Figuring out what current you should charge your LiFePO4 battery is easy.
There are two factors to consider:
- The recommended charge current of the cells
- The maximum allowable charge current from the BMS (battery management system)
Let’s explore the first.
Recommended charge current of the cells
If we take a standard 100Ah 3.2V EVE Lithium cell (we need 4 of these to make a 12V battery). We can see it has the following specifications:
- Typical Capacity: 100Ah
- Typical Voltage: 3.2V
- AC Impedance Resistance(1KHz): ≤0.5mΩ
- Standard Charge/discharge current: 0.5C/0.5C
- Operating Voltage: 2.5V~3.65V
- Maximum continuous charge/discharge current: 1C/1C
- Maximum pulse charge/discharge current(30s): 2C/2C
As we can see, the standard charge/discharge current is 0.5C.
Now, what is C?
C stands for C-rate. To know more about C-rate, I recommend watching my video about it.
The battery capacity (in Ah) multiplied by the C-rate gives you the recommended charging current.
In the case of a 12V 100Ah battery, the maximum charge rate is as follows:
100Ah * 0.5C = 50 Amps
If you have a 12V 200Ah battery, the maximum charge current is as follows:
200Ah * 0.5C = 100 Amps
Now if you have a 48V 100Ah battery (5kw server rack) the charge current is the following:
100Ah * 0.5C = 50 Amps
We can see that the maximum recommended charge current depends on the battery capacity (Ah), not the voltage.
If we use a larger battery cell, the 280Ah EVE cell for example, we can see that the recommended max charge current is 1C.
Let’s calculate the recommended charge current for this cell:
280Ah * 1C = 280Amps
We see that the c-rate is double. This is because the cell is much larger and can dissipate heat better.The higher the cell’s capacity, the higher the charge current can be.
Maximum charge current of the BMS
Another limiting factor will be the charge rate of the BMS.
Each battery management system (BMS) has a maximum charging current. Take a popular Chinese BMS brand, for example.
If we take a 100A BMS, we can see in the datasheet that it can only charge at 50 amps.
If you have a 100amp charger, it won’t work. The BMS will shut down to protect the battery. This is because too much current gets sent to the battery cells.
Charging at a lower C-rate is not bad. It is better for the battery’s lifespan. Refer to my article about my recommended chargers for LiFePO4 batteries.
Figuring out at what amp you should charge your LiFePO4 battery is straightforward.
Multiply the C-rate of the battery by the capacity of the battery.
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C-rate (usually 0.5) * Capacity (in Ah) = Recommended max charge current of a LiFePO4 battery.