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IUoU is an DIN-designation and acronym for an automated, three phase procedure to charge a lead-acid battery by means of a battery charger. The three phases are: I-phase, constant electric current; Uo-phase, constant over-voltage; U-phase, constant voltage.
The term 3-Step Charge Profile refers to the same procedure but is not part of the DIN terminology. At least one equipment manufacturer refers to the three phases as Bulk Phase, Absorption Phase and Float Charge.
The purpose is to fully charge the battery in a relatively short time without reducing its life span and to indefinitely keep the battery charged as long as the charger is connected. The three phases (or stages) have no connection with three-phase electric power.
When an IUoU charger is connected to an empty battery, it will enter the I-phase and provide a constant current. The current is determined by the charger and possibly its settings and should be in a reasonable relation to the battery's capacity. For longest battery life, the current would not exceed one tenth of the battery's capacity (C/10), although many battery specifications allow for a fast charge with a higher current than C/10. As a result of the current, the battery absorbs the charge and its voltage rises. The charger limits the maximum voltage to Uo, a constant or temperature-dependent maximum, typically 2.4 V to 2.6 V per cell. As soon as Uo is reached, the charger enters the Uo-phase.
In the Uo-phase, the battery is continued being charged at a constant voltage, but the charge current is decreasing. The decrease is imposed by the battery. The voltage in the Uo-phase is too high to be applied indefinitely but it allows charging the battery fully in a relatively short time. So it is important that the end of the Uo-phase is detected reliably. Depending on the charger, the Uo-phase is concluded when the charge current goes below a threshold, typically 0.8 times the rated I-phase current and the U-phase is entered.
In the U-phase, the voltage is decreased to typically 2.3 V per cell. This phase is intended to first fully charge the battery and to keep the battery charged, if desired indefinitely. The charge current decreases to a value much lower than C/10 and compensates for the self discharge.
When an IUoU charger is connected to a full or partially full battery, the I-phase and Uo-phase are brief and the U-phase is entered quickly.
A bad battery will have short I-phase and Uo-phase, but there is a risk of gassing, further damaging the battery.
If a battery is connected to a significant load during charging, the end of the Uo-phase may never be reached and the battery will gas and be damaged, depending on the charge current relative to the battery capacity.