Automotive Battery Taper Chargers: There are several types of chargers in this group. They provide a high charging current per dollar of cost but are hard on batteries. They produce a pulsing DC current that really is the same as a steady DC current with an AC component superimposed on it. This AC component causes damage to the battery.

A more serious problem occurs when the battery is fully charged. Some chargers continue to trickle-charge the battery for as long as the charger is connected to the battery. This trickle-charging current is always too high for proper battery maintenance and causes the battery electrolyte to evaporate. Another method used is to automatically turn off the charger when the battery reaches a specific voltage or after a specific number of hours. The charger may then automatically restart if the battery discharges to a specific voltage. Again the battery is damaged by repeated overcharge and discharge cycles.

Dual Level Chargers: These units are available as several variations. Most models on the market are very gentle to batteries when the chargers are used as the designer intended. They do however, cost more than the automotive-style taper chargers. A review of what happens during the charging cycle can explain why improper use of one of these chargers can still cause battery damage.

When initially charging a discharged battery, this type of charger starts off in constant-current (bulk) mode. The battery is charged at the maximum current that the charger can supply. The charger remains in this mode until the battery voltage reaches the mode-switching voltage (approximately 14.7V at room temperature for a 12V system). At this point, the battery is between 75% and 95% fully charged. Some types of chargers (2-mode types) then immediately switch to a constant-voltage (float) mode, set to the battery's float voltage (close to 13.8V at room temperature).

Other designs ( standard 3-mode types) switch to a constant-voltage (absorption) mode, set to the 14.7V mode-switching voltage, until the battery charging current drops to a low level (typically 10% of the maximum charging current). At this point, the charger assumes that the battery is fully charged and switches its output voltage to the battery's float voltage. This extra time spent at an elevated voltage adds the final 5% to 15% of charge to the battery.

As expected, there are pros and cons with both of these dual-level designs. Spending extra time at an elevated voltage to add an additional 5% to 15% of charge to the battery, gets the battery to (almost) 100% charge more quickly. However, most 3-mode chargers gauge the required extra charging time by measuring the battery charging current. When the measured current has dropped to approximately 10% of the charger's rated current, the charger assumes that the battery has reached 100% charge. This will work if the battery is within a certain range of sizes matched to the charger's current-rating, and nothing, other than the charger, is connected to the battery.

If an external circuit diverts appreciable current from the battery while it is being charged, the standard 3-mode charger has no way of knowing that the battery is not the object drawing the current. If the circuit current-draw is above the 10% point that the charger is relying on, the charger assumes that the battery still is not 100% charged. The charger keeps its output voltage high (14.7V), waiting for the charging current to drop to 10%, which it never does. This over-charging reduces battery life in battery-maintenance applications.

A dual-level charger design (2-mode) that switches abruptly from constant-current (bulk) mode to constant-voltage (float) mode cannot be "faked-out" by an external circuit that is drawing current. The charger switches from bulk mode to float mode based only on the battery terminal voltage. The charger doesn't care how much current is being diverted by an external circuit. Also, the size of the battery that can be charged by this type of charger is essentially irrelevant. As long as the charger can make up for the battery's internal leakage current, it will charge the battery without ever overcharging it.

A new type of charger is now available with an enhanced 3-mode operation. This charger uses a microprocessor and can determine if a load is connected to the battery during charging, and to calculate the proper charging times for each of the 3 modes (bulk, absorption, and float). This offers the best of both worlds - the battery is charged as quickly as possible using all 3 charging modes, yet the charger never "locks up" in absorption mode because it can't be fooled by a load placed across the battery during charging.