EEVBlog #176 Lithium Ion Protection Issues

[dbrateris]

Hi All,

After watching Dave's video, I have ran into a few issues with my Li-Ion system, I figured this problem might be common and help some people or maybe I am just having a mis-understanding. Any help would be greatly appreciated.

I am having an issue recharging a single cell Li-Ion battery that contains a protection IC, after the IC has gone into protection mode from an over-discharge of the battery.

I am using an MCP73831 charger to charge the battery. Off of the battery, I have a comparator with a built in voltage reference that monitors the battery voltage and shuts off the main system regulator if the battery voltage drops below 2.9V.  At this point, the total system current is about 5 microamps.  It takes a long time, but eventually, this small current draw reduces the battery voltage enough to cause the protection IC to go into over-discharge protection mode.  The problem is, that once the protection IC is in over-discharge protection mode, the charger will not cause the system to come out of protection mode, and hence the battery will not charge.

The datasheet for the protection IC states that when the system is in over-discharge protection, to get the system out of protection, the terminal voltage must rise above the "Over Discharge release Voltage" (2.9V - 3.0V).

I don't understand how this situation will ever occur, since the battery is disconnected and the charger is feeding the battery with current, there is no current path because the protection IC's discharge FET is off, I don't understand how the charge current can ever cause a voltage to develop and release the battery from protection?

Any help, solutions, suggestions would be very appreciated!!

Thanks so much!
Dan

Here is a link to the protection IC datasheet:
http://www.hycontek.com/attachments/Battery/DS-HY2110_EN.pdf

And the datasheet for the charger:
ww1.microchip.com/downloads/en/DeviceDoc/21984a.pdf

[dbrateris]

Hi All,

I did figure out this issue, after speaking with the manufactures of a few Li-Ion protection IC's the general answer is that the over-discharge protection is more of a safety feature and not intended to cut off the system when the battery dies, it is only a last resort to stop permanent damage to the battery.  Only some chargers will be able to get the battery out of over-discharge protection, and even then the battery could be damaged from being discharged too low.

Since the discharge curve after about 3 volts is so steep, it is recommended that the application circuit stop or significantly reduce current consumption and battery discharge after the battery has reached around 3 volts. Most manufactures are recommending that the application make use of main regulators that have extremely low shutdown currents and can truly disconnect the load from the regulator. Basically, the use of a very low power comparator and voltage reference can be used to measure the battery voltage and make sure it is above 3 volts, then as soon as the comparator detects the battery has dropped below 3V, it will flip its output and turn off the main system voltage regulators, this will reduce total system power consumption to only a few microamps, and leave the battery with months of discharge time before the protection circuit kicks in.  In the mean time the battery can be charged normally, and the comparator helps the application by ensuring that the system voltage is sufficient.  Additionally, end users of the product should be warned that the product should not be left uncharged for an extended period of time.

Thanks everyone,
Dan