Alternator Charging basics

The battery is the heart of your electrical system. Your various on-board gadgets draw power from it, and it is the role of your alternator to recharge it.

 The alternator is just a type of generator; it converts mechanical energy into electrical energy. Coupled to your engine, it will generate an electrical current to recharge your battery. For a given amount of rotation, the amount of current the alternator produces is controlled by the field current fed to the alternator. See the following diagram.

 The purpose of an alternator regulator is to continuously adjust the field current in order to maintain a desired battery voltage. So it is the field current that is being controlled in order to set a charging current, which in turn establishes a desired battery voltage. Confusing I know.

 The dilemma of a standard ‘dumb’ regulator is twofold: it only offers single voltage target, and that target is often not even correct outside of the automotive use! Ideally a regulator should acknowledge the chemistry occurring in the wet-cell batteries and provide two different set points.

 When the engine starts up, we would like the battery to charge as fast as possible. This is accomplished by setting a voltage target of ±14.4V (the actual depends on your specific battery type). Once the battery reaches this value, it is at approximately 80% of full charge capacity, and gassing occurs. This signals that the highest safe level of charging has been reached. We should not exceed this gassing voltage (or we could damage the battery), but neither should we go too far below it (or we will take longer than necessary to charge the battery). At this stage the battery will continue to accept or absorb charge at a gradually decreasing rate.

Since the charging rate tails off, we should choose some sensible point to stop the intense charging and switch to just maintaining or floating the battery at a charged state. We do this by reducing the voltage set point by 0.6V and it is vital for the long-term health of the battery.

Typical target values for ‘dumb’ regulators are between 13.8 and 14.2V, which is too low for the absorption stage and too high for the float stage. So you have the worst of both worlds; during motoring bursts the battery won’t fully charge, but long term it will overcharge, damaging the battery.

A standard charging curve looks something like this: