Battery chargers: Top 5 questions for design, discovery and selection

By Kevin J. Harrigan, April 2025

Battery chargers restore the electrochemical potential in a battery cell or system by applying direct current to the battery cathode. This initiates the flow of ions across the battery electrolyte and returns useful electrical capacity to the battery, which can be stored and deployed on-demand by an electrical device.

A battery charger is essentially an AC-DC converter. The DC conversion is accomplished by a rectifier after a transformer, which provides isolation between supply and battery. In smart chargers, additional circuitry provides feedback to a control board, to ensure output voltage and current matches the charging needs of the battery.

Battery charging is only for rechargeable (secondary) batteries, which use an electrolyte that supports ion reversal. This includes many lead acid, lithium-based and nickel-based battery chemistries, as well as emerging types such as solid state, flow and quantum batteries.

Battery chargers are truly innumerable; designs are often discrete to charging specifications, electrical performance, chemistry type, application, interconnects and local grid service, among other factors.  

1. What are the electrical specifications of the battery to be charged?

Battery voltage and amperage will be determined by the required electrical potential and amp-hours of the device or equipment to be powered by a battery. A charger must mirror, or adapt, to the battery’s specifications.

Voltage is a measure of the electromotive force of the ions within the battery; it is the strength of the electrical current and measured in Volts (V). Battery voltage drops as it is discharged. To restore potential in a battery, charger voltage must match the nominal voltage of the battery. Some chemistries, like lead-acid, may tolerate a slightly higher voltage, although this is inadvisable for most battery types as it will damage the battery.

Current measures the flow rate of electrons in the circuit; it is measured in amps (A). The maximum current that can be discharged or accepted by the battery is known as its C-rating. A higher C-rating means a faster charge acceptance rate, which is an important parameter for high-demand applications. C-rating is typically dictated by the battery’s thermal management performance and electrolyte surface area.

This is especially important for EVs. A “fast charger” will deliver more current in a short period than a standard charger; a “rapid charger” supplies current at or near the C-rating.

Chargers often have a current limit. This limit is a range that is sometimes configurable by the user. Typically, chargers deliver somewhere between 100-120% of the rated current, which doesn’t affect most chemistries as the battery cannot charge faster than its C-rating, provided the battery is otherwise stable. This can help power constant loads on the charging circuit, compensate for recharge factors (see Question 4, below) or mitigate current losses from derating or supply disruptions. This is also helpful in the event the charger output exceeds the AC supply’s amperage, if the user can derate current to prevent tripping circuit protection devices.

Amp hours (Ah) is a measure of how long equipment will operate based on the available electrical potential in a battery. This may be represented as a percentage in a state of charge measurement. Ultimately, the charger only needs to replace the amp hours lost from discharge. This also can be expressed in a percentage as the depth of discharge.

Watt hours is similar to amp hours, although it is more energy-use oriented; it expresses how much energy is consumed within a period. Amp hours measures how long equipment will run, which is often more practical.

Additionally, the main electrical service current frequency must be considered. Most consumer, commercial and industrial battery chargers will rectify from common AC frequencies. Direct DC recharging typically only occurs at renewable generation sites, which often store power in adjacent large-scale batteries.

2. What is the charge profile of the battery to be charged?

Charge profile is important because batteries have different charge profiles based on the chemistry and circuit construction. A charger needs to apply a specific voltage and current, at different phases of the charging cycle, to safely restore the battery’s electrical charge. Voltage and current values fluctuate based on the battery’s type, circuit resistance, number of cells, temperature and more.

Common stages of battery charging

• Precharge or conditioning: This slowly applies current to battery, to bring the battery’s voltage to a level where it can accept higher current values, without overheating or stressing the battery’s circuitry.
• Constant voltage or absorption stage: Phase where the charger can apply the rated voltage to battery for a duration.
• Constant current or bulk stage: The charger applies a consistent current until the battery’s voltage level has risen, when current begins to drop.
• Termination or absorption: Current is reduced once the charger senses the battery is near capacity.
• Trickle or float charge: A maintenance current charge for batteries that are prone to self-discharge when left for long periods, such as lead-acid.
• Equalizing: Applies current to the battery after bulk and float charges to raise battery voltage for a short period. This can help reduce electrolyte crystallization and acid stratification in lead-acid types.

