# Float and boost battery charger

Canonical URL: https://eloragrid.ai/symbols/float-boost-battery-charger
Publisher: Elora Grid
Updated: 2026-10-28
Lookup label: BC
Drawing use: SLD, Schematic
Reference families: IEC, IEEE / ANSI

A rectifier charger that holds a stationary battery at float voltage and can apply a higher boost or equalise voltage.

## In plain English

### What does Float and boost battery charger mean?

A float and boost charger is the rectifier that maintains a stationary battery, typically for switchgear tripping supplies, telecom or a microgrid DC bus. In float mode it supplies the standing DC load and a small trickle current, while boost or equalise mode raises the voltage to recharge the battery quickly after a discharge or to equalise lead-acid cells.

### A simple example

Drawings show a rectifier block with an AC input breaker, DC output to the battery and the DC distribution board, and often a blocking diode or dropping diode chain to keep load voltage within limits during boost. Alarm contacts for charger failure, earth fault and low voltage are drawn on the control schematic.

### What the drawings help you do

The symbol does not state the current limit, whether the charger alone can carry the full load without the battery, or temperature compensation of float voltage. These matter for sizing and for battery life, so they must be in the specification.

### This particular reference

A rectifier charger that holds a stationary battery at float voltage and can apply a higher boost or equalise voltage. A station charger is a power rectifier supplying load and battery; a battery charger controller is an integrated circuit inside smaller equipment.

### Terms

- DC: Direct current: electricity flowing in one direction.
- AC: Alternating current: electricity whose direction changes periodically.
- Normally open / normally closed: The contact state in the normal condition defined for the drawing.

Concept diagram: Identify the device and form → Read the connections and state → Check the equipment reference
A guide to reading this reference. The original symbol illustration and its drawing notes appear below; these boxes are not a wiring diagram.

- [Skylla-IP65 official product and documents](https://www.victronenergy.com/chargers/skylla-ip65)
- [DC UPS, AC UPS and capacitor buffer modules](https://www.phoenixcontact.com/en-gb/products/uninterruptible-power-supplies-ups)


## What it represents

A float and boost charger is the rectifier that maintains a stationary battery, typically for switchgear tripping supplies, telecom or a microgrid DC bus. In float mode it supplies the standing DC load and a small trickle current, while boost or equalise mode raises the voltage to recharge the battery quickly after a discharge or to equalise lead-acid cells.
Drawings show a rectifier block with an AC input breaker, DC output to the battery and the DC distribution board, and often a blocking diode or dropping diode chain to keep load voltage within limits during boost. Alarm contacts for charger failure, earth fault and low voltage are drawn on the control schematic.
The symbol does not state the current limit, whether the charger alone can carry the full load without the battery, or temperature compensation of float voltage. These matter for sizing and for battery life, so they must be in the specification.

## How to read it

- Look for voltage-dropping diodes that protect loads during boost charging.
- Check whether two chargers share one battery for redundancy.

## Information to show or reference

- State float and boost voltages and current rating.
- Show AC supply, battery and load connections.
- Reference alarms and temperature compensation.

## Important distinction

A station charger is a power rectifier supplying load and battery; a battery charger controller is an integrated circuit inside smaller equipment.

## Facts

- Modes: Float, boost or equalise
- Supplies: Battery and standing DC load
- Typical alarms: Charger fail, earth fault, low voltage
- Preview: Independent educational illustration; consult the applicable standard and project legend for the exact graphic

## Public references

- [Skylla-IP65 official product and documents](https://www.victronenergy.com/chargers/skylla-ip65) — Victron Energy. Public battery charger documentation context.
- [DC UPS, AC UPS and capacitor buffer modules](https://www.phoenixcontact.com/en-gb/products/uninterruptible-power-supplies-ups) — Phoenix Contact. Public DC supply and UPS function context.

## Graphic scope

Independent original educational concept illustration by Elora Grid. Not a certified standards graphic or manufacturer footprint. Unitless; not to scale. Original artwork licence: https://creativecommons.org/licenses/by/4.0/. Publisher material is linked only.