Battery Chargers and Energy Storage Power Supplies

A charger and a power supply are different instruments even when they produce the same voltage. A power supply holds its output constant and supplies whatever current the load draws. A charger follows a profile: it moves through defined stages, it responds to what the battery does as it fills, and it decides when to stop.

Using a bench supply as a charger works until the battery approaches full, at which point nothing terminates the charge. For lead-acid that means continuous overcharge, gassing and water loss. For lithium it is more serious, because the chemistry has no mechanism to absorb the surplus.

What a charge profile actually does

Nearly all modern chargers are built on constant-current followed by constant-voltage. During the bulk stage the charger delivers a set current and the battery voltage climbs as it accepts charge. When the voltage reaches the absorption setpoint the charger switches to holding that voltage, and the current it delivers falls away as the pack fills.

That taper is the signal that the pack is full, and it is how termination is decided: when current drops below a threshold, charging is complete. A supply with no current sensing has no way to detect this and simply continues.

Lead-acid chemistries then take a third stage. Float holds a lower voltage indefinitely to offset self-discharge and prevent sulphation, which is what allows a standby battery to sit on charge for years. Lithium iron phosphate does not sulphate and does not need it — several pack manufacturers specify that float be disabled outright, or set low enough that the pack is not held at the top of its voltage curve.

Chemistry decides the voltages, and the margins are narrow

The numbers below are the widely published starting points for 12 V nominal packs. They are a sanity check against a pack manufacturer's specification, not a substitute for it — the cell manufacturer's datasheet always takes precedence.

• LiFePO4 (4 cells in series) — absorption 14.2 V to 14.6 V, which is 3.55 V to 3.65 V per cell. 3.65 V per cell is an absolute ceiling rather than a target, and 14.2 V is the setting associated with longer pack life. Float is either disabled or set around 13.4 V to 13.5 V.
• AGM and other sealed lead-acid — absorption typically 14.4 V to 14.7 V, float 13.5 V to 13.8 V.
• Flooded lead-acid — absorption typically 14.4 V to 14.8 V, float 13.2 V to 13.8 V.

Two settings differ by more than voltage. Temperature compensation, conventionally around −3 mV per °C per cell, is correct for lead-acid and is set to zero for lithium iron phosphate, whose charge voltage is not meaningfully temperature dependent in the same way. Equalisation — a deliberate controlled overcharge that mixes the electrolyte in a flooded battery — must never be applied to AGM, which is sealed and cannot vent the gas, nor to lithium.

This is why a multi-chemistry charger needs its profile selected rather than assumed. A charger left on a lead-acid profile and connected to a lithium pack will hold it above its proper float indefinitely; the same charger on a lithium profile connected to a flooded battery will never fully charge it.

Charge current: how fast is too fast

Charge current is conventionally expressed as a fraction of the pack's capacity — 0.5C on a 100 Ah pack means 50 A. What the pack will accept depends on chemistry and on construction.

Lead-acid is generally charged at 0.2C to 0.3C. Pushing beyond that produces gassing and heat rather than a faster charge, because the chemistry cannot absorb the energy at that rate. Lithium iron phosphate tolerates considerably more, commonly up to 0.5C, with the practical limits set by the pack's battery management system and by whether the enclosure can remove the resulting heat rather than by the cells alone.

Sizing the charger is the calculation in the other direction: the current needed to return the pack in the time available, given the depth of discharge expected. A standby system that discharges rarely can be charged slowly; a vehicle on a two-shift pattern cannot. Where the charger also carries a live load, as most standby installations do, the supply has to deliver the full load current and the charging current at the same time, and that total is what its rating must cover.

The charger and the BMS have to agree

Every lithium pack has a battery management system that monitors cells and disconnects to protect them. It is a protective device and not a charge controller, and the distinction matters when the two are chosen separately.

If the charger's absorption voltage sits above the BMS high-voltage cutoff, the BMS will disconnect at the end of every cycle. The pack is protected, but a contactor opens under load on each charge and nothing in the system reports that anything is wrong. Over years, that is a great many operations on a component not intended for them.

Setting the charger below the BMS thresholds so the protection stays a backstop is the intent. Where the pack supports a communication bus and the charger can read it, that path is better still, because the pack rather than the charger's fixed configuration determines what it will accept.

What is in the charger category

The charger category covers several distinct patterns of product:

• NPB and NPP — wide-range chargers from around 120 W up to 1,700 W, the NPB family built to charge across a span of battery voltages from one unit and the NPP adding a power-supply output alongside the charging function.
• ENC and ENP — enclosed chargers in the mid power range, the ENP variants combining charging with a supply output.
• GC series — portable adaptor-style chargers for equipment charged away from a fixed installation.
• DBU, RPB and HEP-600C — higher-power and harsh-environment units for larger packs and exposed locations.

The bidirectional categories are a different proposition. A conventional charger moves energy one way; a bidirectional converter also returns energy from the pack, which is what a storage system doing peak shaving, backup supply or grid support requires. Specifying one means defining both directions — charge profile going in, and voltage, power and control behaviour coming out.