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How to Charge Batteries Properly?

Introduction

A battery is an electrochemical device. During the battery discharge process, the stored chemical energy is converted to electrical energy that is used to power an electrical load. The charging process is the opposite reaction, where the electrical energy from the charging source is being stored into the chemical energy of the battery.

A number of methods for charging batteries have been developed to meet the “rules for proper charging”. Some of these methods are known as the constant-current method, constant-voltage method, modified constant-voltage method, float charging method, and trickle charging method. Charging methods largely depend on the type of battery.

Using the BAC series of devices all three mayor battery chemistries (Lead-Acid, Nickel-Based and Li-ion) can be charged. The BAC25A device has the ability to generate charging currents of up to 25 A, with output voltages for battery strings of up to 300 V. The model BAC30L is intended for charging battery strings of voltages up to 60 V, with charging currents up to 30 A.

Battery Charger Connected to UPS
Figure 1 – Connection of BAC device to a battery string during the charging process

Charging of Lead-Acid Batteries

The most common method for charging lead-acid batteries is IU mode, which is available for all models of the BAC series. Charging in the IU mode is a 2-step process. The first step s charging the battery with the constant, user-selectable current (Charge I). The process continues until the battery voltage reaches the predefined Charge U. After the Charge U voltage is reached, charging continues with constant voltage, equal to the pre-selected charging voltage value.

As soon as the battery reaches the selected charging voltage, the device switches to the constant voltage charging and the charging current begin to decrease. Charging is stopped when a preset minimum current value, which indicates a full charge, has been reached.

An example of battery charging stages is presented in Figure 2.

Battery Charging States Graph
Figure 2 – Battery charging states

The crucial part of the charging process is to follow the battery’s manufacturer recommendations for maximum charging currents and voltages.

The standard charging voltage for lead-acid cells is from 2.30 V to 2.45 V. Therefore, a 12 V battery (consisting of 6 cells connected in series) would have the charging voltage equal to:

6 x 2.45 = 14.7 V

Another common method for charging lead-acid batteries is using UU, which is also available on the BAC series. This method is rarely used and is recommended only when the battery is heavily discharged.

Charging of Lead-acid batteries in UU mode is a 2-step process. The first step is constant voltage charging until the battery reaches the preselected Boost U voltage. A maximum charging current that can be generated during the boost charging is user-defined (Boost I) which protects the battery from being damaged of high current. Charging will not terminate if Boost I value is reached – BAC device will proceed to charge at constant current (Boost I)
until the voltage Boost U is reached. Otherwise, if the battery current doesn’t reach the current limit, the charger will charge at the selected boost voltage and the current will decrease over time.

The correct setting of the boost and float voltage is critical and typical values are 2.45 V per cell for boost voltage and 2.25 V per cell for float voltage. For instance, float and boost voltages of 12V 70Ah battery string comprised of 6 lead-acid batteries shall be:

6 x 2.25 = 13.5V – Float voltage
6 x 2.45 = 14.7V – Boost voltage

Another parameter that needs to be predefined is the boost current. The boost current represents the maximum current (current limit) value at which the BAC Series device will boost (equalize) the battery. Correct setting depends of C-rate.

Charging of Nickel-Based Batteries

Nickel-based batteries are more complex to charge than Li-ion and lead-acid. Lithium- and lead-based systems are charged with a regulated current to bring the voltage to a set limit after which the battery saturates until fully charged. On the other hand, a fully charged state in Ni-Cd batteries can be detected by a slight voltage drop that occurs at the end of the charging process.

One way of charging Nickel-Based batteries is by using the IU charging mode on the BAC devices.

The first step is charging the battery with the constant user-selectable, current (Charge I). The process continues until the battery voltage reaches the predefined Charge U. Full charge detection occurs by observing a slight voltage drop after a steady rise. After this voltage drop is observed, charging should be terminated manually on the BAC device.

An example of the charging procedure for Nickel-based batteries is shown in the following diagram.

Charging Nickel-Based Battery Graph
Figure 3 – Charging Nickel-based battery

For a Ni-Cd cell with rated voltage 1.2 V, the recommended setting of the charge voltage limit is 1.5 V per cell.

Charging of Li-ion Batteries

As for the lead-acid batteries, Li-ion batteries are charged using IU mode. The first step is charging the battery with the constant, user-selectable, current (Charge I). The process continues until the battery voltage reaches the predefined Charge U. After the Charge U voltage is reached, charging continues with a constant voltage which equals the Charge U voltage.

As soon as the battery reaches the Charge U voltage, the device switches to the constant voltage charging and the charging current begin to decrease. Charging current decreases continuously during the constant voltage charging. Charging is stopped when a preset minimum current value, which indicates a full charge, has been reached.

An example of the charging procedure for Nickel-based batteries is shown in the following diagram.

Charging Li-Ion Battery Graph
Figure 4 – Charging Li-ion battery graph

For a Li-ion cell with rated voltage 3.7 V, the recommended setting of the charge voltage limit is 4.2 V per cell.

The full charge occurs when the battery reaches the voltage threshold and the current drops to around 3 % of the rated current, depending on manufacturer’s recommendations. A battery is also considered fully charged if the current levels off and cannot go down further. Manufacturers of Li-ion cells are very strict on the correct charging voltage settings since Li-ion batteries cannot accept overcharge. To make sure that the battery is not being overcharged, the BAC devices have the ability to terminate the charging process once the preset maximum voltage is reached. This can be configured in the limits menu (STOP U), as shown in the following figure.

BAC Screenshot
Figure 5 – BAC settings menu

Another method for detecting a fully charged state of Li-ion batteries is using the STOP T limit on BAC device and selecting the total charging time.

The correct setting of the current and voltage is as described above, and the same voltage limitation needs to be included. STOP T also must be confirmed.

For example, for a 1Ah li-ion battery with a charge voltage of 4.2 V per cell and current of 1 A (1C) (as on diagram above), voltage limitation shall be 4.5V/cell and STOP T shall be adjusted to 2.5 – 3 h. Also, STOP I can be confirmed and defined between 3 and 5 percent of the Ah rating. (0.05 x 1). Limitations are presented in the figure below.

BAC Limits
Figure 6 – BAC limits

In case multiple limit options are enabled on the BAC device, BAC will terminate the charging process once the first limit is reached.

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December 17, 2019

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