BEFORE WE DISCUSS what the dc output current limit feature does, let’s start with what it doesn’t do: it can’t affect the ac inrush current on startup (see the discussion of inrush current in SECTION 2.4.3).

As the name implies, the dc output current limit feature controls the maximum dc output current that a charger can deliver to the battery and/or load. In most modern phase-controlled chargers, current limiting is electronically controlled. This control normally allows the user to adjust the current limit setting over a limited range. Typically, there is an upper limit to the allowable range built into the control circuit, to protect the system against risky maladjustment.

Why is there an upper limit? It isn’t, as you might guess, to protect the battery, which is largely self-limiting as long as the charger output voltage is set properly.* But there are some practical reasons to limit the current:

1. To keep the output current within a safe range for the ratings of the charger components (rectifier diodes or SCRs, for example), and for the dc wiring connecting the battery to the charger.
2. To control the ac input current to ensure that site wiring ampacities and circuit breaker or fuse ratings aren’t exceeded.
3. To prevent nuisance trips for input or output circuit breakers in the event of a bus fault, or during unusual operating conditions such as starting a dc motor.