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

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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:

--- Quote from: tunk on July 31, 2020, 01:05:49 pm ---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.

--- End quote ---

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:

--- Quote from: tunk on July 31, 2020, 02:02:10 pm ---Does the boost converter output 8.4V when you disconnect the BMS+battery?

--- End quote ---
Yes

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