Lead Acid Battery Charging Basics Reviewed
As discussed in How a Lead Acid Cell Works, a fully charged lead acid battery has a negative electrode made of lead, a positive electrode made of lead dioxide, and an electrolyte that is made of sulfuric acid and water.
During the discharge cycle (when the battery is being used), the chemical composition of the battery changes until it becomes completely discharged.
A completely discharged battery is comprised of two electrodes that have been transformed into lead sulfate and an electrolyte that is almost 100 percent water.
To charge a discharged battery, a voltage greater than the battery voltage is applied to the battery terminals.
During the course of charge cycle, the chemical reaction that occurred during the discharge is being reversed. Charging and thus the chemical reaction continues until the battery becomes 100 percent charged.
Charging and Internal Resistance
The internal resistance of a battery does become greater as a battery becomes discharged. However, this type of increased resistance refers to an opposition to battery discharge current.
During the charge cycle however, it is important to consider another kind of internal resistance. Specifically the resistance to a charge current. This resistance increases as the battery becomes more charged.
As the battery becomes more charged, the voltage produced by the individual cells increases and opposes the charge current. This behavior is frequently referred to as “series opposing” and does represent a resistance to current flow coming from the battery charger.
Three stage battery chargers use this phenomenon to determine which stage of charging to be applied to a battery during the charge cycle.
These three stages of charging are known as bulk, absorption and float charging.
Bulk Charge Values
The bulk charge current and voltage values will vary from charger to charger. The type of charger selected will be driven by the capacity and type of battery being charged. For example, what is required for a Gel battery can differ significantly from an AGM battery.
The Bulk Charge
The first stage in a three stage charge cycle is known as the bulk charge. In this stage, the battery charger will rapidly return the battery to an 80 to 90 percent state of charge.
It accomplishes this by maintaining a constant relatively high current. The current is held constant against the rising internal resistance to charge current by raising the battery voltage.
Consider Ohms Law where Voltage = I (current in Amps) x R (resistance in Ohms).
A quick survey of this equation shows that if we want to maintain a constant current in a circuit with rising resistance, we must raise the voltage.
The bulk charge continues until the voltage output by the charger reaches a specific level. At that point, it switches to the absorption charge.
Absorption Charge Values
Like the bulk charge stage, the absorption charge current and voltage values will vary from charger to charger.
The Absorption Charge
In this stage, the voltage applied to the battery by the charger is held at a constant level. As the charger holds this level, it is also monitoring the current being supplied.
Remember as the battery becomes increasing charged, it’s opposition or resistance to a charge current increases. This will cause the current flow to tail off.
Consider Ohms Law once again where Voltage = I (current in amps) x R (resistance in ohms)
If voltage is held constant and resistance increases, current must decrease.
During the absorption charge, the three stage charger monitors the falling current until a specified point is reached that indicates that the battery is about ninety eight percent charged.
When this specified level of current is reached, the three stage battery charged will switch to the float charge stage.
The Float Charge
During the float charge, the voltage is dropped to a level lower than what was applied during the absorption charge. The float charge serves two purposes…
- First, it brings the battery from a 98 percent state of charge to a 100 percent state of charge.
- Second, it maintains the battery in a 100 percent state of charge condition.
It is this second stage that deserves some discussion as there are a couple ways of maintaining the battery at 100 percent state of charge.
The first is to simple apply a voltage that is theoretically ideal to the type of battery being charged. The idea is that this voltage is low enough to keep the electrolyte from boiling off, yet high enough to counteract the phenomenon known as self discharge.The problem with this is that it does not account for the affect that temperature has on batteries, nor does it account for those small differences caused by battery age and construction.
The second is to actually monitor the state of charge and apply voltage when its needed. Indeed, some of the smarter chargers use temperature sensors. Others monitor the voltage of the cells and send a short pulse of charge using a technique called and a pulse width modulation.