Battery cell isolators and battery cell separators are used to control current flow in multi-battery systems in a range of vehicles, including recreational vehicles, boats, utility vehicles, airplanes, large trucks, and off-road vehicles that accommodate auxiliary loads and high current loads such as a recovery winch. Battery isolators are designed to prevent auxiliary batteries from draining the main battery. On the other hand, battery separators are designed to protect the charging system from excessive electrical loads by separating (disconnecting) the main and auxiliary batteries from one another. Disconnecting the two allows the separator to prioritize the charging of the batteries.

If a single battery system is used, the battery can become drained when the engine is off, and auxiliary loads are drawing power, resulting in an inability to start the engine. Adding a second battery system for auxiliary loads can resolve that concern, but it must be properly wired into the vehicle’s electrical system. There are system performance tradeoffs between battery separators and battery isolators and between various implementations of battery isolators.

Battery isolators

A battery isolator is used to separate the DC bus into multiple (isolated) branches and only allow current flow in one direction in each branch. This supports simultaneously charging more than one battery from a single alternator or other power sources without connecting the batteries in parallel. Isolating the batteries prevents a weak or dead battery (usually the auxiliary battery) from draining charge from a strong battery (often the primary battery). If a common diode-based isolator is used, lower system efficiencies result from the diode drop added to the circuit between the charging power source and the batteries.

While simple general-purpose diodes are the most common battery isolator technology, primarily for cost reasons, they are also the least efficient and require large heatsinks, making them relatively large. More efficient battery isolator designs employ Schottky rectifiers, power MOSFETs, and conventional mechanical relays. The second most common battery isolator technology, after the use of general-purpose diodes, uses mechanical relays. The relays provide a good balance between reasonable cost and high efficiencies.

Battery separators

While battery isolators are relatively simple devices, battery separators (sometimes called “smart battery isolators”) are more complex and perform several functions. For example, when the engine is started, the battery separator monitors the voltages of the main battery, the auxiliary battery, and the charging system voltages. When the engine is first started, the battery separator disconnects the auxiliary battery from the system and senses the charge needs of both batteries. If the overall charge needs are greater than the alternator can support, the battery separator disconnects the auxiliary battery and sends all charge current to the main battery.

When the charging system voltage on the main battery reaches about 13.2Vdc, indicating a charged main battery, the battery separator will engage and enable the auxiliary battery to be charged. Typically, at a voltage of about 12.8Vdc, the auxiliary battery will be disconnected from the charger to protect the charging system. If left connected, a damaged auxiliary battery can potentially drain the main battery. This process of managing the charges on both batteries continues as long as the engine is running.

While battery isolators are typically diode-based devices and allow current to flow in only one direction, battery separators support bidirectional current flow. In addition to protecting the charging system and the main battery from damage, a battery separator can assist in engine starting. Upon engagement of the engine starter, the battery separator compares the voltage in the main and auxiliary batteries. If the main battery voltage is lower than the voltage of the auxiliary battery, the separator will engage both batteries to help an engine start.

A summary of the performance benefits of battery separators include:

• Smaller and lighter than diode-based isolators
• The use of relays, solenoids, or solid-state switches eliminates the diode drop
• Allows bi-directional charging from the alternator or an auxiliary source such as shore power
• Reduces the load on the charging system by not connecting the auxiliary battery until the primary battery is charged to 13.2V, which can extend the life of charging components
• Protects the primary battery in the case of a faulty auxiliary battery
• Can aid in engine starting
• Typical quiescent current under 10mA

Choosing

Battery isolators and battery separators can be selected based on various use-cases considerations. The main differences between the devices lie in the direction of current flow, efficiency, and quiescent current draw. An isolator is essentially a diode system that allows one-directional current flow. At the same time, a separator can select to allow current to flow in either direction depending on various system parameters and needs.

An isolator can provide users with a simpler and lower-cost solution. Auxiliary equipment can be run without considering whether the engine is running or not. The isolator automatically disconnects the vehicle’s main power system. When the engine is running, all equipment is powered from the main power system. When the engine is turned off, the equipment is automatically switched to the auxiliary battery.

There are also considerations for vehicles that may be kept in storage for extended periods: Isolators have zero quiescent power draw, while a battery separator has a small quiescent draw. As a result, the use of a separator will slowly drain the battery, adding to the battery’s self-discharge and reducing the time that a useful level of charge main remains in the battery. However, an isolator will not allow an external charger to charge both batteries, while a battery separator will allow an external charger to charge both batteries.

Isolators are useful for multi-battery systems where redundancy is needed, such as utility vehicles that experience many engines, starts, and stops during a workday. With an isolator, no battery will drain the other batteries in the system, enabling redundancy for a system with multiple auxiliary batteries plus the main battery on a single alternator. On the other hand, a larger alternator may be needed since an isolator will charge all batteries evenly.

If some parasitic load on the auxiliary battery is acceptable, separators can provide backup power in specific application scenarios. For example, some systems with deep-cycle batteries can use separators to charge auxiliary batteries quickly or use the batteries in parallel. A separator can be useful in applications such as tow trucks or snowplows that have dual battery systems. The separator can ensure the availability of maximum current by connecting the auxiliary battery with the main battery to support high surge power demands.

Summary

When choosing between battery separators and battery isolators and between various implementations of battery isolators, system performance demands must be considered. While both devices can be used to control current flow in multi-battery systems in a range of vehicles, including recreational vehicles, boats, utility vehicles, airplanes, large trucks, they do so in different ways, which result in a range of tradeoffs. Battery isolators are designed primarily to prevent auxiliary batteries from draining the main battery. On the other hand, battery separators are designed primarily to protect the charging system from excessive electrical loads by separating the main and auxiliary batteries from one another.

References

Battery Isolators, Littelfuse
Battery Separators, Eaton
How to Choose Between Battery Isolators and Separators, Waytech Wire