Today, lead-acid and lithium batteries are widely used in various devices. Some people use lead-acid battery chargers to charge lithium batteries. However, we don't recommend this. There are significant differences between lead-acid and lithium batteries. Charging lithium batteries with an original lead-acid charger can cause several problems. Here's why we don't recommend it.
How does a lead-acid battery charger work?
The working principle of a lead-acid battery charger can be divided into three stages: constant current charging, constant voltage charging, and float charging.
Constant current charging
In the initial phase, the charger provides a fixed charge current. The battery voltage will gradually increase while the charge current remains constant. This helps to fill the battery quickly until the voltage of the individual cells reaches approximately 2.4V. At this point, the charge current begins to decrease, but the voltage remains relatively constant.
Constant voltage charging
When the battery is close to a full state of charge (battery voltage reaches 2.4V per cell), the charger enters the equalization phase. During this phase, the charger will gradually reduce the output current to avoid overcharging. The charger will monitor the voltage and temperature of the battery to ensure that the battery is fully charged within a safe range (single cell voltage around 2.6V).
Float Charging
When the battery is fully charged, the charger enters the float charge stage. During this stage, the charger maintains a lower charge voltage between 2.25V and 2.3V to compensate for the self-discharge of the battery and to keep the battery in a fully charged state. It maintains the battery in a fully charged state at a lower constant voltage until use. This condition does not cause overcharging.
How to charge a lithium battery?
Now that we understand how a lead-acid charger works, let's look at how a lithium battery is charged.
Trickle charge
This stage is for fully discharged batteries. At this point, the battery voltage is too low and needs to be pre-charged with a small current of 0.1C to reach a safe minimum charge voltage.
Constant current charging (CC)
Once the voltage has reached a safe level, the charger will charge the battery at a constant current of 0.2C to 1.0C. During this phase, the battery voltage gradually increases until a specific value is reached.
Constant voltage charging (CV)
When the battery voltage rises to 4.2V, the charger switches to constant voltage mode and charges the battery at a fixed voltage. When the battery is almost fully charged, the current gradually decreases until it falls below a certain value, at which point the charge is considered complete.
End of charge: After the battery is almost fully charged, some chargers may enter trickle charge mode or periodically perform a maintenance charge.
Note: Lithium battery chargers work on a similar principle, except that lithium batteries have a higher single-cell voltage and a narrower voltage tolerance. This requires more precise voltage control during charging to avoid overcharging or undercharging.
In addition, Li-ion batteries generally do not require a trickle charge when fully charged. This is because overcharging can damage Li-ion batteries, so trickle charging is not usually included in charger designs.
Why can't I use a lead-acid charger to charge a lithium battery?
Voltage and current mismatch
The charging characteristics of lead-acid batteries and lithium batteries are different. Lead-acid batteries and lithium batteries have different parameters such as nominal voltage, maximum charging voltage, charging current, etc. If a lead-acid battery charger is used to charge lithium batteries, the voltage and charging current may be too high, causing damage to the lithium battery.
Charging algorithm not applicable
Lead-acid and lithium batteries have different charging algorithms and charging curves. Lithium batteries must be charged using constant current and constant voltage charging, while lead-acid batteries have different charging characteristics. If a lead-acid charger is used to charge Li-ion batteries, the mismatch in charging algorithms may lead to problems such as incomplete charging, overcharging, or undercharging.
Lack of communication protocols
Many modern Li-ion battery charging systems use 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 can result in incorrect charging.
Lack of safety mechanisms
Lithium batteries require very strict safety protection, including overcharge protection, overheating protection, and short circuit protection. The lack of these safety mechanisms during the charging process can pose many safety risks to Li-ion batteries. Lead-acid battery chargers may not have these features.
What happens when charging lithium batteries with a lead-acid charger
Many lead-acid chargers have a built-in desulphurization and equalization stage that pulses high voltages into the battery. This is important for the correct charging and maintenance of lead-acid batteries to prevent electrolyte delamination and to ensure that the battery voltage is equalized, but does not apply to lithium batteries. Frequent overcharging can seriously shorten battery life or cause irreversible damage to the battery.
Lead-acid chargers usually have an equalization function to ensure that each battery cell reaches the same state of charge. However, this equalization may not apply to lithium batteries as the charging characteristics of lead-acid and lithium batteries are different. If the equalization function of a lead-acid charger causes the voltage of a lithium battery to reach the high-voltage cut-off protection mode, the battery may go into an open-circuit state, causing the charger and the load to be disconnected. This may require manual intervention or waiting for the battery to reset itself. In the case of an on/off/on/off condition, this may result in reduced charging performance or even the inability to charge properly for several hours. This can affect the charging efficiency and performance of the battery.
Incorrect charging of a lithium battery can cause the battery to overheat. An overheated lithium battery poses a risk of explosion and fire, especially if the charger does not stop charging properly.
Precautions when charging lithium batteries
Regular charging: Even if you don't use the device often, the lithium battery needs to be charged and discharged frequently to prolong its life. Don't wait until the battery is flat before charging it to preserve its characteristics. It is recommended that the battery be fully charged and discharged every few months.
Follow the manufacturer's recommendations: Strictly follow the charging guidelines and recommendations provided by the lithium battery manufacturer. These guidelines include information such as recommended charging voltage, current, charging time, etc.
Avoid overcharging: Avoid overcharging lithium batteries as overcharging can damage the battery and shorten its life. Make sure you use a charger with a constant current and constant voltage charging method.
Avoid over-discharging: It is also important to avoid over-discharging lithium batteries as over-discharging can damage the battery and reduce its performance. Charge promptly to keep the battery within the correct charge range.
Avoid overheating: Avoid overheating the battery while charging. Ensure that the charger and battery are well ventilated and avoid charging in a hot environment.
Consider the charging environment: When charging, make sure the charging environment is safe, keep away from flammable objects, and avoid spilling water or other liquids into the charger or battery.
Use the original charger: Wherever possible, use the charger supplied by the original manufacturer. Due to the different raw materials and manufacturing processes of lithium batteries, the technical requirements of the charger are also different, so only the use of special chargers can prolong the use of lithium batteries and prevent the occurrence of potential safety hazards.
