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From this site and many others, it seems to be universally recommended that a lithium ion battery not be discharged lower than 3.0 volts. Most actually recommend to stop discharging at around 3.3 volts as there is little usable energy beyond this.

However, every lithium ion PCB or BMS stops the discharging of a lithium battery or battery pack at a voltage of 2.7 or even as low as 2.4.

Here is an example:

http://www.batteryspace.com/PCB-for-3.7V-of-Li-Ion-Battery-2.0A-limit.aspx

In this protection circuit, it stops the discharge at 2.4 volts/cell.

The reasoning I am given for not over discharging the lithium ion batteries is that it will hasten their demise and increase the probability of catastrophic failure such as fire or explosion.

So why the low voltages on the protection circuits and are our lithium ion drills etc being allowed to go to this low a voltage?

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  • \$\begingroup\$ Links to eBay items do not make good examples, because they don't have the long term value. The auction will expire and the link will die. Try to use manufacturer's product pages, or [ideally] reference designs as examples. \$\endgroup\$ – Nick Alexeev May 20 '14 at 22:44
  • \$\begingroup\$ @Nick Alexeev - Good point, I hadn't thought of that. I've edited it and added a better link. \$\endgroup\$ – Filek May 21 '14 at 3:32
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The PCBA that you give as an example is intended as a protection unit that will (attempt to) save the battery from excesses of treatment by the system.
It assumes that a "proper" controller will handle the actual system management and that its role is as an emergency watchdog.

If its parameters were set conservatively it might interfere with design decisions made by a system designer for the main controller. As this unit is usually hard connected inside a battery pack, having it stop the designer doing sensible things would be "a bad thing" [tm] SO it is set to values that no sane designer will reasonably approach.

If you rely on it as the main controller your battery's days will be short on the face of the land.
eg 4.35V +/- 0.025 V for Vmax during charging will at minimum greatly decrease available battery cycle lifetime and worst case will set remaining cycle lifetime to zero.

V_min_protection at 2.4V is unlikely to be fatal but will decrease cycle life if used regularly.

The "restart after protection operates" values gives a good idea of what the designer of this protection board think that you should REALLY be doing - ie 4.2V recharge restart and 3V discharge restart. These correspond to typical industry practice for actual limiting values.

ICs intended for actual battery management in systems will use end point values around 3V and 4.2V.

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  • \$\begingroup\$ Your answer certainly makes sense. That would mean that most lithium ion batteries would have "double" protection from over charge and over discharge - the PCB and the main controller both prevent it. The odd thing is that there seems to be thousands of the PCB available, but the actual controllers or Battery management IC seem to be hard to find! \$\endgroup\$ – Filek May 21 '14 at 4:58
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    \$\begingroup\$ @Filek: That bit of smarts belongs in the MCU, via ADC. \$\endgroup\$ – Ignacio Vazquez-Abrams May 21 '14 at 5:02
  • \$\begingroup\$ @IgnacioVazquez-Abrams - thank you. I learned something new yet again at the stack exchange. \$\endgroup\$ – Filek May 21 '14 at 5:37
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"Lithium ion" is actually a family of chemistries. The highest discharge voltage of them (NMC+graphite) is 3.0V, so that is the only universal "safe" threshold. Some of them can survive to 2.7V or even 2.4V, but I would not use a circuit designed to do so unless I knew that the cells could handle it.

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