How Battery Isolators Work

What is a Battery Isolator?

Most vehicles include something known as a starting battery or chassis battery. Typically this battery provides enough power to start an engine, headlights and engine computers.

The chassis battery is not typically designed for frequent deep discharge cycles. This is because the alternator supplies power to vehicle circuits once the engine is running.

However, some vehicles, like boats and RVs, will include additional batteries to power other devices. These batteries are often known as house batteries. As a result, there is also a need to recharge these batteries.

Because the vehicle already has a battery charger known as an alternator, the vehicle will frequently be wired so that these additional batteries are charged by that alternator.

Simply put, the Battery Isolator allows this charging to occur while keeping the house devices from accidentally discharging the starting battery.

The Problem of NOT Isolating Batteries

Provided the alternator is sufficiently sized, there is not much of a problem when the vehicle engine is running. The alternator will in fact charge both batteries (albeit slower).

As pictured below, the real problem occurs when the alternator is not running.

Any house device that is drawing power will draw from both batteries.

Given enough time, these house devices WILL drain both batteries. In other words, if you play that stereo long enough, you will be looking for someone with a car and jumper cables.

What a Diode Based Battery Isolator Does

Diodes are devices that only allow power to flow in one direction. If you’re mechanically inclined, they’re a lot like check valves that only allow fluid to flow in one direction.

The battery isolator incorporates two of these diodes. As shown below, these diodes permit the alternator to charge both batteries while preventing the devices connected to one battery from draining the other.

The Battery Isolator During Discharge Cycle

The discharge cycle occurs when the alternator is off and the batteries are being used to power the house and chassis devices.

During this cycle, current from the house battery is blocked from supplying chassis devices and current from the chassis battery is blocked from powering house devices.

 Battery Isolator During Charge Cycle

During the charge cycle, the power from the alternator passes through the isolator to charge both batteries and to power both house and chassis circuits.

Potential Issues with a Battery Isolator

There are a few potential problems associated the use of a battery isolator that users should be aware of.

  • First – It will take longer to charge the batteries. The alternator in the vehicle may have been sized to charge only one battery. The addition of a second will slow that down.
  • Second – The alternator may have been sized to only charge the chassis battery and supply chassis circuits. The addition of an additional battery and circuits may create a situation where the alternator can not keep pace with the demands. This problem can be mitigated by keeping house loads to a minimum when the alternator is charging.
  • Third – Voltage is lost as current passes through the battery isolator. For isolators based on silicon diodes, the lose may be as great as 0.6V. While this may seem insignificant, that 0.6 volts is important when it comes to charging lead acid batteries. If the alternator is putting out 13.5 volts, the battery will only see 12.9 volts. Such a condition could lead to an undercharged battery and thus cause permanent battery damage.