Examples of charge profiles

Stage durations and values are individual to battery type and manufacture. For example, lead-acid and lithium batteries require a precharge, before transitioning to a constant current and constant voltage phases. Nickel-based batteries often begin with a low conditioning current before sharply increasing it once the battery’s voltage climbs to a permissible level; however, current remains constant throughout the charging cycle.

3. What system integrations are needed for the battery chemistry, application or user-friendliness?

Most battery chemistries will be damaged due to overvoltage and overtemperature conditions. Generally, chargers are engineered to restore the electrochemical potential in one type of battery chemistry.

With some exception, such as for lead-acid and nickel-cadmium types, nearly all batteries benefit from a battery management system, which continuously monitors battery cell voltage, current, temperature and cell balancing, and will curtail or shut-off recharging if it senses out-of-spec values. BMSs are essentially embedded systems that maximize battery health and longevity, as well as charging safety.

BMSs are also present on equipment where the energy storage technology is integral, such as on an EV, although the BMS is part of a larger electrical architecture.

User features

• Fast or rapid charger: Returns the battery charge at a hastened rate, albeit with higher inefficiency and increased risk for battery damage.
• Interface: An analog, digital or screen instrument communicates charger and battery state to user.
• Inductive charging: Inductive charging returns charge to battery by wirelessly conducting current through a compact charge transmitter antenna and a receiver integrated into the device or battery.
• Alarm/alert: The charger notifies users based on operations or measurements. It is connected to users and equipment by SCADA, internet or other communication protocols.
• Polarity protection: The charger registers when the terminal connections have been errantly swapped – and fails to deliver a recharging current to protect the battery and user.
• Abnormal/fault: Detection of a battery or circuit defect prevents the charger from delivering current.
• Timed charge: The charger delivers current for a predetermined interval.
• Desulfation: A charger setting for lead acid batteries that reduces sulfate crystallization. Conducted during equalizing charge phases.
• Smart charger: These chargers can measure battery state-of-charge, depth-of-discharge and other parameters, and automatically apply the correct voltage and current ratings per the battery charge profile. Smart chargers do not necessarily communicate with other equipment.
• Ground bus: The charger can be ground through a busbar connection.
• Blocking diode: Prevents reverse current from sending battery power to the supply, which is important in renewable applications.

4. What is the charger’s electrical efficiency and thermal management requirements?

All types of battery chargers experience efficiency loss, which can be due to myriad factors – resistance, noise or losses at the connectors, transformers, switches or rectifier are common. Depending on the type of battery charger and applicable standards, energy efficiency rates of 70-95% are common. Each battery is subject to a recharge factor, which requires a certain additional percentage of current, typically 10-40% additional, above the battery’s nominal current output. This is typically factored in by the charger’s current limit.

Generally, regulations and standards have tightened to reduce the acceptable efficiency losses for all electrical systems, including chargers. Regulator marketing programs, such as Energy Star, may designate chargers meeting high-efficiency criteria. Battery charger efficiency can be increased through design techniques, such as:

• Using high-quality, high-efficiency power electronics, such as converters.
• Using PID feedback loops to inform the system about the precise voltage and current being applied to the battery.
• Reducing sources of noise by designing PCBs to keep traces short, smooth and straight.
• Minimizing interference potential, such as from high and low voltage components, with shielding materials.

Regardless, all battery chargers will need to expel some wasted electrical energy as heat. This heat is routed away from electronics via dielectric thermal interface materials, where it is conveyed to cooling technologies, likely a heat sink.

Battery chargers often must be derated at high ambient temperatures and high elevations, as the ability to exhaust heat decreases, performance in filtering capacitors degrades and circuit breakers or fuses might be more prone to trip. Due to the higher currents used, fast charging equipment often creates additional heat.

Active cooling technologies, such as fans or coolants, are sometimes used in challenging applications, although for applications between 50° - 70° C current derating is usually sufficient, provided the longer recharge times are acceptable.

5. What are the relevant standards and regulations for the battery system and application at hand?

Due to the innumerable applications for battery systems and the safety risk for any electrical system, battery chargers must meet stringent regulatory and industry standards – although those standards bodies, scope and authority vary highly.

