One question we get repeatedly is “Can I top balance at 3.7V?”
The reason for this question is that many DC power supplies have a minimum voltage adjustment range of 0.1V. It is impossible to set the voltage to 3.65V. The only option is to top balance with 3.6V or 3.7V.
This gives rise to two concerns.
- whether the battery cells will not reach true top balance if they are charged with 3.6V.
- if 3.7V is used for charging, the maximum rated voltage of the battery cell is exceeded, and whether this will damage the battery cell.
Here, we recommend using 3.6V for top balancing when your DC power supply cannot be set to 3.65V.
While 3.7V over the rated maximum voltage of 3.65V for LiFePO4 battery cells is not too high, it is, after all, over the limit. Although it will not necessarily cause damage to the LiFePO4 battery cells, there is always a risk.
For LiFePO4 battery cells that we plan to use for more than 5 years, we need to take this risk out of the equation.
So is charging with 3.6V enough to bring LiFePO4 battery cells to top balance?
The answer is yes, 3.6V is enough to bring LiFePO4 cells to top balance. According to the charging curve of LiFePO4 battery cells, when the battery cells are charged to 3.4V, the voltage rises very fast, and by 3.6V, the curve has also reached the rightmost part near the vertical end. The SOC(State of Charge) is close to 100% at this point.
The correct way to top balance LiFePO4 using 3.6V
In general we recommend the user to operate like this.
1, Connect LiFePO4 battery cells in parallel and charge them with 3.4V or 3.5V.
2, Continuously monitor the voltage of the battery cell. And fine tune the charging voltage to 3.6V when the voltage rises to 3.4V or 3.5V.
3, pay close attention to the change of charging current, when the current gradually decreases to close to 0. disco
4, (OPTION) charge again with 3.6V and observe the current. If the current is close to 0, it means the top balance is done. If the current is not close to 0, you will also observe that it drops quickly. When it drops close to 0, disconnect and leave it for some more time.
After the above steps, the SOC of the battery cell is over 99.9%. In fact it can already be seen as achieving top balance.
Finally, you can connect the battery cells in series and form a battery pack with BMS. In the future, the BMS will balance all the cells without interruption to maintain the efficient output of the whole battery pack.
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