosting circuit from the actual project to keep things as compact as possible.

(...) which will do the job properly and not burn down your house. Lithium ion charging is something you do NOT want to mess up.

By my understanding the BMS module is there for exactly that reason? (To secure the charging process and prevent "burning my house down")
Beside that I did not expect using a boost converter would be an issue, it is basically a switched-mode power supply, right? So what is the difference between that and a 8.4V power brick?

Just to clearify to prevent my reply from coming across wrong: I am not denying your claim or searching for excuses or whatever. I simply did search for a solution that matches my requirements to the best of my abilities.

[Siwastaja]

You need a CC-CV supply to charge a battery. A CV supply is not sufficient. High quality CV supply shuts down, or provides very limited current in hickup mode; an improper one might burn.

[Forsaken]

You need a CC-CV supply to charge a battery. A CV supply is not sufficient. High quality CV supply shuts down, or provides very limited current in hickup mode; a improper one might burn.

So something like this would be fitting? https://www.amazon.com/Adjustable-Converter-5V-30V-0-5V-30V-Supply/dp/B073B3MJCF
What should I look out for to make sure not to take unnecessary risks?

[Siwastaja]

Yes, it's of a correct type, but the quality of those cheap modules vary. The very least what you can do is to derate the current, i.e., don't turn it to full current but maybe 50-60% of it.

Having a BMS is an extra layer of safety in case the CC-CV supply - which can be called "charger" - burns out and outputs excess voltage.

Make sure the BMS is what you expect it to be, though. It needs to be able to actually disconnect the batteries in case of battery overvoltage.