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Dual Battery Systems 

Before looking into any dual battery system, the question you need to ask yourself is “Why do I want a dual battery system?”  Once you have this answer, then you can decide on what type of system you need.

Option 1 (direct wired – non isolated):

If you want to purely use it for a winch (because the main battery is to small) you can directly parallel wire (+ to + and – to -) a second battery in.  If you go down the auctions in Brisbane or Sydney you will see that most of the ex-council/government vehicles with winches are wired this way.  Alternately
you can replace the main battery with a larger one.  If you are looking for battery leads, try obtaining leads for an arc welder (available from an engineering shop) or cut up a set of jumper leads.

Why Isolate?

You isolate the second battery so your additional electronic devices (fridges, lights, CD players, inverters etc) don’t empty your main starting battery while the engine is switch off.

Option 2 (Manual isolate)

The cheapest way to manually isolate the second battery is to remove one battery terminal connection each night or time you stop.  If you go to most marine or automotive stores they will have some sort of  manual isolation switch you can purchase (usually about $10-$15) which makes life a bit easier.

Here are two that I have seen used by many 4wders:-

The problem (or my problem) with these type setups is if you forget to reconnect them in the morning, your second battery doesn’t get any charge during that days driving.  I used to have the removable red key type isolator that I used to tie to the steering wheel each night so I would remember to put it back in the morning.

Why Auto-Isolate?

You install an auto-isolate system so you don’t have to manually do it (or remember to do it).  This type of system normally has a solenoid or relay to connect the batteries together during charging.  The solenoid is usually triggered of the vehicles “accessories on” power or the “ignition on” power.  The addition of a switch can make the system more controllable.

Option 3 (Auto-isolate with winch and switch)

The following is the circuit diagram for a system that I currently use.  The main items used are the “200amp continuous solenoid” (I paid approx $28 at Supercheap Auto for one) and a “dpdt h/d centre off toggle switch” ($3.75 from ST0576).

The switch gives me the options of:
  • Positon 1 (on – solenoid trigger power from Acc+ or Ign+): Connects both batteries when the engine is running, thus charging both at the same time.  When the engine is turned off, the second battery is auto-isolated via the solenoid opening).
  • Postion 2 (off): Second battery is always isloated and never gets any charge.  Use this when 2nd battery is out of vehicle.
  • Positon 3 (on – solenoid trigger power from second battery): Use this when main battery is flat and second battery is needed to start (direct wired jumper leads).

The finished setup looks something like this:-

Because the solenoid allows a minimum of 200 amp passage, it will aid the main battery in winching operations (wire winch direct to main battery).  The solenoid only draws 0.7amp to operate, so uses very little power.

Option 4 (Auto-isolate and switch, no winch)

If your not running a winch, use Option 3 but change the solenoid to a large relay (say 30 amp) and replace connecting battery leads with 30amp wire.  This will save you a heap $$$.

Option 5 (After market black boxes)

There are plenty of after market systems available, all have different options.  Some will suit your needs, others will not.  Prices vary from $60 (solder yourself) kits up to $300+ units with all sorts of flashing lights.  (Just a personal experience with the KA1782 “Smart Dual 12VBattery Isolator Kit For 4Wders”,  This unit does work but when the motor is running, it only allows an 8 amp charge to go to the second battery.   This can make the charge time of the second battery quite long.

Mick F

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