Battery Charger Symbol
Definition: The Battery Charger symbol represents an AC-to-DC converter that rectifies mains power and regulates it through charge stages (bulk, absorption, float, and optionally equalize) to charge and maintain a battery, drawn as a rectangle with AC input terminals (AC L, AC N) on one side and DC output terminals (DC+, DC−) on the other, marked with a rectifier or battery symbol.
Also known as: charger, float charger, battery charger-rectifier, smart charger, multi-stage charger, trickle charger, station battery charger, charger-eliminator.
What the Battery Charger symbol means
The Battery Charger symbol denotes the power-conversion block that keeps a battery system charged from the AC supply. On the diagram it is the boundary between the AC world (fused line and neutral, typically 120/230 V AC) and the DC battery bus (12, 24, 48, or 110/125 V DC). Unlike a plain power supply, a charger's output behaviour is battery-aware: it regulates current and voltage according to the battery's state of charge rather than holding one fixed voltage regardless of load.
Modern chargers implement a multi-stage algorithm. Bulk stage: constant current at the charger's rated output until the battery reaches the absorption voltage. Absorption stage: constant voltage (e.g. 14.4–14.7 V for a 12 V flooded lead-acid battery, 14.2–14.4 V for AGM) while current tapers as the battery fills. Float stage: a reduced maintenance voltage (13.2–13.8 V for 12 V lead-acid) that keeps the battery full indefinitely without gassing. Some chargers add a periodic equalize stage — a controlled overcharge around 15.3–15.8 V (12 V flooded only) that stirs the electrolyte and balances cell voltages. In standby-power schematics the charger, battery, and load all hang on the same DC bus so the load transfers to battery seamlessly on AC failure.
How to identify the Battery Charger symbol
The symbol is a rectangle with the AC terminals (marked ~ or 'AC L / AC N') entering one side and DC terminals (marked +/− or a battery cell symbol) leaving the other, usually with a rectifier mark inside — a diode symbol, or the IEC AC/DC converter notation of a tilde over a solid-plus-dashed line separated by a diagonal. A small battery symbol at the output is the giveaway that distinguishes a charger from a generic power supply block.
IEC 60617 practice composes the converter square with ~ on the input side and ⌗ (straight/dashed DC lines) on the output side; ANSI/IEEE one-line diagrams often just label a rectangle 'BATTERY CHARGER' or 'CHGR' with kW/A ratings, particularly in substation DC-system drawings. Polarity marking on DC+ and DC− is essential and always shown — reversed connection is the most common installation fault.
Function in a circuit
Internally, the charger rectifies the AC input (transformer plus bridge rectifier in ferroresonant and SCR designs; high-frequency switch-mode conversion in modern units), filters it, and regulates the output under microprocessor control. The controller measures battery voltage (and often temperature, via a sensor lead) and steps through the charge stages: full rated current in bulk, voltage-clamped taper in absorption, then drop to float. Temperature compensation shifts the voltage set points by roughly −3 to −5 mV/°C per cell to prevent overcharging hot batteries and undercharging cold ones.
In engineered DC systems (substations, switchgear tripping supplies, telecom plants, UPS rectifiers) the charger is sized to carry the continuous DC load plus recharge the battery within a specified time — the classic sizing rule is charger amps ≥ continuous load + (amp-hours discharged × 1.1 to 1.25) ÷ recharge hours. Alarms for AC fail, low DC volts, high DC volts, and earth fault are standard on industrial chargers and are often shown as contact outputs on the schematic.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 provides the AC/DC converter symbol used for chargers (square with ~ input and DC output notation). Product safety and performance fall under IEC 60335-2-29 (battery chargers, household/similar) and IEC 62477 / EN 50272 series (stationary battery installations, ventilation and safety). |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE one-line practice labels the charger as a converter block ('CHGR'); IEEE 946 gives recommended practice for DC auxiliary power systems (charger sizing and configuration), IEEE 485 covers lead-acid battery sizing, and UL 1236 governs battery charger product safety in North America. |
| Key difference | IEC drawings use the graphical converter symbol with ~ and DC-line marks; North American one-lines lean on a labelled rectangle with ratings. Engineering practice differs mainly in sizing references (IEEE 946/485 versus IEC/EN 50272 and manufacturer rules), while charge-stage terminology — bulk, absorption, float, equalize — is universal across both. |
Terminals / pins
| Pin | Name |
|---|---|
| ac_l | AC L |
| ac_n | AC N |
| dc_pos | DC+ |
| dc_neg | DC- |
Typical values
Common system voltages: 12 V, 24 V, and 48 V for vehicles, boats, solar, and telecom; 110/125 V DC for substation and switchgear batteries. For a 12 V lead-acid battery the standard set points are: absorption 14.4–14.7 V (flooded) or 14.2–14.4 V (AGM/gel 14.1–14.4 V), float 13.2–13.8 V, equalize 15.3–15.8 V (flooded only); double these for 24 V, quadruple for 48 V. Charger output current is typically sized at 10–25% of battery amp-hour capacity (C/10 to C/4) — e.g. a 20 A charger for a 100–200 Ah bank. Lithium (LiFePO4) profiles use ~14.2–14.6 V absorption with no equalize and often no float. Temperature compensation is about −3 to −5 mV/°C/cell. Input is 120 or 230 V AC, 50/60 Hz, with efficiencies of 85–94% for switch-mode designs.