For example, EV chargers are outdoor, high-voltage electrical equipment in an automotive environment – challenging climate, exposure to water, oils and solvents, and severe wear and tear requires an electrical system with high safety and durability criteria.

In one example, North American OEMs have settled on the SAE J1772 charger plug for standard, single-phase AC charging; the SAE J3068 connect can be used for three-phase AV or DC recharging. 

Meanwhile, most power tools operate on one of just a few different voltages - such as 18 V, 20 V, 40 V – yet include discrete interconnects and battery docks intended to keep buyers within the brand’s product ecosystem. It was this sprawling product strategy that forced regulators in the early 2020s to standardize the USB-C connector for many consumer electronics.

Common parameters of battery charger standardization include:

• Interconnects: Terminal types for mating the battery with the device and charger’s electrical circuit, which are often distinct per application or industry.
• Safety: Including how the charger addresses overvoltage, overcurrent, high temperature and short circuits, as well as defective batteries.
• Power specifications, such as:
  • Current, input type and output amperage
  • Voltage, including input and output
• Chemistry: Compatible battery chemistry for the charger or application.
• Operating temperature: Permissible environmental temperature, which often factors in temperature rise when recharging batteries.
• EMI/EMC: Shielding to mitigate the effects of EMI or EMC on or created by the charger.
• Construction materials: Including the enclosure or pack; circuitry and component materials; thermal interface materials, and potentially much more.
• Design criteria: Specialty engineering for demanding or critical applications, such as for spacecraft, which will have additional limits on chemistries, dimensions, power and size of batteries destined for low-Earth orbit. Or EVs, which will have specific advisories on safety, due to the proximity of passengers to potential thermal events.
• Test and measurement: Designated practices to assess battery and charger system operation and performance.
• Lightning protection: Arrestors, groundings or surge protection devices might be required for chargers exposed to outdoor environments, such as EV chargers.
• Enclosure: Use of conformal coatings, seals, gaskets, glyptol solutions and other design techniques prevents contaminants, such as dirt, dust or debris, cannot intrude on the device’s operating components. IP ratings have become near universal and are central to IEC enclosure ratings. NEMA-style cabinets have similar specifications and equivalents. For example a NEMA 1 is a basic industrial-style cabinet for indoor locations; an IEC IP00 would be its counterpart.

Some notable standards for common battery charging systems include:

• UL 1564: Standard for Industrial Battery Chargers
• IEC 63370: Lithium-ion batteries and charging systems
• SAE ARP 1816D: Charger for battery-powered ground support equipment (aka chargers)
• IEC 62880: USB battery charging specification
• IEEE 446: Emergency and standby power systems
• IEEE 946: Design of stationary DC power systems
• NEMA PE 5: Utility-type battery chargers

Additional battery charger information

Notable formulas and equations

How to calculate battery charging time

Where,

H = Hours

BAH = Battery amp hours

CA = charge amperage

How to calculate charger amperage

Where,

AHr = amp hours removed

Cf = battery charge factor; consult manufacturer for specific charge factor values. Common values include: lead acid, 1.1 or 1.15; or NiCd: 1.3, 1.35 or 1.4

T = Recharge time, provided time is within nominal charge profile

CLA = constant load amperage; static current needs of equipment running on battery power while recharging

How to calculate amp hours removed

Where,

BT = battery time; in hours, time of battery use

Key battery charger circuit components

• Input/output: Connection to power supply
• Circuit protection: Such as a breaker or fuse
• Transformers: Which isolates the battery from the supply and steps down voltage
• Rectifier: Converts current to DC
• Filter: such as capacitors to reduce noise
• Test circuit: monitors voltage and current levels from the rectifier
• Control: Such as a discrete IC or microcontroller to administer voltage and current

Types of battery chargers

The following is an incomplete list of common battery charging apparatuses.

Automotive/marine

Chargers that recharge 12 V batteries that are common in automotive, small watercraft and personal vehicles. Batteries are typically lead acid and are designed to deliver either high peak current to initiate a starter solenoid or power simple appliances.

