Wednesday 8 July 2015

The LiPo Charger that wants to burn down your house

For a tiny LiPo package, I needed a charger.
Sounds like an easy one? I beg to differ.

Rather than reinventing the wheel I thought it was a good idea to get some tiny, ready made USB charger. A suitable circuit with a MCP73831 should come cheap enough from the PR of C. For around €2.50, I thought I could expect just that.
Wrong again.

I found one that was advertised as: 3.7V Lipo Battery USB charger JST plug 500ma output. It has a charge control LED that comes up when the battery is fully charged. - Sounds good. It arrived a few weeks later.

Just to be on the safe side, I took my meter (thou shalt check voltages) and was surprised that the USB-side seemed to be shorted to the charging side of the circuit. The meter read 5V on the charging connector. Way beyond safe for LiPos.

So two minutes a little prying later, I was in. And stunned.

Inside the plug

What did they do there? Is there any design rule not violated by this piece of ****? A 2.4 Ohms SMD resistor can't possibly pass for a LiPo charging circuit. I mistook it for a short.
Carefully tracing the circuit (because my initial impression just had to be wrong), this turned out to be the schematic diagram:

This "charger" is outright dangerous. I can't even remotely meet the proper charging conditions.
I have no idea where the "3.7V" and the "500mA" in the product description might come from. Possibly from the fact that some USB ports are limited to 500mA and that the nominal voltage of a LiPo cell is 3.7V.
And what are all those unused solderpads for? Maybe the original design idea was good and some electrical engineering genius found a way to cut the cost. At the expense of a few burnt down houses.

I suppose the idea might me to babysit the charger and the pack and wait for the LED to come on. Then disconnect the pack immediately. - That just won't work for me.

Watch my rant about this piece of **** on YouTube.


  1. Liions with built in protection, like smartphone batterys, can be loaded with 5V + resistor. see at B Kainkas Labortagebuch for more infos

  2. Interesting. I didn't kow there were LiPo packs with built-in protection. I've only seen that on Li-Ion cells. - Also: Cells that had "protected" printed on it, but had no protection circuit.

  3. The 2.5 ohms resistor acts as current limiter. However, that might allow for higher than 4.2V near the complete charge. How about adding a diode in series?

    1. Yes, a 1N-type Si diode should shed around 0.6V, getting us a lot closer to a safe operating range. Worth a try.

  4. I bought one of these off eBay recently too. Luckily I was monitoring the voltage of my 1S battery during the charge, it got to 4.2V (max charge), and simply kept going. I left if plugged in to see if it would cut off eventually - nope! I unplugged it when it reached 4.31V to prevent my house from burning down.

  5. Thanks for the article!

    Pardon me as I am a novice to technical stuff, but this charger is dangerous "because it has some missing protective component" OR "because it is bypassing the transistor and only V1, R4 and R5 are in active circuit"?

    Also, any safe USB chargers of this type from China you have come across?

  6. Very good find! I also had the same charger from 2 different sellers and they both charge at 5v output. Totally sucks!

  7. I tore down the lipo charger of furibee f36, it contains the KX55BX chip.

  8. I think this is not a protection circuit ..instead it's just an indication circuit to light up an led when battery reaches 5v

  9. I think it's not a charging circuit instead its an indication circuit that light up an led when battery is charged up to 5v