Modern charging regimes are intended to recharge the battery as quickly as the charger is able, without excessive gassing & consequent electrolyte loss.

Bulk charging

From the fully depleted condition, the first phase is known as bulk charging. During this phase, the recharge current is the maximum that the charger is able to deliver. This phase continues until the battery can no longer accept the full output from the charger.

Boost charging

Following on from the bulk phase, various techniques may be applied in order to maintain the charge rate; this involves raising the charging voltage applied to the battery. When using sealed batteries, it is important to terminate this phase, often called the boost phase, before the onset of gassing. Of the range of methods used, each has it’s application specific advantages & drawbacks. The presence of a connected load during recharge is a major influence on the choice of regime.

Taper phase

Typically, the battery can be quickly restored to around 85% full capacity using an appropriate boost charging method. Thereafter, the charge voltage needs to be reduced towards the so called float voltage to avoid electrolyte loss. This is known as the taper phase, since the charging current tapers away as the battery rises to the fully charged state.

Float charging

Once fully charged, float voltage is applied, hence this stage is known as the float phase. The battery is fully charged & only the natural losses need to be supplied, together with the demands of any connected load. In fact, it is common to apply a slightly higher voltage than strictly necessary, resulting in slight gassing. By design, recombinant cells (GEL & AGM) recombine these gasses back into water, preventing any significant electrolyte loss. One advantage of this small overcharge current is that it tends to equalise the cells in the stack, bringing them all to the fully charged state (Slight variations in the manufacturing process give rise to slight differences in cell capacity). In practice, the applied float voltage for a 12volt battery would be around 13.6 volts, whereas the true float voltage (ie no gassing) is actually around 13.2 volts.

Battery electrochemistry is temperature sensitive, so temperature compensation of the charge regime will improve battery performance, particularly if a wide temperature range is anticipated.