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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.

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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.

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  • \$\begingroup\$ A1: clear answer. Thank you! A2: I dont have a multicell configuration. I have a pouch cell and fused my charge circuit so i dont think the current will get really and warm up the battery very much. But I still have to test the charging at higher ambient temperatures. \$\endgroup\$ – user3689010 Oct 22 '15 at 7:08
  • \$\begingroup\$ Pouch cells are usually li-ion polymer cells. If your fuse is the right size & type, it could protect your cell/s from excess discharge, however, the heat/vent/exhaust/fire problem with rechargeable cells is always a risk because, when some of them start to go bad (and some of them will DEFINITELY go bad) they do not dis/charge properly. They can seriously overheat from even normal dis/charging currents. I just recharged a cell recently with a slow charging rate (.5A) that overheated enough 'halfway' through the charging process to trigger a thermal circuit shutdown. \$\endgroup\$ – zeffur Oct 22 '15 at 17:58
  • \$\begingroup\$ Without protection, that cell may have vented toxic gas, possibly exploded, & also possibly caught fire. Most of the time you won't encounter such a situation--but when it does happen, you'll be very glad that you have protection circuitry to deal with it. \$\endgroup\$ – zeffur Oct 22 '15 at 17:58
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Clear question :-)

How low do you want to go ? I think the 2.4 - 2.7 v limit is more for protecting against deep discharge than anything else. Deep discharge is bad for Lithium based batteries, it shortens their lifetime. Indeed I would also add an additional "battery empty" detection. If you already have a microController with an ADC in your application you could use that to detect the battery voltage and warn around 3.7 V and shut down below 3.5 V for example. Then the battery's protection should never activate which is good.

Regarding the NTC for temperature monitoring: I think that is only needed if you fast-charge the battery. By that I mean charge with a large current so that the battery can be fully charged from empty in 1 - 2 hours. In case the battery cannot absorb the charge it will heat up quickly, you want to detect that. If you only charge slowly (it takes 5 hours or more between empty and fully charged) then the battery could still get warm but not so much that it can be a problem. So no temperature detection is needed.

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  • \$\begingroup\$ All of the Li+ batteries I have worked with should only initiate a charge cycle between 0C and +40C, and continue the charge cycle between 0C and +45C. I would say that is the primary reason for the thermistor (although fast charging will definitely affect cell temperature). \$\endgroup\$ – Peter Smith Oct 21 '15 at 13:41
  • \$\begingroup\$ I agree, although that 0C to +40 C you could also cover with a sensor elsewhere, it does not need to be on the battery itself like it would need to for fast charging. \$\endgroup\$ – Bimpelrekkie Oct 21 '15 at 13:47
  • \$\begingroup\$ Thank for you answer! But wouldnt controlling the switching of the battery be impossible with the CPU that is being powered from the battery? I prefer some sort of hardware circuit, this is also works in cases when the CPU stalls.. Hm oke, so it can be used for multiple uses. But detecting ambient temperature is only useful at start otherwise the charging of the battery would affect the measurement. So then it would make more sense to place the on a different place than the battery. \$\endgroup\$ – user3689010 Oct 22 '15 at 7:07
  • \$\begingroup\$ Yes correct and that's why the battery switch indeed must be controlled by a dedicated chip and is usually integrated in the battery itself (when it is user replaceable). It's not so much that you would need an accurate temperature reading but that you need a way to detect when things go wrong. \$\endgroup\$ – Bimpelrekkie Oct 22 '15 at 11:21
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The first question has already been answered well. To re-iterate the built in protection is emergency protection to avoid things going bang. These are emergency limits that you shouldn't hit under normal operation. Running the battery to these limits will reduce it's capacity and operational life.

The second question I can't see a correct answer to. It's not for multi-cell setups or only for fast charging. The temperature sensor is there because charging a Li-Ion battery below 0C or over 40C will significantly reduce it's life and capacity. The temperature sensor allows the charger to cut off if these limits are hit in order to prevent damage to the battery. 40C sounds hot but remember that the charger IC will be generating heat and is often close to the battery.

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  • The higher the voltage you charge up to and the more times you do it, the more damage you do the battery (reduction in permanent capacity and cycle life) and the more swelling pouch and prismatic (flat) cells incur.
  • Over discharging the battery also leads to permanent damage called "dissolution" which starts to separate electrode materials from one another, also causing permanent damage to the battery's performance.
  • Both of these can result in safety issues. Even charging 0-3mV over the manufacturers recommended limits can have a significant impact on the battery over time. I know this from experience.
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