Lead battery charging: The IUoU charging cycle

Nicolas Venance - Boat Technical Journal

Tags: Battery, Lead-acid battery, Electrical circuit, Electricity, Electrical network

IUoU? UOIU? It's hard not to get confused by this acronym, which describes the process of charging a battery in three phases, with distinct characteristics in terms of voltage, current and duration. But what is it? What's it for? How do you adjust your charger? Find the answers here!

Perhaps you've already noticed, if you're equipped with a battery monitor (a battery capacity gauge), that this device doesn't indicate 100 % even after hours of motoring with the alternator running?
This is because an alternator alone is not capable of delivering a full load with its 14 V maximum voltage.
Without special equipment, it will not be possible to charge batteries at the alternator to more than 80 % of their capacity.
To reach the next 20 %, a slight voltage overload must be applied to the accumulators: this is the second phase.
Then, once a preset time has been reached, the charger switches to a third, maintenance phase, designed to compensate for self-discharge.

3-phase loading IUoUo

A lead-acid battery (12 V) fully charged to 100 % will have a stable voltage of 12.7 V after one hour's disconnection from any consumer or charger.
To achieve this, chargers need to alternate between different voltage and current scenarios.

Today's chargers can modulate voltage, current and time, and apply these parameters to the charging of a battery bank.

As the characteristics of these phases vary slightly from one chemistry to another (AGM, Gel, liquid electrolyte lead, etc.), they must be set on the charger before it is put into service.

DIN 41773 describes this IUoU load.

The three loading phases in detail

Phase I
During this phase, a high voltage is applied to the battery. This can be as high as 14.6 or even 14.9 V in the case of fast charge settings.
The charger then delivers the highest possible current, giving its name to phase I, also known as the Bulk phase.
At the end of this first phase, the battery will be charged to 80 %.

The Uo phase
Uo is the overvoltage voltage.
To exceed the charging limit of phase I, chargers apply a slight momentary overvoltage at Uo, with high current. This is the absorption phase.

With an absorption voltage as high as 14.4 V, compared with that of a standard alternator at 14.2 V, it's easy to understand why it's impossible to exceed the 80 % SOC (state of charge) loading level with a single alternator.

The U phase
In this phase, known as maintenance, the voltage drops, as does the current.
This sequence corresponds to maintenance of the voltage of the charged battery, the float mode.

Additional phases delivered by certain loaders

Battery-safe mode

On some devices, this sequence can be followed by the charger simply going into standby mode, if no consumption is detected, in order to place as little strain as possible on the battery plates. The voltage will then be automatically raised to float level, once a week only, as in Victron Energy's "Battery Safe" mode.

Equalization mode

An equalization mode is intended for liquid electrolyte batteries only. In this mode, an overvoltage of 10 % is applied to the battery to limit the stratification process of the liquid electrolyte. It's a bit like shaking the contents of the battery to keep the mixture homogeneous.

Desulfation mode

When applied by a consumer, the lead plates (electrodes) combine with the acid electrolyte solution to form lead sulfate crystals. These crystals are deposited on the surface of the plates and prevent the electrochemical process from taking place properly, thus severely degrading the performance of the battery in question: this is "sulfation".

To get rid of them, a forced process is triggered which emits a series of electrical impulses designed to disintegrate the crystals in question by breaking the molecular bonds between lead and acid sulfate ions.

Setting charging sequence parameters on the charger(s).

The different types of battery are designed to benefit from a well-defined charging voltage, depending on the charging sequence. Gel batteries, for example, require lower voltages than AGM batteries.
Depending on the load phase, these voltage differences can reach 0.5 V.
Charging a Gel battery with AGM parameters means applying a major overvoltage that will send it straight to the scrap heap!
Charging AGMs with Gel parameters will prevent them from cycling, which will considerably limit their service life.
These parameters are set by computer via an adjustment interface, or by manipulating microscopic DIP switches inside the device.

In the top left corner of this picture (below), DIP switches on a Victron Energy Multiplus 12/3000 inverter/charger.

On other devices, settings are made via an integrated control screen.
In any case, great care must be taken when defining these settings.
If you're not sure, it's a good idea to refer to the battery manufacturer's documentation, which specifies the voltage and current ranges per sequence.
These instructions may also be found on the batteries themselves.