Chargers, rectifiers and DC power

[how to size a battery charger · charger sizing formula] A working rule for lead-acid: charger amps = battery Ah ÷ (hours to charge ÷ 2). A 100 Ah battery to be charged in 8 hours needs 100 ÷ 4 = a 25 A charger. Two refinements matter in practice: AS/NZS 4509.2 uses the C₁₀ rate for charger sizing, and the charger must carry the normal load as well as the recharge current if the load remains connected, a point utility specifications are explicit about.

[float vs boost charge · equalise charge battery] Float is a continuous low voltage that holds a fully charged battery against self-discharge without over-charging it. Boost (or equalise) is a temporary higher voltage used to restore a discharged battery or to equalise cells that have drifted apart. Utility specifications typically set boost to initiate when battery voltage falls below a threshold. Sydney Water uses 80% of nominal, and require float to be set for minimal over-charge and water loss.

[temperature compensated battery charging · why temperature compensation charger] Because the correct float voltage changes with temperature. A charger holding a 20 °C float voltage at a 45 °C battery is over-charging it, which accelerates dryout and is a recognised thermal runaway mechanism. This is now the modern standard in utility specifications: AusNet notes that only around half its existing charger fleet has the functionality and that the current standard requires it. If a bank is in an uncontrolled environment, temperature compensation is not optional.

[DC power system vs UPS · rectifier vs UPS] A DC power system (rectifier plant) converts AC to DC, feeds DC loads directly, and floats a battery on the same bus, so if the AC fails, the battery simply continues to supply the DC bus with no transfer event. A UPS converts AC to DC and back to AC to feed AC loads, and involves a transfer when the mains fails. Where the equipment runs natively on DC, telecommunications, signalling, substation protection and control, most industrial instrumentation, a DC power system is simpler, more efficient and has fewer failure modes. Where the load is AC, a UPS is required. This is the highest-value definitional question in the whole hub, and no Australian competitor answers it.

[rectifier vs battery charger difference] A rectifier in a DC power system continuously supplies the DC load and keeps the battery on float, the battery is a reserve, not the primary customer. A battery charger is designed primarily to charge the battery. The distinction matters when specifying: a charger sized only for the battery will not carry the load, and a rectifier plant must be sized for load plus recharge current simultaneously. ARTC’s DC power supply specification, for example, requires the unit to supply full load with the batteries removed and to supply normal load while recharging a fully discharged battery, restoring it to at least 75% capacity within 10 hours.

[N+1 redundancy DC power system · hot swap rectifier] Sizing the rectifier complement so total load plus battery recharge current can still be carried with one module out of service. Combined with hot-swappable modules, it means a rectifier can fail or be replaced without dropping the DC bus. It is the standard resilience requirement in telecommunications, rail and utility DC systems, and it is a specification to state explicitly rather than assume.

[why is telecom -48V · why substation 110V DC 125V DC] Telecommunications standardised on a negative 48 V bus, positive earth, because it reduces electrolytic corrosion of buried and exposed metalwork. ETSI EN 300 132-2 sets the normal interface range at −40.5 V to −57.0 V, derived from 24 lead-acid cells at 2.35 V per cell on float with allowance for distribution drop. Substations use higher voltages, commonly 110 V or 125 V DC, with AusNet reporting 125 V as the most common across its fleet, because tripping and protection circuits are long, and higher voltage reduces conductor size and voltage drop over distance. Rail signalling adds its own conventions: 50 V DC for signalling logic, 24 V DC for electronics, 12–15 V DC for level crossings.