Today, both lead-acid and lithium batteries have a wide range of applications and can be seen everywhere. There are also an increasing number of people using lithium batteries as a replacement for lead-acid batteries to power their devices. So, after replacing a lead-acid battery with a lithium battery, can you still use the original lead-acid battery charger to charge it? We do not recommend doing so! Let’s analyze why the answer is ‘no’.

How Do Battery Chargers Work?

Let’s first understand how a lead-acid battery charger works. The working principle of a lead-acid battery charger can be divided into three stages: the constant current charging stage, the constant voltage charging stage, and the float charging stage.

Constant Current Charging

The charger initially provides a fixed charging current, allowing the voltage of the battery to gradually increase until the voltage of the individual cells reaches approximately 2.4V. At this point, the charging current begins to decrease, but the voltage remains relatively constant.

Constant Voltage Charging

Once the individual cell voltage of the battery reaches 2.4V, it enters the constant voltage charging stage. As the battery approaches full charge, the current gradually decreases to a very low level, and the charging is essentially complete. At this time, the individual cell voltage of the battery is around 2.6V.

Float Charging 

The float stage is to compensate for the loss caused by the battery’s self-discharge. At this point, the charger reduces the output charging voltage to maintain it between 2.25V to 2.3V. It maintains the battery in a fully charged state at a lower constant voltage until it is used. This state does not result in overcharging.

How To Charge Lithium Battery?

After understanding the working process of the charger, let’s organize the charging process for lithium batteries. This process is mainly divided into two stages: constant current and constant voltage. However, the charger will adjust according to the actual condition of the battery:

Trickle Charging 

This stage is intended for a fully discharged battery. At this point, the voltage of the battery is too low and requires an initial pre-charge with a small current of 0.1C to reach the safe minimum charging voltage.

Constant Current Charging (CC)

 Once the voltage reaches a safe level, the charger charges the battery with a constant current ranging from 0.2C to 1.0C. During this stage, the battery voltage gradually increases until it reaches a specific value.

Constant Voltage Charging (CV)

 When the battery voltage rises to 4.2V, the charger switches to constant voltage mode, charging the battery at a fixed voltage. As the battery nears full charge, the current gradually decreases until it drops below a certain value, at which point the charging is considered complete.

Termination of Charging: After the battery approaches a full charge, some chargers may enter a trickle charge mode or periodically perform maintenance charging.

Why Can’t You Use A Lead-Acid Charger To Charge Lithium Batteries?

Given that both have constant current and constant voltage stages, why is it not recommended to use a lead-acid battery charger to charge lithium batteries? This involves several aspects:

Voltage Compatibility Issue

The nominal voltage of a single lead-acid battery cell is 2V, with a full charge state at about 2.4V.

The nominal voltage of a single lithium battery cell is 3.6-3.7V, with a full charge state at 4.2V.

The nominal voltage of a single lithium iron phosphate (LiFePO4) battery cell is 3.2V, with a full charge state at about 3.65V.

The charging voltage of lead-acid batteries is typically lower than that of lithium batteries, which may result in the lithium battery not being fully charged. However, if the output voltage of the lead-acid battery charger is too high, it could lead to overcharging damage to the lithium battery, and might even cause a safety incident due to thermal runaway.

Charge Mode Mismatch

Lithium batteries typically use a constant current/constant voltage charging mode. The charging characteristics of lead-acid batteries differ from this and may not provide a suitable charging mode for lithium batteries, thus affecting the battery’s performance and lifespan.

Lack of Necessary Communication Protocol

Many modern lithium battery charging systems utilize specific communication protocols to exchange information between the charger and the Battery Management System (BMS). Lead-acid battery chargers do not support this type of communication, which could result in the charging process not being carried out correctly.

Lack of Safety Mechanisms

Lithium batteries require very strict safety protections, including overcharge protection, overheating protection, and short-circuit protection. The absence of these safety mechanisms during charging can pose numerous safety risks to lithium batteries. Lead-acid battery chargers may not be equipped with these features.

Battery Damage

Inappropriate charging methods can lead to damage to the internal chemical structure of lithium batteries, not only shortening the battery’s lifespan but also causing a decline in performance.

Safety Risks

Incorrect charging methods can damage the battery and also lead to safety accidents. This is because lithium batteries can swell, catch fire, or explode when overcharged or overheated.

Conclusion

To protect battery safety and performance, it is recommended that you use a charger specifically designed for lithium batteries to charge them.