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I am trying to figure out how this lithium battery charger works.

enter image description here

How exactly do those discharge/bleeding resistors work? Are they only "activated" when no power is present so that each battery can equalize its voltage via those resistors? Are they disconnected from the circuit when a load is connected?

I don't see an MCU on the PCB, so what is monitoring the three voltages in order to make it safe?

What are those 6-pin ICs used for?

Does anyone know where I can find a schematic for the PCB?

Edit 1: So the datasheet for the HY2213 says it a "1 Cell Li-ion/Polymer Battery Charge Balance IC". This component is used in the above device (The 6 pin ICs close to the bleed resistors). The other 6 pin IC is a DW01-P or One Cell Lithium-ion/Polymer Battery Protection IC.

HY2213

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    \$\begingroup\$ That looks like the sort of BMS sold on the likes of Amazon. It's not a charger. \$\endgroup\$
    – Finbarr
    Mar 21 at 19:03
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    \$\begingroup\$ This is not a charger. It's a BMS. Don't try to use it as a charger. \$\endgroup\$
    – StarCat
    Mar 21 at 19:33
  • \$\begingroup\$ The six SOT23-6 you speak is likely a buck/boost DC-DC converter. It's difficult to say without its part numbers but considering that there's no other component like that one, I'm fairly certain that's what it is. \$\endgroup\$
    – Colin
    Mar 21 at 21:18
  • \$\begingroup\$ @Finbarr so then what is the difference between a bms and charger? Also is the TP4056 a type of charger I could use for this bms application? \$\endgroup\$
    – JoeyB
    Mar 22 at 13:29
  • \$\begingroup\$ No, a TP4056 is designed to charge a single Li ion cell, not three in series. You would need three of them together with three isolated power supplies. \$\endgroup\$
    – Finbarr
    Mar 22 at 13:39

1 Answer 1

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First, that is not a "lithium battery charger". That is a li-ion battery management system".

A charger regulates current and voltage. This circuit doesn't. It can only turn the battery off. In addition to this protector BMS, you also need a separate device: an actual charger.

How exactly do those discharge/bleeding resistors work?

They are turned on when the cell voltage exceeds a threshold.

Are they only "activated" when no power is present so that each battery can equalize its voltage via those resistors?

No. They work regardless and independently.

Are they disconnected from the circuit when a load is connected?

No. They work regardless and independently.

I don't see an MCU on the PCB, so what is monitoring the three voltages in order to make it safe?

That is a "hat": it's added to a single-board computer (e.g., Raspberry Pi) that does the control. (Note connector RP11 at the bottom right of the schematic diagram.) By the way, that is extremely unreliable because you have no idea when that computer becomes unavailable. Do not do that because it's very dangerous. Instead, buy a standalone protector BMS.

What are those 6-pin ICs used for?

They are not ICs, they are MOSFETs (transistors). They are power switches to turn off the battery current. There are two in parallel to increase the current handling. There are two in anti-series (back to back) to control the charging current and the discharging current independently. The third pair (on the right) looks like an ideal diode.

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