General A battery is recharged by applying a voltage to the battery that is higher than the battery's own terminal voltage. This forces a reverse current through the battery which recharges it. This must be done in a way that causes the desired electrochemical reaction to occur in the battery without causing undue damage to the battery.

Quick charging a battery (under 5 hours) requires special techniques and usually a custom-designed charger. All off-the-shelf chargers from Ibex Battery Systems are intended for 5 hour (or longer) charging. These chargers use 2-mode or enhanced 3-mode charging techniques.

Enhanced 3-Mode Chargers This new type of charger offers the best features of the 2-mode chargers and the standard (non-Ibex) 3-mode chargers (see below). It charges the battery to 100% during the three charging modes (bulk, absorption, and float).

Because the charger is controlled by a microprocessor, it has the "intelligence" to determine whether or not a load is connected across the battery as it is being charged and to compute the ideal time for remaining in each of the three charging modes.

Because of this, the charger can be used to charge stand-alone batteries or as a combination charger/power supply. In either application, the charger will properly charge the battery to 100%, using each of the three charging modes, with no danger of the charger locking itself into the absorption mode (as can happen with a standard (non-Ibex) 3-mode charger (see below).

Mode 1 - Bulk Mode: The charger is in a current-limit state and delivers its maximum rated current to the battery. Because the charger can deliver only its maximum rated current to the battery, the voltage at the charger's output terminals is clamped to some level. This is determined by the battery (usually between 12V for a discharged battery rising to 14.7V as the battery re-charges).

It is in this mode that the battery receives most of its recharge. While the battery is being recharged, its terminal voltage slowly rises. When the battery voltage rises to approximately 14.7V (for a 12V system), the battery is at a 75% to 90% recharged level and the charger switches to absorption mode.

Mode 2 - Absorption Mode: The charger is in a constant-voltage state delivering approximately 14.7V to the battery. The battery draws less and less current from the charger as the battery reaches 100% recharged. When the charger calculates that the battery is 100% recharged, the charger switches to float mode.

Mode 3 - Float Mode: The charger is in constant-voltage state at a lower voltage than in absorption mode (approx. 13.8V for a 12V system). This allows the battery to draw just enough current to make up for its internal leakage current. When the charger is in this mode, the battery may remain connected to the charger for all of the battery's service life with no damage to the battery.

The charger, the battery, and a user's circuit may all be directly and permanently connected together. While AC line power is available, the charger recharges and maintains the battery while supplying the circuit with power. If the AC line power fails, the battery supplies the circuit with power with no interruption. The charger will not draw an appreciable reverse leakage current from the battery while AC power is off. When AC line power is restored, the charger once again charges the battery.

2-Mode Chargers A 2-mode charger can be used as a combination charger/power supply. An external load may draw current from the charger while the charger is recharging a battery.

However, a drawback to a 2-mode charger is that the battery is not completely recharged when the charger switches from bulk mode to float mode. After switching to float mode, the battery will, in most cases, be 75% to 90% recharged. The battery receives the remaining recharge while in float mode over several days and, even then, the battery is not truly "topped off" for several weeks. For this reason, 2-mode chargers are best used for maintaining a battery backup system which will not be subjected to a discharge cycle more often than once a month (on average). For rapid charging, an enhanced 3-mode charger is more appropriate.

Bulk Mode The charger is in a constant-current state. If the battery is partially or completely discharged, the charger delivers its maximum rated current to the battery (and external circuit). During this time, the battery and external circuit (if any) compete for the available current. The battery acts like a very large zener diode clamping the charger's output to some voltage. Since a 12V battery should not be discharged to less than 10.8V (open circuit) this voltage is the minimum that appears at the battery's terminals when charging begins. In practice, the voltage usually jumps to 12V or so when charging begins unless the battery is very large. The external circuit (if any) draws whatever current it requires at this voltage and the battery gets the rest.

The charger remains in constant-current bulk mode until the battery voltage rises to approximately 14.7V (for a 12V system). At this point, depending upon the charge rate, the battery is between 75% and 100% recharged. The following list shows the % recharged level as the charger switches from bulk mode to float mode where "C" is the Ah rating of the battery. For instance, a 3Ah battery connected to a 1A charger is being charged at a C/3 rate.

C/3 charge rate --> 75% recharged
C/5 charge rate --> 85%
C/10 rate --> 90%
C/20 rate --> 95%
C/100 rate --> 100%

The charger then switches to constant-voltage float mode. The absorption mode of the 3-mode type charger is eliminated in this type of charger.

Float Mode: The charger's output voltage is reduced to approx. 13.8V (for a 12V system). The battery is brought up to an almost 100% recharged level over the next 2-3 days and is constantly maintained at that level with the charger supplying just enough current to compensate for the battery's internal leakage. When the charger is in this mode, the battery may be left connected to the charger for all of its service life with no damage occurring to the battery.

Standard (non-Ibex) 3-Mode Chargers: This type of charger is designed to recharge a stand-alone battery only. It recharges a battery to 100% more quickly than a 2-mode type, however, it must only be used to charge a battery and should not be used to supply current to an external circuit while charging a battery. Connecting a load across the battery during charging can cause this type of charger to "lock-up" at an elevated voltage and can damage the battery. Ibex Battery Systems no longer makes this type of charger.

The charger measures the amount of charging current being delivered to the battery. When the charging current drops to approximately 10% of the charger's maximum-current rating, the battery is assumed to be 100% recharged and the charger switches to float mode. It is because of the absorption mode that this type of charger should not be used as a power supply (see App Note 6 for information about how this type of charger can damage a battery if used as a power supply).

In addition, because the charger switches to float mode too soon (when the current drops to 10%), the battery isn't truly 100% recharged. This makes for poor recharging performance in a cyclical application.

A standard (non-Ibex) 3-mode charger should not be used to charge a very large battery at a very slow rate. This means that a tiny standard 3-mode charger should not be used to charge a large battery. This is because of the possibility of a large battery's leakage current causing the lockup condition described above.

Actually, there is nothing to be gained by using a standard 3-mode charger to slow-charge a battery because there is little difference in charging time between a 2-mode and standard 3-mode charger when charging at a less-than C/10 rate (C is the Ah rating of the battery).

Batteries Over 12V Regardless of the charger type, batteries over 12V should be at least partially discharged and then recharged every 3 months or so. This is to equalize the charge of the battery's individual cells.