Battery charge controllers regulate charge rate and cut off charge as necessary
Well-designed charge control circuitry helps ensure a long, safe life for a battery pack. As we discussed in our previous article on return on investment[PD1] , you can’t measure the value of a battery pack based solely on initial cost, capacity, efficiency, or any other single parameter. Rather, you need to consider how much energy a battery can deliver over its whole lifetime.
Many elements of battery pack design help extend battery cycle life and ensure safe operation. We’ve highlighted many of them in our previous articles, including cell chemistry, cell count and arrangement, thermal management, and battery management.
In this article, we’re highlighting the importance of charge control. Whether part of a BMS or as a stand-alone component, charge control protects a battery from unsafe conditions during charge and ensures that the charge cycle happens as efficiently as possible. Some charge control functions include:
- Preventing overcharge
- Controlling the rate of charge
- Keeping battery capacity topped off as needed
First, we’ll dive in to the importance of charge control for battery health and safety. Then, we’ll look basics of charge control, including hardware and circuitry. Finally, we’ll look at different charge algorithms and other charge control functions and how they help protect battery health and safety.
Let Aved’s expert team design, test, and manufacture your custom battery packs. We’ll ensure that you battery’s charge control circuitry is calibrated to provide the longest useful life. Contact us to discuss you project, or request a quote directly.
The importance of charge control—ensuring long battery life, preventing dangerous conditions
Charge control is essential for both the long-term health of a battery and the immediate safety of users. Rapid charge, overcharge, and charging at elevated temperatures all create problems for battery performance. Charge control circuitry prevents batteries from entering these danger zones.
The dangers of overcharge
Every battery has a nominal maximum capacity. In real life, an artificial maximum is often set with a charge controller because charging to real max capacity can cause harmful chemical reactions to occur at the electrodes, truncating battery cycle life.
Besides the harm to battery cycle life (and thus long-term return on investment), overcharge can lead to temperature and pressure buildup that poses a threat to users. Lithium-ion batteries are especially susceptible to fiery failure in the event of overcharge. Cell phones and laptops don’t burst into flame despite being connected to chargers overnight because they have built-in charge control.
The dangers of rapid charge
Similar to overcharge, rapidly charging a battery is harmful in several ways. Charging a battery rapidly can reduce the capacity can hold during the charge cycle. Additionally, repeated rapid charging can shorten the cycle life by promoting undesirable chemical reactions. In lithium-ion batteries, this take the form of the lithium plating, the formation of metallic lithium on the surface of the anode. Fast charging is also harmful to nickel cadmium and nickel metal-hydride chemistries. A charge controller that limits charge to an appropriate rate reduce unwanted chemical reactions like lithium plating.
Moreover, rapid charge also increases temperature. This is a hazard on its own (potentially leading to thermal runaway). Charging at high temperatures also shortens cycle life.
Charge control basics—switching and voltage regulation
Charge controllers are circuits that manage the input of charge into a battery pack. Frequently, battery management systems (BMS) provide charge control services alongside other BMS functions in one integrated circuit board. But sometimes, stand-alone charge control devices are made for direct sale to consumers. This is most common in the residential solar power industry and for power storage in RVs and boats.
Depending on the application, different hardware and circuitry choices can be more advantageous than others. Let’s take a look at how switches, voltage regulators, and other components can be arranged in a circuit to achieve different goals.
Transistors provide on/off switching
A core function of a charge controller is to connect and disconnect the battery from the charger. Thus, the central component of a charge controller is a switch. The switch can be a mechanical relay, but transistors like field-effect transistors (FETs) are more common.
The logic of a FET is simple. There are three terminals: a source, a drain, and a gate. The source and drain terminals for the main channel for current flow. A FET checks the voltage across the source and drain (the main channel) against the voltage across the gate and source. Depending on how these values compare to one another and to the FET’s fixed threshold voltage, the FET is either “on” or “off.”
Thus, a FET can be used to connect and disconnect a battery from the charger based on the gate voltage. The gate voltage depends on separate output, such as from a controller, which will in turn depend some cutoff condition determined by the BMS.
Charge cutoff conditions
Cutoff conditions can be based on battery voltage sensing, temperature sensing, current sensing, time, a combination of these factors, and more. The method of determining when to end charge will depend on the battery chemistry. Voltage sensing, for instance, is not appropriate for lithium-ion chemistries who have exceptionally flat charge/discharge voltage curves.
