Why Can’t Batteries Be Charged as Fast as They Can Discharge?

That’s a good question. And it’s true. Despite the impressive capabilities of modern lithium-ion batteries, particularly in electric vehicle (EV) applications, the rate at which a battery can recharge is often slower compared to how quickly it can discharge. This limitation is mainly due to the deep-rooted reason involving high internal heat, which can significantly compromise the battery’s ability to be safely and effectively recharged multiple times.

Battery Management System (BMS) and Rapid Discharge

Even with advanced technology, such as in the case of EV batteries, the battery management system (BMS) controls the discharge rate to prevent damage. In a scenario where the BMS is disabled, an EV battery can rapidly drain its charge. However, attempting to charge the battery back up in the same manner would quickly lead to overheating and potential damage. Lithium-ion batteries, in particular, slow down their charging process as they get closer to full capacity to minimize internal heat generation.

Heat and Battery Efficiency

The simple answer to the question is that internal heat wastage limits the speed at which a battery can be charged. When a battery is subjected to a heavy load, it quickly discharges but, in practice, is often charged much faster than it is discharged. Overheating is, therefore, the main issue that needs to be addressed to improve charging efficiency and longevity.

Lithium-ion Batteries and Charging Limits

For many types of lithium batteries, the charging rate can exceed the rate at which they are discharged. This is evident in systems like the Tesla supercharger, which is designed to fast-charge batteries at high rates up to 80% of their capacity to avoid overheating. However, even in these systems, there is a need to reduce the charging rate near the end of the charge cycle to prevent heat buildup and potential damage to the battery cells.

Lead Acid Batteries and Fast Charging

Lead-acid batteries, such as Absorbed Glassmat (AGM) deep cycle batteries, have a higher internal resistance and are more prone to overheating when charged rapidly. Safe charging rates for AGM batteries are usually around 20% of the ampere-hour (Ah) rating. For instance, a 100 Ah battery should not be charged at more than 20 amps. Although fast charging can be done, it is not very efficient, and the battery will take around 120 charging cycles to reach a full 100%.

Impact of Fast Charging on Battery Life

While fast charging can be convenient, particularly in situations where charging time is limited, it can also lead to a reduction in the battery's lifespan. Battery manufacturers often warn that conventional charging, which is slower, can extend the battery's working life from 5 years to 3 years when subjected to frequent and rapid charging cycles.

Conclusion

Neither should we outright avoid fast charging entirely nor should we condemn it definitively. It depends on the circumstances and design of the charge controller and vehicle. For instance, in a car where charging time is scarce, rapid charging might be the only option. Conversely, a slower, more conservative charging method, such as a 5W charger, is less stressful on the battery during regular, overnight charging sessions.