I'm looking for true independent 2-cell charging chip

[dusan]

I bought HY2120-CB 2-cell battery protection strip. The output is 7.4V (2*3.7V) but it doesn't charge batteries independently, as soon as any battery reaches 4.2V the charging stops, this leaves other battery half-charged. Looking at datasheet[1] it's obvious why, those paths with 330R resistors are for sensing voltage only. I need true independent 2-cell solution where baterries are charged independently and the output voltage is sum of both voltages (7.4V). I need this to be reasonably small (few parts, not hundreds of parts).

I asked copilot but it told me to connect them in parallel to get 7.4V and when I asked for chip solution it told me MP2615A and LTC4002, neither have middle voltage sensing, let alone balanced charging.



[1] https://www.hycontek.com/wp-content/uploads/DS-HY2120_EN.pdf

[shapirus]

Use two single-cell chargers. The easiest way would be to power one of them from an isolated DC-DC converter so that they don't have common ground and can sense each its own cell's voltage independently.

[tunk]

Two identical cells shouldn't behave like that.
First you should find out why.

[Kim Christensen]

I bought HY2120-CB 2-cell battery protection strip. The output is 7.4V (2*3.7V) but it doesn't charge batteries independently, as soon as any battery reaches 4.2V the charging stops, this leaves other battery half-charged.

That's because it's not a charger IC.

[thm_w]

I bought HY2120-CB 2-cell battery protection strip. The output is 7.4V (2*3.7V) but it doesn't charge batteries independently, as soon as any battery reaches 4.2V the charging stops, this leaves other battery half-charged.

That's because it's not a charger IC.

Yeah.
OP: what you want is a balancing BMS. If you just need one or two you can buy a module for $2: https://www.aliexpress.com/item/1005003614781969.html
For charging use whatever 2S charger, or even just a current limited supply set to 8V.

If you really need cell by cell charging, then look at hobby chargers with multiple cell connections on them.

And even without a balancing circuit, its never going to be one cell fully charged and one cell half charged. Unless you either didn't match the cell capacities at all, or, started them off completely imbalanced.

[dusan]

That is the exact strip I have. It has HY-2120 inside and 2 mosfets 8205LA.

[thm_w]

That is the exact strip I have. It has HY-2120 inside and 2 mosfets 8205LA.

Thats not enough to balance, it needs more components. See the link again and compare the "Balance" and "Standard" models.

[dusan]

According to this: https://electronics.stackexchange.com/questions/476188/identify-chip-on-2s-bms-balance-card the main SOT-23-6 chip has marking PAYG which is BATTERY PROTECTION IC FOR 2-SERIAL-CELL PACK
S-8242A Series, datasheet on page 18 of https://img.hqew.com/file/Others/1090000-1099999/1098111/Electronic/201251816354378771.pdf
has exactly the same application schematic as my HY-2120 with 300ohm resistors.

[thm_w]

OK but the main chip doesn't matter, its the supporting balance IC's seen at the top of the board.
If your board does not have those supporting IC's, and doesn't have the large resistors, then its not going to balance the cells.

[Kim Christensen]

The documentation on those AliExpress PCBs is terrible. Multiple/different products on a single listing.    It's no wonder the OP is confused.
I did find some info on one of the balancing/protection PCBs (HX-2S-JH20). With that one you'll still need a 2 cell charger PCB for the entire pack because this one only does protection and balancing.

Here's a link to someone's attempt to reverse engineer this PCB.
Another attempt at a schematic.

[pcprogrammer]

I discovered this too recently. 

Own a Husqvarna battery pack that died and found two of the 20 cells to be dead, so took an interest in lithium ion batteries and charging. Bought a  cheap multiple cell board for what I thought to be for balanced charging and protection, to indeed discover that it does not have a balancing function. So yes it cuts off when one of the cells reaches the set over protection voltage.

The used IC is capable of balancing, but needs an additional mosfet and discharge resistor per cell. This method is not the same as active per cell charging, and I'm not sure that this even exists in a standard IC setup.

In balancing mode it will discharge cells that are to high and then continue normal charging, and rinse and repeat this cycle until all cells are at the same charged level. Thus wasting energy.

To get my Husqvarna battery pack working again I had to manually equal the cells with a separate power supply because the official charger would fall back into error mode every time it detected the unbalance.

[thm_w]

The documentation on those AliExpress PCBs is terrible. Multiple/different products on a single listing.    It's no wonder the OP is confused.
I did find some info on one of the balancing/protection PCBs (HX-2S-JH20). With that one you'll still need a 2 cell charger PCB for the entire pack because this one only does protection and balancing.

Here's a link to someone's attempt to reverse engineer this PCB.
Another attempt at a schematic.

As mentioned above, yes a charger is still required, these boards simply protect the cells.
The documentation is definitely poorly translated and not for beginners. For advanced users its fine.

The used IC is capable of balancing, but needs an additional mosfet and discharge resistor per cell. This method is not the same as active per cell charging, and I'm not sure that this even exists in a standard IC setup.

In balancing mode it will discharge cells that are to high and then continue normal charging, and rinse and repeat this cycle until all cells are at the same charged level. Thus wasting energy.

Yes balance ICs exist, active per cell charger ICs also. Many on LCSC.
The trade off for a tiny amount of energy lost is a much simpler and cheaper circuit, compared to active balancing or having an additional connector for active balancing on the charger side. If the cells capacities/temperatures are equivalent, then no balancing is needed at all, and no energy is lost.

[pcprogrammer]

Yes balance ICs exist, active per cell charger ICs also. Many on LCSC.
The trade off for a tiny amount of energy lost is a much simpler and cheaper circuit, compared to active balancing or having an additional connector for active balancing on the charger side. If the cells capacities/temperatures are equivalent, then no balancing is needed at all, and no energy is lost.

You are right. I found this article that explains it and shows some IC's suited for it.

Requires quite a bit of additional components to make it work.

[dusan]

Charging single cell is trivial (solved problem), could I solve the discharge by using 2-input buck converter? For example https://www.monolithicpower.com/en/mp5461.html

[Kim Christensen]

No. that'll make things worse.
Since you're willing to use a buck/boost, you could put the two cells in parallel and use a boost convertor.

[dusan]

If I connect 2 cells in parallel there like 1A flowing between them (from the stronger, freshly charged 4.2V to slightly weaker 4.1V  battery which will never reaches 4.2V and eventually the stronger battery can loose significant amount of charge.

[sleemanj]

this leaves other battery half-charged

If your cells are so drastically imbalanced, you have a problem which is solved by replacing a bad cell.  Balancing will not help you.

If I connect 2 cells in parallel there like 1A flowing between them (from the stronger, freshly charged 4.2V to slightly weaker 4.1V  battery which will never reaches 4.2V and eventually the stronger battery can loose significant amount of charge.

Largely ditto, you have a cell which is in poor condition.  Replace it.

[Kim Christensen]

If I connect 2 cells in parallel there like 1A flowing between them (from the stronger, freshly charged 4.2V to slightly weaker 4.1V  battery which will never reaches 4.2V and eventually the stronger battery can loose significant amount of charge.

Start with two new identical cells that are fully charged. Then this won't be a problem for the life of the pack.