Additionally, the threshold voltage needs to function properly with whatever controller is used. For instance, logic voltage from microcontrollers tops out at about 5V. If this isn’t enough to close the transistor, a FET with a lower threshold voltage will be needed.
Transistors also provide voltage regulation
Another critical function of a charge controller is to regulate voltage. Voltage can be regulated even in conditions of varying current by components that provide variable and toggleable resistance. This can be either a shunt regulator or (more commonly) a transistor.
Circuitry layout for simultaneous charge and discharge
Finally, FETs for charge control can be arranged to allow for simultaneous charge and discharge. Such functionality is found in applications like uninterruptible power supplies and laptops. To enable simultaneous charge/discharge, charge/discharge FETs must be arranged in parallel.
Charge control algorithms—how you charge matters
In order to deliver charge at the appropriate voltage and current, charge control circuitry uses different charging algorithms. Depending on how the charge controller is constructed, charging can either single-stage (having only one setting for delivering charge) or multi-stage.
Besides being appropriate for some applications more than others, different charge control algorithms are also better for certain battery chemistries than others.
The simplest charge algorithm is a single-stage on/off algorithm. In this kind of charging, the battery receives power at a constant voltage from the charger until cutoff conditions are reached. Then, the battery is disconnected from the charge.
Instead of breaking the circuit, another single-stage charge control method is to short the circuit. In some renewable energy storage applications, a load dump is used. Rather than opening the circuit, the circuit is shorted to pass through a shunt, wasting the charge.
Switch-mode power supplies for voltage regulation
Switch-mode power supplies use the on/off mechanism of a single-stage switch to achieve the voltage control of a series regulator. This is accomplished with pulse-width modulation. Instead of constantly supplying the battery with current, the current is rapidly switched on and off. “Pulse width” refers to the length of time the power is switched on in one cycle.
By modulating the width of the pulse, the average amount of current and voltage received by the battery. In other words, the charge is rapidly turned on and off, and the exact frequency of the pulsing determines the average voltage and current the battery receives.
Slower charge rates and trickle charging
Rapid charging is not always the best way to charge. In fact, slow rates of charge are especially useful for two purposes: fully charging a battery and maintaining a full charge.
First, it’s dangerous to overcharge a battery, but valuable to charge the maximum acceptable state of charge. In the early stages of a charge cycle, the battery receives power relatively quickly and at a high voltage. This is sometimes called “bulk charge.” But when the battery approaches full charge, bulk charge is undesirable. At this time, the charging voltage can be decreased to allow the battery to charge more slowly and reach the target SOC.
Second, a slow-charging algorithm called trickle charge helps maintain a battery a full charge when it has to be left on the charger. Once a battery is fully charged, the charge controller should automatically cut off the charge. But if the battery is left on the charger for long enough, self-discharge will sap the capacity. Trickle charging solves this problem by matching rate of charge to the rate of self-discharge to keep the battery topped off.
Trickle charging can be advantageous, but only for certain battery chemistries. In lithium-ion chemistries, for instance, trickle charge is unsafe for battery health and battery users.
Float charging is very similar to trickle charging, but refers to a situation where the load and the battery are in parallel. The battery is trickle-charged while the power source supplies power directly to the load. In the event that the power source is disconnected, the battery provides immediate power to the load. Float charging is used in uninterruptible power supplies.
We’ve discussed several different charge methods here, and each one has its uses. But no single method works for all situations. This is why many charge controllers use multi-stage charging for optimum performance.
Multi-stage charging switches between different rates and methods of charge based on state of charge. For instance, a multi-stage charge algorithm might have three steps:
- Bulk charge when SOC is low
- Pulse-width modification or slower charge to aid the final absorption of max charge
- Trickle charge for maintenance
Battery charge control ensures ideal charging conditions, protects user safety, and prolongs battery life
The conditions under which a battery is charged play a major role in battery life and safety. Charge control, whether provided by battery management circuitry or a standalone component, controls the rate and voltage of charge and cuts off charge when the battery is fully charged and when conditions become unsafe.
Battery design involves many interrelated choices. Major initial choices like battery chemistry and cell arrangement determine core things like voltage and capacity. But peripheral circuitry and devices provide services that ensure the battery works as intended for a long and safe working life. Thermal management is one example of this. Battery management systems and charge control are other examples.
For batteries that perfectly meet your operation’s or your customers’ needs, turn to the design experts at Aved. We’ll match the charge control settings to the chemistry and application and provide your batteries with appropriate thermal management and battery management systems. Contact us for more information or request a quote to get started on your project.