Dual battery pack

QUESTION: What is a dual-battery and how do you pick the right system?

ANSWER: (Supplied by National Luna)

Established in 1987, National Luna is a leading specialist in 12V outdoor products, and the preferred supplier to all major off-road trailer and caravan manufacturers in South Africa.

In its simplest form, a dual-battery system is nothing more than a second battery in your 4×4 that allows you to power various electrical components (camping lights and/or a 12V fridge) without running the risk of discharging the main (starter) battery of your vehicle.

The dual battery (also known as auxiliary or slave battery) is generally connected to the vehicle’s main (starter / cranking) battery so that both batteries can benefit from the vehicle’s automated charging system − the alternator.

The problem, however, is finding a way to isolate the two batteries to ensure that when the auxiliary battery is in use, it doesn’t also drain the main / cranking battery.

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In the early 1990s, many campers, boaters and 4×4 enthusiasts used a very basic dual-battery set-up that consisted of a manual switch (usually a Cole Hersee) which was turned on when you wanted the battery to charge (while driving) and turned off once you were in camp and needed to isolate the auxiliary system.

Although this set-up is still regarded as absolutely bulletproof and highly effective, in reality many users forget to disengage the switch (which results in a dead battery); so the system isn’t all that practical.

Sometime in 1995, National Luna tried to solve this problem by automating the isolating system; but in order to do this we needed a switch that could handle high amperage while also remaining absolutely reliable and long-lasting. The answer lay in the Warn Winch, which (at the time) used a heavy-duty solenoid that could handle more than 85 amps continuously.

A dual battery solenoid system
Dual battery systems are generally installed inside your vehicle’s engine bay – as high as possible and away from moisture.

We adopted the same solenoid system, and created South Africa’s first fully automated dual-battery system. More than 20 years later, the solenoid system has remained unchanged, and has proved to be the most reliable (and safe) means of recharging and isolating two batteries.

The debate (or contention) that often follows any mention of a dual-battery system on an open forum, is whether or not the solenoid-based system has been replaced by newer (far more expensive) technology, specifically the DC to DC chargers.

Unfortunately, there has been no evidence to confirm this theory, so the arguments are entirely subjective and opinion-based. What’s more, the subject of dual-batteries is further complicated by many other questions which need to be answered before a system is installed. These include:

  • Battery application: deep-cycle versus high-cycle versus hybrid.
  • Battery construction: wet cell, dry cell and gel.
  • Charging / Isolating system: solenoid versus DC to DC.
  • Vehicle alternator system and output (voltage & current) rating
  • Typical driving time / distances between stops

“The point to remember is that one man’s dual-battery needs aren’t necessarily the same as another’s; there is no one-size-fits-all answer. Applications vary considerably, as do people’s budgets.”

Anyone considering a dual-battery system should first consider the following points:

  1. Start your vehicle’s engine and measure the alternator’s charge / voltage output when cold. Be sure to load the system by turning on the headlights, radio and air-con. (Typical readings should be between 13.7 and 14.5 volts).
  2. Repeat the same test when the engine is warm and running at its normal operating temperature. (Some alternators have compensating intelligence which drops the voltage when a high engine-bay temperature is detected)
  3. If your alternator’s output is below 13.7 volts, then you should consider a DC to DC charger. Alternatively, you may be able to increase the alternator’s regulated voltage – consult an auto electrician.
  4. If your alternator’s output is above 13.7 volts, an intelligent solenoid system is the preferred set-up; and it will generally recharge your battery much faster than a DC to DC system.
  5. DC to DC chargers can be up to nine times more expensive than an automated solenoid system.
  6. Regardless of which dual-battery system you opt for, neither set-up will fully recharge your battery in less than 12-hours, which is an impractical driving time for most people. For this reason, it is vitally important to trickle / float charge your battery in order to prolong its life and achieve a 100% recharge state. This can be done either by using a high-quality AC (mains) charger, or through the use of a solar panel system.
  7. In most instances, a high-cycle battery will recharge significantly faster than a deep-cycle battery, and so may be a better solution for some people – depending on your typical driving durations per day. However, the downside to using a high-cycle battery is that it doesn’t offer as many deep discharge cycles and/or stored power.
  8. Most dual-battery systems will not restore battery power to a practical level in less than three hours of driving time. This limitation is often due to the battery itself.
  9. A specialist battery (such as an AGM dry cell unit) can offer a shorter recharge time than a flooded / wet-cell battery.
  10. A battery is a consumable item that degrades with use and time. Never leave a battery (regardless of type) partially discharged, as this will reduce its life.

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