Ibex Manufacturing, Inc.

Application Note 4
So What Are 1-Mode, 2-Mode, And 3-Mode Chargers?

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. 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 Manufacturing are intended for 5 hour (or longer) charging. These chargers use 2-mode, standard 3-mode, or enhanced 3-mode charging techniques.

Standard 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.

Also, a standard 3-mode charger should not be used to charge batteries at a slow rate. This means that a tiny standard 3-mode charger should not be used to charge a large battery. Actually, there is nothing to be gained by using a 3-mode charger to slow-charge a battery because there is little difference in charging time between a 2-mode and 3-mode charger when charging at a less-than C/10 rate (C is the Ah rating of the battery). The listed recommended battery sizes for each 3-mode charger results in a battery charging rate of from C/10 to C/3. For instance the recommended battery size for a 0.5A charger is from 1.5Ah to 5.0Ah.

Some battery manufacturers don't allow their batteries to be charged at a higher than C/5 rate. Check with the manufacturer. If this is the case, use a larger battery or a smaller charger.

Mode 1 - Bulk Mode: The charger is in current-limit and delivers its maximum rated current to the battery. It is in this mode that the battery receives most of its recharge. When the battery voltage rises to approximately 14.7V (for a 12V system), the battery is at a 75% to 90% recharge level and the charger switches to absorption mode.

Mode 2 - Absorption Mode: The charger is in a constant-voltage mode delivering approximately 14.7V to the battery. 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 considered 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).

Mode 3 - Float Mode: The charger is in constant-voltage mode 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.

2-Mode Chargers: A 2-mode charger can be used as a combination charger/power supply and can charge and maintain very large batteries. An external circuit may draw current from the charger while the charger is recharging a battery. This type of charger can charge a battery with an Ah rating of up-to 100 times the current capacity of the charger.

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 interuption. When AC line power is restored, the charger once again charges the 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.

Bulk Mode: The charger is in constant-current mode. 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 % recharge 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 a 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.

Enhanced 3-Mode Chargers: This new type of charger offers the best features of the 2-mode chargers and the standard 3-mode chargers. 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 charging time for each charging mode. 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.

Batteries Over 12V: Regardless of the charger type, batteries of 14V or higher 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.