The reliable performance and long service life of Keheng sealed lead-acid batteries depend on the correct charging method. Incorrect charging methods or improper equipment will lead to shorter battery life and reduced performance. Choosing the right SLA battery charger and charging method is as important as choosing the right battery for your application. KEH recommends that when charging a sealed lead-acid battery, a charger designed specifically for the chemistry of that battery, such as a sealed lead-acid battery charger, be used.
Battery charging technology
ACE-sealed lead-acid batteries can be charged by a variety of technologies:
- Constant voltage
- Constant current
- Taper current
- Two-step constant voltage
Constant voltage-limited current charging is recommended in order to extend battery life and capacity with acceptable charging time and cost-effectiveness.
A DC voltage between 2.30 volts/cell (float) and 2.45 volts/cell (fast charge) should be applied to the battery terminals to charge a sealed lead-acid battery. Depending on the state of charge (SoC), the discharged battery voltage may be lower than the applied voltage. However, the voltage should stabilize over time.
When charging, the lead sulfate in the positive plate is converted to lead dioxide. As the battery nears full capacity, the positive plates begin to produce lead dioxide, resulting in a lower internal resistance and a sudden rise in voltage. Constant voltage charging can detect the voltage rise and thus control the amount of charging.
Battery Charging Characteristics
During constant voltage or taper charging, the battery’s current acceptance decreases as the voltage and state of charge increase. The battery is fully charged when the current has stabilized at a low level for several hours. There are two criteria for determining whether a battery is fully charged: (1) the final current level and (2) the peak charging voltage at which the current flows.
- Typical Characteristics of Sealed Lead-Acid Battery Charging for Cyclic Service with Discontinuous Charging and High Peak Voltage.
- Typical battery charging characteristics for standby-type batteries are where charging is continuous, and the peak charging voltage must be lower.
Battery charging method
The correct charging method selection for a Keheng sealed lead-acid battery depends on its intended use (cycling or float), economic factors, charging time, expected frequency and depth of discharge (DoD), and desired service life. The purpose of any charging method is to control the charge current at the end of the charge.
Constant Voltage Charging
Constant voltage charging is the preferred charging method for sealed lead-acid batteries. Depending on the application, the battery can be charged continuously or discontinuously. Continuous float charging is recommended for applications requiring backup power, such as security systems or uninterruptible power supplies (UPS). Non-continuous cyclic charging is suitable for portable equipment such as electric wheelchairs and mobile medical carts where intermittent charging is appropriate.
This method applies a constant voltage to the battery, limiting the initial charging current. Setting the charging voltage according to the specified charging and temperature characteristics is critical. Inaccurate voltage settings may result in overcharging or undercharging. This method is suitable for cycling and standby applications.
- Constant Voltage Charging Circuit
- Constant Voltage Charging Characteristics
Constant Current Charging
Constant current charging is used when the number of discharge ampere hours from the previous discharge cycle is known. Charge time and charge volume can be easily managed using this method.
Charging method of Keheng lead-acid battery
Constant current charging
Although constant current charging can be calculated efficiently, expensive circuitry is required to achieve a highly accurate constant current. The charging voltage must be monitored to prevent the battery from being overcharged, or the charging time must be limited. This method is particularly effective for restoring capacity to a Kay Heng sealed lead-acid (SLA) battery, which may have been in storage for a long time, or for occasional overcharging that requires balancing of the battery’s capacity. However, it lacks certain characteristics required for today’s electronic applications.
Tapered Current Charging
Cone current charging is not normally recommended for Tech Haven sealed lead-acid batteries because it can cause abuse and shorten the life of the battery. Nevertheless, the conical current charging method is widely used for multiple battery charging and cycle charging applications because of its simplicity and low cost. When using a tapered current charger, it is necessary to limit the charging time or incorporate a charge cutoff circuit to prevent overcharging.
In a tapered current charging circuit, the current decreases proportionately to the voltage rise. Supply voltage fluctuations should always be considered when designing a tapered charger. In this case, the drop in internal resistance will be converted into heat, which should be monitored. If necessary, a heat sink should be included in the design to control heat generation.
Keheng lead-acid battery overcharging
Excessive charging voltage will lead to a current surge when the battery is fully charged, leading to the decomposition of water in the electrolyte and premature aging of the battery. With a high overcharging rate, the battery will gradually heat up. The battery absorbs more current as the temperature rises, leading to further heating. This phenomenon is known as thermal runaway and will destroy the battery in just a few hours.
Insufficient charging of Keheng lead-acid battery
Low charging voltage will cause the current to stop flowing before the battery is fully charged. This situation will lead to lead sulfate residue on the electrodes and eventually reduce the battery capacity. Batteries in a discharged state or long time idle may initially show an “open circuit” phenomenon or accept a much lower current than the normal level due to the “sulfation” phenomenon. In this case, it is recommended that the charger remain connected to the battery. Normally, the battery will begin to receive more and more current until it reaches normal levels. If the charging voltage is higher than the recommended level and the battery still does not respond, the battery may have been discharged too long to recover, and the KAHN SLA battery must be replaced.
Cyclic charging of KAHN lead-acid batteries
Cycling applications usually require charging to be completed in a relatively short period of time. However, the initial charging current must not exceed 0.30 x C amps. Just as the battery voltage decreases during discharge, it gradually increases during charging. Full charging depends on the voltage and the inflow current. A battery is considered fully charged when the charging voltage reaches 2.45 ± 0.05 volts per cell and the current received by the battery drops to less than 0.01 x C amps (1% of rated capacity). At this point, the charger should be disconnected or switched to a float charge voltage of 2.25 to 2.30 volts per cell. It is critical to ensure that the voltage does not exceed 2.45 ± 0.05 volts per battery.
