Bulk Charging Explained

Constant Current Bulk Charging

Bulk Charge LogoAs the name suggests, bulk charging is where most of the charging occurs.  In other words, its where most of the energy is transferred during the charging process.

In multistage charging, it is known as the first stage and will account for 70 to 80 percent of the charging.

The process is also known as constant current charging or constant current, voltage limited charging.

Simply put, the current is held at a constant value (or rate) until the voltage reaches a defined level.   At that point, bulk charging stops.

Basic Bulk Charging Theory

When a battery is nearly dead, its resistance to a charge is very low.  Because it is low, the voltage necessary to keep a constant voltage flowing through the battery is low as well.

As the state of charge on the battery increases,  its resistance to a charge also increases.   As result, keeping the current constant requires more and more voltage.

This is all governed by Ohm’s Law and is illustrated in the graphic below.

Bulk Charging Theory

Why Bulk Charging is Stopped Before Reaching a Full Charge

The thing about current flowing through resistance is that it creates heat.  If you increase voltage to maintain a current flow through an increasing resistance, you will also increase the heat dissipated through a resistance.    At some point this increase in heat will cause the electrolyte in the battery to boil or evaporate.   In a lead acid battery, this can cause permanent damage.