Superior Technology Integration Knowledge
The best method to recharge a lead-acid battery is a multi-stage (typically three-stage) charging process. Regardless of the charging source—grid (AC) connection, solar panel, or even an automotive alternator—this method takes three parameters (current, voltage, and time) and sequentially applies each one at specific rates and durations.
Three Stages in Multi-stage Charging
During bulk charging, the charging device applies a constant current charge, raising the cell voltage to a preset level. The charge rate depends on the difference between the charging voltage and the battery voltage. This first stage of charging keeps the voltage difference between the charger and battery constant so that as the battery voltage rises, the charging voltage rises and the charging current remains constant.
This stage typically takes the battery to about three-quarters charged, and at a rate that usually does not exceed 25% of the battery’s amp hour capacity.
In the acceptance (or absorption) stage, the charge current is gradually reduced as the cell becomes saturated. Charging current is reduced to about half of the bulk charge rate. Since battery charging is an electrochemical process that has a very long reaction time (particularly in the case of deep-cycle batteries), the charging voltage in this stage is reduced for a few hours to enable the charge that is now concentrated in and around the plates to become homogeneously distributed through the entire battery.
The acceptance stage is typically maintained for several hours.
The final stage is the float charge, which compensates for the self-discharge of the battery. Following the acceptance stage, the charging voltage drops further to a level that eventually counterbalances the battery’s internal losses. Depending on ambient temperature and battery type , float charge will usually be between 13.2 V and 13.3 V (VRLA batteries) or 13.6 V to 13.8 V (flooded-cell batteries).
Batteries can usually be left in float stage indefinitely. During the float stage, a battery will typically be 95 to 97% charged.
Battery Charging Devices
Aging affects each battery cell differently. Since the cells are connected in series, it is virtually impossible to control the individual cell voltages during charging. Even if the correct overall voltage is applied, a weak cell will generate its own voltage level and in turn affect the battery as a whole. To slow the aging process, lead-acid batteries must always be stored in a charged state.
Most garage and consumer (automotive) battery chargers are bulk charge only and have little, if any, voltage regulation. They are fine for a quick boost to low batteries but should not be left connected for long periods of time. Damage to the battery can occur when a battery is left connected to this type of charger for extended amounts of time.
A voltage-regulated charge control is designed to supply constantly regulated voltage to batteries. If these are set to the correct voltages for the batteries, they will keep the batteries charged without damage. These are sometimes called taper chargers.
These are regulators that go between a charging source, such as solar panels, and the storage batteries. Solar charge controllers are designed to keep the batteries charged to peak voltage without overcharging.
Parameters for Recharging
When recharging a lead-acid battery, you need to consider the battery’s voltage, maximum current, and the temperature of the battery’s environment.
A lead-acid battery is charged by applying a voltage across its positive and negative terminals that is higher than the voltage it already has across them. The greater the difference between the applied voltage and the battery voltage, the greater the charging current that will flow into the battery and the quicker the battery will be charged.
All batteries have a maximum current at which they can be charged safely. High charging current means less time is necessary to complete the recharging process. However, charging at the maximum allowable, or higher current, can also shorten battery life. Cases of extreme overcurrent could result in a hazardous condition due to battery overheating and thermal runaway.
To improve charge performance of lead-acid batteries at colder temperatures and to avoid thermal runaway during hot spells, controlling the charging voltage is important. Implementing temperature-compensated charging can prolong battery life by up to 15%. Higher temperatures require slightly lower charging voltage, while lower temperatures require slightly higher charging voltage. Three-stage chargers that have a battery temperature sensor are excellent for batteries that are exposed to large fluctuations in temperature.
Battery testing can be done in several ways, and the method often is chosen according to the type of battery and the tools available. The most popular methods include:
- Measurement of battery voltage
- Measuring the specific gravity of the electrolyte
- Load testing
Different batteries, and especially different types of batteries, age at different rates. Like the links of a chain, the battery with the lowest capacity will determine the runtime. Do not use different types of batteries together in a bank.