Where the Battery Charger symbol is used
- Substation and switchgear DC systems, where a 110/125 V DC station battery and charger supply breaker tripping and closing power (IEEE 946 practice)
- Fire alarm, security, and access-control panels maintaining 24 V standby batteries on float
- Telecom and data installations floating 48 V battery plants that carry the load through AC outages
- Marine and RV house-battery systems, often as a combined inverter/charger with automatic transfer
- Forklift and golf-cart charging bays delivering high-current bulk charges to 24/36/48 V traction batteries
- Generator sets keeping the 12/24 V engine-start battery float-charged so standby gensets crank on demand
Example
In a standby-power diagram, the Battery Charger symbol's AC L and AC N pins are fed from a fused 230 V AC circuit, while DC+ and DC− connect to a 24 V, 100 Ah AGM battery bank and the DC load bus in parallel. The 20 A charger (C/5 rate) bulk-charges at 20 A until the bank reaches 28.6 V absorption, holds that voltage while current tapers below 2 A, then drops to 27.2 V float — keeping the batteries full so the load rides through any AC failure without interruption.
Key facts
- The charger symbol marks the AC-to-DC boundary: AC L / AC N terminals on the input side, polarity-marked DC+ / DC− on the battery side, with a rectifier or battery mark inside the block.
- Multi-stage charging sequence: bulk (constant current) → absorption (constant voltage, current tapers) → float (reduced maintenance voltage); equalize is an optional periodic overcharge for flooded lead-acid only.
- Typical 12 V lead-acid set points: absorption 14.4–14.7 V, float 13.2–13.8 V, equalize 15.3–15.8 V — scale ×2 for 24 V and ×4 for 48 V systems.
- Charger current is conventionally sized at 10–25% of battery Ah capacity (C/10–C/4); undersized chargers never finish absorption, oversized ones overheat flooded cells.
- Never equalize sealed AGM, gel, or lithium batteries — the elevated voltage vents AGM/gel cells and triggers BMS protection (or damage) on lithium.
- Temperature compensation of roughly −3 to −5 mV/°C per cell prevents thermal runaway on hot batteries and chronic undercharge on cold ones.
- In standby systems the charger, battery, and DC load share one bus, so the load transfers to battery instantly and seamlessly when AC fails.
- IEEE 946 sizing rule for station chargers: rated amps ≥ continuous DC load + (Ah discharged × 1.1–1.25) ÷ required recharge hours.
Frequently asked questions
What is the difference between float, absorption, and equalize charging?
Absorption is the constant-voltage stage (about 14.4–14.7 V on a 12 V flooded battery) that completes the charge while current tapers. Float is a lower continuous maintenance voltage (13.2–13.8 V) safe to apply indefinitely — it replaces self-discharge without gassing. Equalize is a deliberate periodic overcharge (15.3–15.8 V) applied only to flooded lead-acid to balance cell voltages and stir stratified electrolyte; it must never be applied to AGM, gel, or lithium batteries.
What is the difference between a battery charger and a DC power supply?
A power supply holds one fixed voltage regardless of what is connected; a charger regulates according to the battery's state of charge, stepping through current-limited bulk, constant-voltage absorption, and reduced float stages, often with temperature compensation. Powering a battery from a plain fixed supply either chronically undercharges it (if set to float voltage) or boils it dry (if set to absorption voltage permanently).
How do I size a battery charger?
The rule of thumb for lead-acid is 10–25% of the bank's amp-hour capacity: a 100 Ah battery suits a 10–25 A charger. For engineered standby systems, IEEE 946 practice sizes the charger to carry the continuous DC load plus recharge the discharged amp-hours (with a 1.1–1.25 inefficiency factor) within the required recharge time. Lithium banks accept much higher rates — often 0.5C — so the charger, not the battery, is usually the limit.
Can I leave a float charger connected permanently?
Yes — that is precisely what float mode is for. At 13.2–13.8 V (12 V lead-acid) the charger only replaces self-discharge and standing-load current, without significant gassing or grid corrosion. Standby applications (gensets, fire panels, substation batteries, boats in marinas) float their batteries continuously for years. Quality chargers add temperature compensation and periodic refresh cycles to avoid float-induced stratification.
What do the AC L, AC N, DC+ and DC− terminals mean on the charger symbol?
AC L (line/live) and AC N (neutral) are the fused mains input — 230 or 120 V AC. DC+ and DC− are the regulated output to the battery, and polarity is critical: reverse connection blows the charger's protection (or the battery's fuse) and is the most common installation fault. Industrial chargers add an earth terminal, a temperature-sensor lead, and alarm contact terminals not shown in the simplified symbol.
Why does a 24 V or 48 V system just double or quadruple the 12 V voltages?
Because lead-acid chemistry is per-cell: a nominal 2 V cell absorbs at about 2.40–2.45 V and floats at about 2.25–2.30 V. A 12 V battery is six cells, 24 V is twelve, 48 V is twenty-four — so every set point scales linearly. That is also why chargers specify per-cell temperature compensation (−3 to −5 mV/°C/cell): the total correction scales with the cell count too.
Related symbols
- Battery symbol
- Full-Wave Rectifier symbol
- Inverter (DC-AC) symbol
- Power Supply 24V DC symbol
- Transformer symbol
- UPS Block symbol
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