Ibex Battery Systems

Application Note 6
So Why Do Some Chargers Kill Batteries?

Automotive Battery Taper Chargers There are a several types of chargers meant for automotive (wet type) batteries. The very oldest design is a transformer with a full wave selenium rectifier. This provided unregulated half-sine-wave pulses to the battery. Wet type batteries can tolerate this kind of mistreatment if not done too often.

Sealed Lead-Acid (SLA) batteries absolutely cannot tolerate this. The gross over-current that occurs at the peak of each sine wave overheats the battery and causes corrosion of the battery plates.

A newer version of this design uses SCR's or Thyristors to partially regulate the amplitude of the peaks, but it's still too crude for SLA batteries.

An even newer design is a switch mode charger that is highly regulated and is much more gentle to batteries. However, if designed for automotive batteries, these chargers are usually set to a too-low voltage which causes an SLA battery to be undercharged.

Undercharging an SLA battery actually can cause faster damage than overcharging. Never use a charger designed for wet type automotive batteries to charge an SLA battery.

Standard 3-mode (non-Ibex) types These start out in higher-voltage current-limit bulk mode. When the battery voltage reaches 14.7V, the charger switches to a 14.7V constant-voltage absorption mode. The charger remains in this mode until the charging current drops to 10% of the charger's maximum current rating. At this point, the charger assumes that the battery is fully charged and changes to a 13.8V constant-voltage float (maintenance) mode.

If an external circuit is connected across the battery while it is being charged, problems arise. Because the external circuit diverts 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 (to signal the switch to float mode), 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 considerably.

Enhanced 3-mode (Ibex) types 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. It can't be fooled by a load placed across the battery during charging. This is the only type of 3 mode charger manufactured by Ibex Battery Systems.