Aviation/aerospace charger

Large aircraft are typically connected to airport power systems through a ground power unit. This will recharge the aircraft’s batteries to initiate engine or auxiliary power units. Personal and small aircraft may connect their 24 V battery systems to a dedicated charger type

EV or hybrid charger

Equipment dedicated to the recharging of battery storage systems in electric or partially electric vehicles, which are usually lithium or sodium-ion.

Consumer

Charger is compatible with common alkaline battery sizes, such as AA, AAA, C or D cells. Or might repower larger li-ion 18650 or similar batteries.

Cabinet charger

Common in factories or facilities where equipment is connected to a stationary battery, which can provide intermittent power in the event of disruption to a utility or facility power supply.

Fast charger

Delivers the higher current that a standard charger, which hastens recharge time, but mildy stresses battery circuits, materials and thermal management. Some fast chargers can deliver up to 100% of recharge in 90 minutes, with outputs between 7-22 kW.

Medical device charger

Portable oxygen concentrators, as well as some specialize mobility equipment, commonly run on li-ion batteries. Rechargeable hearing aids have also come a long was in recent years.

Personal electronics

USB-C is slowly becoming the ubiquitous interconnect for recharging the lithium-ion batteries found in cellular telephones, personal and tablet computers, portable speakers and other common devices. Micro-USB

Solar charger

Through photovoltaics, a panel converts photons into electrons, which are then regulated through a solar charger controller, to prevent overcharging, overheating and reverse charge flow.

Power bank

This is a battery that essentially recharges another battery, typically one integrated into a device. Through a mating interconnect and simple power circuitry, this recharges personal devices on the go.

Rapid charger

Delivers maximum recharging current (or C-rating) to the battery, in some cases up to 150 kW or more, which can recharge many EV batteries up to 80% in as little as 30 minutes. The higher current stresses battery circuits, materials and thermal management and reduces battery life. Many rapid chargers limit battery charge to 80% as a result.

Standard charger

Typically refers to chargers or current adapters that plug directly into a wall, with a single wire insulation and two conduits, as well as connector. Also called a slow charger. For an EV, outputs of 3-6 kW are common.

Technological trends and future applications

Battery charging is a fundamental growth technology in 2020s and 2030s. Increasingly many countries, regulators and standards organizations are prioritizing or incentivizing electrified transportation methods, including last-mile solutions like e-bikes and e-scooters. Additionally, batteries are an important means to store electricity stored by renewable assets for high-demand or low-supply periods. And they remain the core technology behind endless personal electronics.

For these reasons, there is ample research into batteries – and charger technologies that conveniently and safely recharge them. This is also guiding engineering advances in battery capacity, chemistries and service life. Solid-state batteries are one of the most likely technologies to displace lithium in transportation applications and have at times been classified as a ‘forever battery’ due to its relatively long service life. Vanadium redox technologies – also known as flow batteries – may become essential for renewable energy storage, although that market is largely held by lithium technologies, which is becoming an increasingly attractive means to repurposing older cells. Quantum batteries leverage the quantum principles of superposition and entanglement to vastly increase energy density and recharge rates, as potentially all ions in the battery are simultaneously addressable. 

This is also requiring a reconsideration of how battery chargers work. No longer are they one-way conductance systems, only for re-energizing depleted cells. Batteries are increasingly seen as new layer of grid infrastructure that can help stabilize a residence’s power supply, or potentially a neighborhood’s.

All of these are stoking additional changes in battery charger engineering, as well as the power electronics, electrical components and materials within electrical systems, which are adapting quickly, with high modularity, to meet an evolving market and demand.

Resources and further reading

The following webpages or reference materials were consulted during the construction of this webpage.

• S. Department of Energy: DOE Explains Batteries
• S. Department of Energy: Test Procedure for Battery Chargers
• GlobalSpec: How to design wireless charging systems for safe, reliable operation
• Journal of Energy Storage: Battery management strategies: An essential review…
• MIT School of Engineering: Ask an engineer – How does a battery work?
• Hindle Power: Hindle Institute: All About Stationary Batteries and Battery Chargers
• GlobalSpec: Will induction charging solve EV range anxiety?
• GlobalSpec: Fundamentals of BMS selection
• Battery University: Charge Methods

Explore more about batteries on GlobalSpec and Electronics360.