Dual Battery System: VSR – DC to DC charger – BMS?

Looking to put an auxiliary battery (dual battery setup) in your vehicle?  In this post I will go over a few different devices  that are used when putting a dual battery system in your vehicle.  So you can run additional electrical accessories even a portable fridge freezer.


What Is An Auxiliary Battery

An auxiliary battery is a separate battery (more on batteries click here) so you can run additional accessories off, so you don’t drain your main cranking (start) battery. You will need away to connect isolate and or charge an auxiliary battery and that is where a VSR, Dc to DC charger or BMS comes in.



Fixed Voltage Alternator

Most vehicles 2000 and prior or somewhere around there, had a fixed voltage alternator charging system.  The alternator had a constant voltage output regulated around 14.0v give or take some,

Then once the cranking battery was fully charge the voltage would be reduced a bit.  The constant alternator voltage output usually being high enough allowing cranking (start) and auxiliary batteries to receive an adequate charge.



VSR (voltage sensitive relay)

A VSR is installed inline on the positive lead between your cranking (start) battery and an auxiliary battery. It is a smart switch to isolate the cranking battery from the auxiliary.

Once the VSR detects charging voltage from the vehicles alternator it will connect the switch. Allowing voltage to transfer over to charge the auxiliary battery.  A VSR also detects when the vehicle is not running and charging the main cranking battery then disconnects voltage to the auxiliary battery.  This is a protection keeping you from discharging the cranking (start) battery.  Allowing you to still be able to start your vehicle.   When you have all those accessories running on the auxiliary battery while the vehicle is off.

A VSR (voltage sensitive relay) is typically used on vehicles with a fixed voltage alternator charging system to isolate an auxiliary battery.


Red Arc Smart Start



  • Always mount the VSR as close as possible to the cranking (start) battery for effective operation
  • Also its is very important to have proper (large enough diameter) gauge battery cables and connections, most charging problems stem from inadequate cable size and connections
  • Leave the vehicle run long enough to charge the batteries.  Quick starts and stops (shut downs) could leave you with an under charged or dead auxiliary battery.
  • You could also see a voltage drop if your auxiliary battery is to far from the cranking battery having a long run on your battery cables. Causing your auxiliary battery to not receive a good enough charge. To help remedy this a BMS or Dc to Dc charger is your only real reliable option.




 Smart Charging System /ECU Controlled Alternators

Smart charging systems, are used in modern vehicles.  To help reduce emissions and increase fuel economy.  The ECU (engine control unit) or PCM (powertrain control module) communicates with the alternator to adjust charging output.

Alternator charging voltage will vary greatly in a smart charging system. Could go up to or above 15v or could go down to 12.3v or so to the cranking (start) battery.  The system will produce a high level of voltage to push a lot of current quickly to recover the cranking battery upon starting the vehicle. Once the cranking battery is recovered from engine start.  It will reduce alternator voltage once the current goes below a predetermined level.

The alternator current will be regulated.  To maintain the target voltage.  The target voltage will go up or down, according to the temperature, engine load, electrical loads and cranking battery SOC (state of charge) SOH (state of health)

During acceleration or cruising the voltage stays low.  Upon decelerating the target charging voltage may increase to replenish the lost battery charge while you where cruising or accelerating.

Smart charging systems do not strive to achieve a full cranking battery state of charge. Only reaching about 80 percent.  Maintaining just enough cranking battery voltage for electrical loads and an engine restart.  So adding an additional auxiliary battery could be troublesome.  Since this system will not give any consideration to its state of charge.

With a (ECU controlled alternator) smart charging system you will need a BMS or Dc to Dc charger to boost the generally low output and or stabilize the fluctuating voltage.  So your auxiliary battery can reach a full state of charge



Also on a current sensing smart charging system you will have to hook all of the additional accessory grounds to the vehicle chassis or body.  You cannot hook them to the negative terminal on the main cranking (start) battery.


DC TO DC Charger

A Dc to Dc charger, can be installed on any year vehicle. (fixed voltage or smart charging system) Connecting inline between the positive cable of your auxiliary and cranking battery.  A Dc to Dc charger is a smart device that will boost or reduce voltage.  Creating a complete controlled fast recharge of your auxiliary battery.  Also acts as a smart switch.  To disconnect (isolate) the auxiliary battery from the main cranking (start) battery when its not being charged by the alternator (or solar panel if equipped.) Will also detect battery type, and charge the battery at the appropriate rate.

A Dc to Dc charger with dual input (alternator and solar) that is solar ready incase you would ever want to upgrade adding solar panels in the future.  An MPPT (maximum power point tracking) style solar controller/regulator is the best. Helps ensure maximum power from portable solar panels when in low light conditions.



Red Arc BCDC1225 is a 25 amp charger


Red Arc BCDC1240 is a 40 amp charger





  • A Dc to Dc charger should be located as close to auxiliary battery a possible
  • Some Dc to Dc chargers can really struggle under heat so take caution where you mount them so the have plenty air flow to keep them cool



Battery Management System (BMS)

A BMS will do it all so to speak.  All things combined in one it is basically a dc to dc charger solar controller/regulator you can also plug it into 110 Ac volts to charge the batteries when you are near a Ac power source.  It can charge different battery types.  And one to multiple batteries hooked together.  A BMS will isolate cranking to auxiliary battery.  Used in vehicles, campers, travel trailers and off road camper trailers.


Red Arc Manager 15


Red Arc Manager 30



 Solar Charge Controller/ Solar Regulator

A solar charge controller regulates the constant voltage from a solar panel to a battery.  So it doesn’t over charge the battery.  Check out more on solar panels here.




Portable Power Station/Solar Generator

No room under the hood for an  auxiliary battery or just want something more portable to put in the back of your 4×4.  Giving you the ability to move it around camp.   So you can run lights, portable 12v fridge, accessories and recharge electronic devices etc.  You can check out my on these units here.