How Battery Equalizers Work

Why Some Vehicles Have 24 Volt Electrical Systems

Specialty vehicles such as boats, trucks, buses and motor homes will frequently have 24 Volt electrical systems.

The decision to build a vehicle in this fashion is often an economic one that has to do with the cost of wire. Simply put, the cost of the wire necessary to build a 24 Volt electrical system is less than the cost of the wire for a 12 Volt electrical system.

On smaller vehicles, the wire cost compared to the cost an extra battery is not enough to justify adding an additional battery. However, on vehicles that approach 40 ft in length and have electrical devices in every conceivable nook and cranny, the cost of wire adds up quick.

The Problem with 24 Volt Electrical Systems

The 24 Volt electrical system in a vehicle is built by placing two 12 Volt batteries in series (or end to end). IF the only devices that are connected to the electrical system are 24 volt devices, the problems are minimal and are generally not worth being concerned about.

HOWEVER, there are occasions when it is desirable to add 12 volt devices to a 24 volt system. Often times, the addition of these 12 volt components has to do with the higher cost and limited availability of equivalent 24 volt devices.

IF 12 volt components are added to a 24 volt system, problems with the batteries can and frequently will develop.

Let’s first examine the discharge cycle in such a system. As shown in the picture below:

  • The Alternator which charges the battery is off. As a result, power being supplied to both 12V and 24 Devices is coming from the batteries alone.
  • Battery A is supplying power to 12V devices and 24 volt devices.
  • Battery B is only supplying to 24V devices.

The net result is that Battery A is being discharged faster than Battery B.

After the discharge cycle, we’re left with two batteries that are at different states of charge.

  • Battery A, which has been powering both 12 Volt and 24 Volt devices is significantly discharged.
  • Battery B, which has been powering only 24 volt devices, retains significantly more charge than Battery A.

Thus we begin the charge cycle by turning the alternator on.

A NOTE HERE: The alternator is a 24 volt alternator. In a sense, it treats both 12 volt batteries as if they were one big single 24 volt battery. It has no way of knowing that each of the two batteries are different states charges. It will just keep charging.

Referring to the picture below:

  • Battery B, will charge up completely.
  • The Alternator will keep charging because the sum of both batteries is still less than the state of charge for a 24 Volt Battery
  • Battery B, will overcharge and start losing electrolyte and thus become permanently damaged
  • Battery A, may never become fully charged, thus making it susceptible to electrode damage caused by chronic undercharging.

What a Battery Equalizer Does

The battery equalizer solves both problems of imbalanced battery discharging and charging.

Let’s look at the two primary modes of operation.

Referring to the picture below, let’s examine the discharge cycle with a battery equalizer.

  • The Battery Equalizer senses that battery A is being asked to work harder by supply power to both 12 Volt and 24 Volt devices
  • The Battery Equalizer then diverts power from Battery B so that the demands being placed on both batteries is equal.
  • The result is that battery A and battery B discharge equally.

Battery equalizer operation is similar during the charge cycle, except that the power being diverted is coming from the alternator.

  • The alternator begins charging and supplying power for the 12 volt and 24 volt loads.
  • The battery equalizer senses and imbalance between the charge voltage applied to battery A and Battery b.
  • The battery equalizer redirects some of the power coming from the alternator so that the charge voltages being applied to battery A and battery B are equal.

Battery Equalizer Operation Summarized

What’s important to remember from this article is that without the battery equalizer, there is a risk of permanent damage to both batteries.

With the battery equalizer, the risk of damage is substantially reduced because because the discharging and charging of both batteries has been equalized.