2S Li-ion charger+protection with reverse protection

Question

I'm designing a device powered by two Li-ions in series and I'm struggling with the charging and protection of the batteries. One of the requirements is to avoid battery packs, the individual cells need to be interchangeable. The load current can reach 6 Amps.

The idea is to use the BQ25887 charger with integrated balancer and S-82C2A protection circuit.

The issue I'm hitting here is the reverse polarity protection. Any or both cells could be inserted in reversed position, damaging the protection circuit and/or the charger.

The to back-to-back mosfets required by the S-82C2A already introduce quite a lot of series resistance (2 milliohm Rdson - 150 mW losses), adding more MOSFETs would increase the losses even more, not speaking about another single point of failure.

Is there a simple way to protect the circuit from the reverse oriented cells I'm missing? Or maybe a different 2s overvoltage/undervoltage/overcurrent Li-ion protection IC that could survive the reverse polarity event?

Edit

I'm thinking, I could place P-MOS between bat1+ and VDD and another between bat2+ and VC. If the battery got inserted in a reverse polarity, the P-MOS would close, protecting the IC. And the IC would not be powered, the FET1 and FET2 would remain closed, protecting the connected circuit and the charger.

And as almost no current would flow through the P-MOS transistors, the efficiency would be unaffected.

Answer 1

First and foremost, never, ever, ever use battery holders for 2 or more Li-ion cells because they could be inserted at different levels of State of Charge (SoC). You can't control what people do.

• If in parallel, they are at best degraded, at worse can catch fire due to the inrush of current between cells.
• If in series, they will be unbalanced. If severely so, the BMS may not be able to bring them back in balance.

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Secondly, to answer your question:

Is there a simple way to protect the circuit from the reverse oriented cells I'm missing?

The simplest is to place a fuse in series with each cell holder, plus a reverse-biased diode across each, past the fuse. If a cell is inserted backward, current will flow through the diode and blow the fuse. Fuses do have internal resistance (just as MOSFETs do), so they will waste power.

A self-resetting fuse (PTC) avoids having to replace a fuse, but will waste more power and over-discharge a backward cell over time if left in place. Don't do it.

For better efficiency and to avoid blowing fuses, you will need to implement a reverse polarity protection circuit for each cell, using two MOSFETs in any-series. Not pretty.

So, again, don't use a holder for two Li-ion cells. Install them permanently in place.

Answer 2

Could use Batteries with Connectors that won't connect in reverse. not like 9 v Battery Connector that still can touch in reverse.. a more rare solution is to design a new Battery shape and Holder so it wont fit or touch both Terminals at same time.

Answer 3

I understand your requirement. The solution is quit simple you just need to use the battery with build-in BMS. so you don't need to worry about over discharge and protection circuit. Then you do reverse-polarity protection for your system.

For charger, make it ack like second power source. Which activate by correct voltage by battery terminal. If the voltage is not correct then it not charging. Also for balancing fet need to be back to back configuration to avoid uncontrollable discharge.

With these complicate requirement for charger. I would implement this by using CC/CV adaptor control by microcontroller. So you can flexibly choose how you want system to behave. Also I can design battery voltage sensing to handle negative voltage which I'm not sure if that possible with any charging IC.

This problem remind me of my RC charger which based on microcontroller too.