Batteries are widely used in our daily lives. I'm sure everyone here has seen one; batteries play a vital role in all aspects of life. From mobile phones and computers to flashlights and remote controls, they all contain batteries, demonstrating their pervasive presence. Now, batteries are no longer limited to ordinary types; many more are being used in our lives. Let me introduce you to the performance and charging methods of lithium iron phosphate batteries!
Lithium iron phosphate battery charging method
Lithium rechargeable batteries are widely used. Lithium iron phosphate (LFP) batteries, due to their unique advantages, are also increasingly used. The chargers for LFP batteries differ from those for ordinary lithium batteries. The maximum termination charging voltage for lithium batteries is 4.2 volts; for LFP batteries, it's 3.65 volts (some online sources say it shouldn't exceed 3.8 volts). The question is: can a mobile phone charger (with slight modifications) be used to charge LFP batteries; or can a discarded lithium battery protection circuit board (with slight modifications) be used for charging protection of LFP batteries?
When batteries in series have the same current, the one with the lowest capacity will fully charge first, but the overall voltage hasn't reached the charging cutoff voltage. Continuing to charge will lead to overcharging, and the same applies to discharging. With too many cycles, this will damage the battery. Therefore, in a series battery pack, there will always be a weakest one. The protection board doesn't charge individual batteries; it protects them. Without a balancing function, it will protect the weakest battery once it's fully charged, even if the entire pack isn't actually fully charged. Even with a balancing protection board, it's a false balance. It bypasses the fully charged battery using a bypass resistor, allowing the other batteries to continue charging to achieve the effect of an overall full charge. However, the bypass resistor has limited power; 100mAh is considered large, which is insufficient for the entire battery pack. Balancing series batteries is a global challenge. Low-cost, high-current balancing solutions are unavailable, and the larger the number of batteries in series, the more difficult it becomes. This is because overcharging and over-discharging of series batteries are difficult to control, and lithium batteries are damaged very quickly and pose safety issues. This is why lithium iron phosphate batteries were developed. Lithium iron phosphate batteries have low capacity and low energy, and are inferior to lithium batteries in many aspects. Their popularity stems from their resistance to overcharging and over-discharging.
The ribbon cable is the detection line of the protection board and does not require thick wire. The red and black wires are the power supply lines, which carry a large current and require thick wire. The protection board detects the voltage of each individual cell, protecting each cell from over-discharge and over-charge. Otherwise, the cells cannot discharge or charge through the protection board, thus fulfilling its protective function.
During charging, the balancing charging board is connected to a ribbon cable, typically charging the entire battery pack directly in series from both ends. The charger voltage is higher than the battery pack voltage. The ribbon cable detects the voltage of each individual cell, essentially acting as a parallel Zener diode. The charging voltage of a single cell will not exceed the regulated value, while other cells continue charging via a bypass circuit through the Zener diode. Because each cell is nearly fully charged at this point, the charging current is small, only balancing each cell to ensure they are fully charged. The charger only protects the overall battery pack voltage, while the balancing charging board ensures that each cell is neither overcharged nor fully charged, preventing the entire battery pack from stopping charging because one cell is fully charged.
1. Some phone chargers have built-in protection that stops charging when the lithium battery voltage reaches 4.2 volts. My idea is to connect a slightly higher power silicon diode (with a forward voltage drop of about 0.6 volts) in series between the lithium iron phosphate battery and the charger. This way, when the lithium iron phosphate battery voltage reaches 3.6 volts, the diode voltage drop will add up to 4.2 volts, which the charger will detect and stop charging.
2. The protection circuit of the mobile phone battery was taken out and used for the charging protection of lithium iron phosphate batteries. It is based on the above principle, with a diode connected in series during charging, which provides 4.2 volt protection.
