Battery Isolators Information

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battery isolatorA battery isolator splits direct current (DC) and divides it into multiple branches, allowing current to proceed in a single direction exclusively in each branch. It provides a simultaneous charge to multiple batteries from one power source, such as an alternator, without requiring connection of the battery terminals in parallel.


A battery isolator avoids the danger of connecting a weak or dead battery to a stronger one. When this occurs, the weaker battery drains the charge from the stronger one when directly linked. While an isolator forestalls this problem, it comes with added expense and also complicates the battery charging process. In diode-type isolators, a further drop in voltage is expected to occur in the circuit linking the charging source to the batteries.


A starting or chassis battery in a vehicle is enough to power an engine, headlights and other parts such as engine computers. However, this battery is not intended to be used for constant deep discharge cycles. An alternator provides power when the engine begins running.


Vehicles such as RVs and boats require additional batteries to provide power for other devices such as refrigerators, lights, and inverters. Often referred to as house or auxiliary batteries, these units also need to be recharged. The alternator already present in a vehicle or boat is allocated for this purpose. However, this presents the risk of auxiliary batteries draining the charge from the main starting battery. To prevent this a battery isolator segregates the charging process for each battery.


Battery isolators are constructed using silicon diodes that act as check valves; they experience a voltage drop approximating 0.7 volts through the diodes. A charging source that monitors its output, such as a regulated one, senses the voltage occurring at the battery is lower than the voltage experienced at the charger. This may lead to charging that is less robust than normal.




Manual switch: This isolator is turned off and on from a switch near the ignition. While they can be quite effective, they normally require opening the hood to engage or disengage. If this task is overlooked it can easily lead to a drained battery. The basic dual battery switch approach has other limitations. It can create additional power demands on the starter battery when the auxiliary battery has diminished power after heavy use. It can also place added demand on the alternator immediately after starting the engine, as it works to charge both the starter and auxiliary battery.


Ignition-switched solenoid: These battery isolators work on the same principle as the manual switch variety but are turned on and off at ignition.


Diode-style isolator: These isolator systems can protect a starting battery and extend the power available for other purposes, such as powering auxiliary batteries to run lights or a refrigerator. Blocking diodes that serve to split the charges to each battery perform this function but at the cost of a drop in voltage from the alternator to the batteries. This reduces the power that can be received from batteries on the system unless measures are taken to circumvent the voltage drop.battery isolator


Smart isolators: These isolators use predetermined points for switching to control the linking and delinking of a solenoid. Using this approach enables the starter battery alone to charge initially. Only after it has reached an acceptable voltage level and a timer has elapsed are the auxiliary batteries linked to the charging process. The timer prevents rapid switching of the solenoid, which has the potential for load spikes in the alternator and arcing, along with functional degradation of the solenoid.


Programmable isolators: Isolators of this type work well if the goal is to extend a power supply as much as possible. By programming the isolator's disconnect points, battery discharge is managed to extract maximum power. They also help avoid full discharge of auxiliary batteries, which is harmful to battery lifespan and can occur when no low voltage cut-off mechanism is used.




When selecting an isolator, it is helpful to calculate the charging load in order to determine the amount of power being used by each appliance. An item used intermittently will draw more power than one that is powered on and unused for long periods of time. Inverters designed to run laptops, phone chargers, or camera equipment may need to be on for hours at a time. Simply turning on these inverters can draw a measurable amount of power. For best results, align power usage needs with an isolator's capabilities.


Important details to consider include the number of batteries to be charged, data display, and remote control.


Considerations for battery isolators include:


  • Does the isolator offer simple dual charging capability or full functionality for charging multiple batteries? 
  • Does the isolator offer link capability to provide added power for starting or performing other functions? 
  • Does the isolator provide system data for monitoring the charge state of batteries? (Is a voltmeter included?) 
  • Does the isolator allow for remote control of the system, or must it be turned on and off manually? 
  • Can both the starter and the auxiliary battery charge from the same battery, if, for example, the auxiliary battery is connected to a solar panel? 
  • Is there a need for the batteries to separate at times? For instance, if it becomes necessary to winch from the auxiliary battery or to unlink the aux battery when it is not needed. 
  • Is there a need to tell if an auxiliary battery is fitted? The majority of isolators connect batteries together even if the auxiliary battery is not connected, which can result in short circuits and harm the isolator. 
  • Will separate battery types be used? If so, this requires changing of the voltage for linking and unlinking to accommodate them.battery isolator



Battery isolators are used in applications where transportation devices support multiple batteries, and backup sources of power may be needed, including:

  • Recreational vehicles
  • Heavy-duty trucks
  • Airplanes
  • Boats
  • Utility vehicles


Isolators help to maintain power to a variety of devices supported by batteries, including:


  • Refrigerators, stoves, and appliances
  • Camp lights
  • Winches
  • Radar
  • Instruments
  • Navigation lights
  • High-power car stereos
  • Inverters
  • Cellular phones
  • Computers, both internal and external




Image Credits:


Littelfuse, Inc.