Electronic – protection circuit Lithium-ion batteries

batterieslithiumlithium ionprotection

I am designing a lithium-ion battery in my project but I am a little confused in regards to certain aspects of the protection circuit of lithium-ion batteries.I know about the different stages of charging a Li-Ion and you have to have a lithium charger IC to do the charging safely for you. If I understand it correctly you need to protect the battery against overcharge, overdischarge and overcurrent(shortcircuit) situations.

The Li-Ion battery(single cell, 1200mAh) I selected has a protection circuit of its own but the shutdown voltage is somewhere around 2.4V to 2.7V and the overcharge voltage is around 4.2/4.3V. My confusion lies around this 2.4V to 2.7V, I understood you shouldnt let Li-Ion discharge below 3.0V. My battery supplier and another source I found are saying it is better the batteries protection circuit should not be activated and they suggested I should also build an "extra/second" protection circuit on my PCB with a higher voltage like 3.0V so this circuit switches of the load first instead of the protection of the battery.

I cant confirm this in any other sources which seems strange to me for such an important aspect if there were correct. So my question basically is what the right design practice is and how this is often done. I know Seiko is pretty big in over/under voltage protection ICs but I can also imagine circuit with supervisors or op-amps.

An other question I have is in regards to a NTC thermistor for Li_ion batteries.
(white or yellow wire in many batteries or packs)How important is this? When should I include this in my design? Because I sometimes see charger IC's with and without a connection for a thermistor.

Best Answer

A1. The built-in protection for li-ion cells should be considered "emergency protection"--it should not be relied upon for normal cycle V & A protection. In a "safe" battery design, V&A protection should "always" be included in your design.

A2. A thermistor protected battery system is wise. Sometimes li-ion cells do not dis/charge at the same rate & cause an imbalance in the series/parallel configuration of larger Ah batteries. A thermistor can be used to detect an overheating condition, prevent a fire, slow the dis/charge rate/s, or indicate a dis/charging fault in a smart battery system. I consider it very important in a safe battery system, but, it is often overlooked or ignored when ICR type li-ion cells are used. If you want to make a safe battery pack, then I suggest you always include a thermistor & supporting circuitry--don't allow the incomplete/shoddy designs of others to affect your design decisions.