In dual battery setups, the common wiring method often causes the first battery to receive most of the charge while the second charges slowly, due to resistance in the positive cable.
The preferred wiring method connects the first battery’s earth to the second battery’s negative terminal, balancing resistance in both positive and negative paths so both batteries charge evenly.
When using an isolator, similar imbalance can occur; matching resistance in the negative wiring to that of the positive wire and solenoid contact helps correct it.
REDARC recommends seeking the support of a qualified auto electrician or technician.
Important
For your safety, REDARC recommends installation by a qualified auto electrician or technician. Our trusted REDNetwork is made up of professional auto electrical businesses certified by REDARC to install the complete of REDARC solutions. Find Your Local Installer
The usual connection
When a vehicle has a second battery installed, even if both batteries are the same size, type and age, it is often found that one battery receives the “lions share” of charge and the other takes a much longer time to become fully charged. A major cause of this is the method of wiring the two batteries together. Most installations involve the addition of the second battery to the standard factory installed battery and the simplest way is to wire it this way.
Because of the resistance in the POS wire connecting the second battery, most of the current from the alternator takes the “path of least resistance” and goes through the first battery. This results in the first battery receiving more charge than the second.
The preferred connection
There is a way of overcoming this problem, eliminating it as a cause of unequal charging. This requires the first battery to be earthed at the second battery negative terminal as shown:
When the batteries are wired this way, the effects of resistance in the POS and NEG wires cancel each other out and both batteries receive a more equal charge.
Isolator connection
Sometimes, when an isolator is used, this same effect is noticed. In this instance, the ideal would be for the resistance in the NEG wire to be the same as the resistances in the POS wire and the solenoid contact combined. A good quality solenoid has extremely low resistance and may not contribute too much to the effect, but if this is suspected as being a problem, an increase in length of the NEG wire will reduce the problem. If solenoid contact resistance cannot measured or required wire length calculated, some "trial and error" may help select any additional NEG wire length.
While this may sound difficult, it will in most cases solve the problem without the expense of a more sophisticated alternator or electronic "black box".