Standby charging of ACE lead-acid batteries
In standby applications, batteries do not need to be recharged as quickly or as frequently as in cycling operations. However, a continuous charge must be maintained to make up for the energy lost due to internal discharge and battery aging. Although these losses are minimal in a Keheng lead-acid battery, they must be replenished at the battery’s self-discharge rate. At the same time, care must be taken to avoid overcharging to avoid exceeding these losses.
Charging Methods for Lead-Acid Batteries
In order to effectively maintain lead-acid batteries, a constant voltage charging technique known as standby or float charging is required. The ideal float voltage is 2.25 to 2.30 volts per battery. By maintaining this float voltage, the battery can autonomously regulate the current level and remain fully charged without disconnecting the charger. For a fully charged battery at the recommended float voltage, the trickle current usually stabilizes at about 0.001C (e.g., 7 milliamps for a battery at 7 amps).
Float chargers operate on the principle of constant voltage power. Similar to a cycling charger, the key to a float charger is an initial charge current limit of 0.30 x C amps that must not be exceeded.
Two-Step Constant Voltage Charging
This charging technology uses two constant voltage settings. During the initial charging phase, a high-voltage setting is used. When charging is nearly complete and the voltage reaches a preset threshold (with a corresponding reduction in charging current), the charger switches to the low voltage setting. This approach facilitates fast charging in cycle and float modes while minimizing the risk of overcharging, even during long charging periods.
Features of the two-stage current-limited SLA battery charger
Series Charging
Lead-acid batteries consist of strings of 2-volt cells connected in series, typically containing 2, 3, 4, or 6 cells each. Charging series-connected strings of lead-acid batteries is safe and efficient, even when configured for up to 48 volts or more. However, as the number of cells in series increases, the potential for capacity variations increases. These variations can stem from factors such as battery life, storage history, temperature fluctuations, or misuse.
When charging in series, never mix fully charged batteries with discharged batteries; make sure discharged batteries are charged before connecting. When using a single constant voltage charger in a high voltage battery pack, the same current flows through all cells. As a result, some batteries may be overcharged while others may remain slightly undercharged.
To mitigate the effects of individual battery differences, it is recommended that batteries with the same date of manufacture, manufacturer, amp-hour rating, and operating history be used. Whenever possible, charge in series at no more than 24 or 48 volts.
Parallel Charging
Lead-acid batteries may be connected in parallel with one or more batteries of the same voltage. In a parallel configuration, the current generated by the charger is almost equally distributed to the individual batteries. No special matching of the batteries is required. However, if batteries of different capacities are connected in parallel, the current distribution will be consistent with the capacity ratio (subject to internal resistance).
When charging batteries in parallel, especially when different charging ratios are expected, careful steps should be taken to ensure that current variations between batteries are kept to a minimum.
Battery temperature compensation
Keheng sealed lead-acid batteries exhibit reliable performance over a wide range of temperatures. However, charging efficiency decreases at lower temperatures and increases rapidly at temperatures above 45°C (113°F), and there is a risk of thermal runaway if temperature compensation is not managed properly.
While the effect of temperature on charging voltage is less important in float charging applications, it becomes very important in cycling, where higher charging currents are required to speed up charging times.
The effect of temperature must be considered when designing or selecting a charging system. It is highly recommended that temperature compensation be incorporated into the charging circuitry, especially when operating outside the optimum range of 5°C to 35°C (41°F to 95°F). The temperature coefficient is -2mV/cell/ºC up to 68°F (20°C) in float mode and -6mV/cell/ºC up to 20°C in cycle mode. For high temperatures, the charging voltage should be adjusted accordingly.
Battery temperature compensation table
The following table lists the recommended charging voltage of Keheng-sealed lead-acid batteries under different temperatures according to the ambient charging voltage of each battery.
| Temperature (°C) | Cycling use (V) | Float charging use (V) |
|---|---|---|
| -40°C (-40°F) | 2.85 – 2.95 | 2.38 – 2.43 |
| -20°C (-4°F) | 2.67 – 2.77 | 2.34 – 2.39 |
| -10°C (14°F) | 2.61 – 2.71 | 2.32 – 2.37 |
| 0°C (32°F) | 2.55 – 2.65 | 2.30 – 2.35 |
| 10°C (50°F) | 2.49 – 2.59 | 2.28 – 2.33 |
| 20°C (68°F) | 2.43 – 2.53 | 2.26 – 2.31 |
| 25°C (77°F) | 2.40 – 2.50 | 2.25 – 2.30 |
| 30°C (86°F) | 2.37 – 2.47 | 2.24 – 2.29 |
| 40°C (104°F) | 2.31 – 2.41 | 2.22 – 2.27 |
| 50°C (122°F) | 2.25 – 2.35 | 2.20 – 2.25 |
Supplemental Charging of KAHN SLA Batteries
All batteries are subject to loss of capacity due to self-discharge. It is recommended that batteries that have been stored for a long period of time be “recharged” before they are put back into service.
To effectively charge batteries that have been stored for more than 12 months, ensure that each battery has an open voltage of more than 2.0 volts. Always check the open voltage before beginning charging.
Battery Charging Efficiency
The charging efficiency (η) of a battery can be expressed by the following formula:
The charging efficiency depends on the battery’s state of charge, temperature, and charging rate. The following chart shows that a Keheng sealed lead-acid battery can exhibit high charging efficiency even at a low charging rate.
It is critical to provide the proper current and voltage for the battery being charged so that the charger matches it. For example, using a 24V charger to charge a 12V battery is not recommended.
If you have any questions about the compatibility of your existing charger with our products, we will be happy to assist you with charging.