How to use a BCDC to charge a secondary battery in any vehicle

A REDARC BCDC*D can be used in either 12V or 24V vehicle systems, to charge 12V auxiliary (aux) batteries to 100% SOC from all common vehicle alternator systems and from solar panels.* They are available in the 25 amp BCDC1225D, 40 amp BCDC1240D and 50 amp BCDC1250D sizes.

Battery types

It is suitable for all common automotive/recreational/deep cycle batteries, lead types (flooded, AGM, Gell, Calcium Lead Crystal) and Lithium (LiFePO4).

Fixed Voltage alternators

While a BCDC* is not essential (as the voltage produced by a fixed voltage alternator is high enough to give a full charge to an aux battery) there are still benefits in using a using one of the REDARC BCDC* chargers. These benefits include:

  • Charging direct from an alternator or through an isolator does not limit the charging current and it is possible for the aux battery to be damaged by being charged above the manufacturer’s recommended maximum current. A REDARC BCDC charger limits the charging current to a safe level and the size BCDC should be chosen to suit the battery manufacturer’s recommended maximum charging current.
  • A fixed voltage alternator can charge your aux battery at 14V or more all the time you are driving, but most battery manufacturers recommend a lower “float” voltage. When a REDARC BCDC charger recognises the battery aux is fully charged, it drops to lower float voltage (13.3V).

When deciding whether to charge direct from a Fixed Voltage alternator or via a DC to DC charger, you should check:

  • What current the aux battery could potentially be charged at, compared to battery manufacturer’s recommended maximum.
  • The alternator’s charging voltage compared to battery manufacturer’s recommended float voltage.
  • The long term effect of charging at that voltage or current (as advised by the battery manufacturer).

Temperature Compensating alternators

When Temperature Compensating alternators started appearing in the recreational automotive market in the early 2000s, it quickly became apparent that deep cycle aux batteries were not being fully charged (in many cases, not much more than 70% SOC in normal use), meaning camping equipment including fridges and lights Etc. were not running as long as they had with Fixed Voltage alternators. To overcome this problem, REDARC developed the BCDC* (Battery Charger from Direct Current input) range of in-vehicle 3 stage battery chargers to ensure recreational aux batteries had a 100% charge, tailored to different types by selectable charging profiles. The original BCDC* chargers were designed to, via start battery voltage sensing, automatically switch on when the vehicle was running and switch off when the vehicle was not running, effectively acting as a battery isolator, ensuring that camping equipment Etc did not flatten the starter battery.

Variable Voltage or “smart” alternators

Around 2011-2012, vehicle manufacturer’s, seeking to decrease fuel consumption and emissions (in line with ever tightening environmental targets) determined that one contribution would be (after ensuring the starter battery was sufficiently recharged) to significantly reduce alternator charging voltage, thus reducing alternator load and therefore reduce engine load to help meet those targets.

This lower voltage (often as low as 12.3V) was too low to charge deep cycle batteries at all and was below the automatic turn off voltage of the BCDC*, meaning that the voltage sensing suitable for previous vehicles could no longer be relied upon to effectively charge the aux battery.

REDARC overcame this problem by re-purposing blue wire of the BCDC* to be activated by vehicle ignition to over-ride the low voltage cut out while the vehicle is running.

This ensured that the REDARC BCDC* with another “D” on the end could ensure that even in “smart” alternator vehicles, the aux battery could be 100% charged.*

The dual input BCDC*D range is suitable for all vehicle alternator types

*large capacity battery banks may require a few cycles to fully charge.

Where to connect the blue wire?

The blue wire only needs to be connected to ignition when the BCDC* is installed in a “smart” alternator system. For all other alternator systems, it should be left either unconnected (or, even better, grounded to stop it accidentally contacting anything else).

There is no “universal” connection point, as it varies from vehicle to vehicle. It is only a trigger input that draws minimal current (in the order one microamp), so it will not interfere with any of these typical connection points provided they are only active when ignition is on:

An original vehicle fuse.
A DC supply socket, such as a cigarette lighter or 12V accessory socket.
The supply line to original vehicle equipment such as wipers, power windows, external mirrors, dashcam, GPS, Etc.

Any potential connection point should be validated by checking for the presence vehicle battery voltage with ignition on and no voltage when ignition off.

Not every vehicle will have all of these options available but should have at least some for the installer to choose from. The choice of connection would normally be the most readily available one.

Solar input

The REDARC BCDC*D chargers feature an inbuilt MPPT solar regulator and incorporate Green Priority.

Which BCDC*D to use?

You should choose a charger that has a current rating within the battery manufacturer’s recommended maximum charging current. As a general rule, battery bank sizes of 75-200AH have a recommended charge rate of less than 40 amps, so a BCDC1225D would be the better choice. A higher current charger should not be used because excessive current flow into a battery can impact it in a number of ways, the worst of which can be the degradation of battery service life and capacity.

For a larger battery or for a bank of two or more batteries, a BCDC1240D or BCDC1250D could be used.

Can it charge 2 batteries in different locations?

For information on charging 2 batteries in different locations, see one BCDC for two batteries in different locations.

Can a BCDC*D also charge the start battery when charging from solar?

While there is no internal provision in the BCDC*D for charging back to the start battery, this can be done with some additional wiring and components.

What is the difference between DC-DC and BCDC?

A DC-DC converyer converts one level of Direct Current to a different level of Direct Current. There are many DC-DC converters with many different purposes world wide. One of them is the DC-DC converter REDARC makes specifically for battery charging, with our part number BCDC**** (standing for Battery Charger Direct Current).

So in a nutshell, the difference between “DC-DC” and “BCDC” is like the difference between “4x4” and “Hilux”.