Lithium Ion Battery Charging And Discharging Tips

Nowadays more and more professional customer can assemble the battery by themselves, namely purchase cell, BMS and other components to DIY a complete battery pack.

So how you manage the charge and discharge limit of your battery? What voltage should be set for BMS to well control each cell? Why need to set it? How you judge your battery SOC?

Let Bonnen Battery‘s engineer take NMC Li-ion cell for example.

The maximum charge termination voltage of a single-cell NMC lithium-ion battery is 4.2V, and it cannot be overcharged. Otherwise, the battery will be scrapped due to too much lithium-ion loss from the positive electrode. When charging the lithium battery, a dedicated constant current and constant voltage charger should be used. After constant current charging, the lithium battery voltage reaches 4.2V, then it is switched to the constant voltage charging mode; when the constant voltage charging current is reduced to 100mA, The charging should be stopped.

The charging current (mA) can be 0.1 to 1.5 times the battery capacity, for example, a 100 Ah lithium battery, and the charging current can be controlled between 10A and 150A. The normal charging current can be selected to be about 0.5 times (0.5C) the battery capacity, and the charging time is about 2-3 hours.

Due to the internal structure of the lithium battery, lithium ions cannot all move to the positive electrode during discharge, and a part of lithium ions must be retained at the negative electrode to ensure that lithium ions can be smoothly inserted into the channel during the next charging. Otherwise, battery life will be shortened. To ensure that some lithium ions remain in the graphite layer after discharge, it is necessary to strictly limit the minimum voltage at the end of discharge, that is, the lithium battery cannot be over-discharged. The discharge termination voltage of an NMC single-cell lithium battery is usually 3.0V, and the minimum can not be lower than 2.5V. The battery discharge time is related to the battery capacity and discharge current. Battery discharge time (hours) = battery capacity/discharge current, and lithium battery discharge current need follow manufacture specification, cannot over its definition.

Regarding the SOC, we can have a simple way to calculate it by voltage.
Below is a rough way for your reference:

4.20V—-100%
3.95V—-75%
3.85V—-50%
3.73V—-25%
3.50V—-5%
2.75V—-0%

Different cell from a different manufacturer that may a bit varied in its parameter, above could be a majority data for current industry level. Welcome you consult Bonnen engineer should you have any query during PACK.