Thread: Dual batteries?

Originally Posted by Daniel

I know that in RVs they never ever put the aux battery inside the cabin.

Yes they do and the most common and safest place, is under an open bed or seats.

Originally Posted by Daniel

I assume that there is some standard (ADR etc) that addresses the issue. I suspect that if a battery is mounted inside a vehicle cabin then a warning sign must be placed.

Hi Daniel and there are no specific rules and no need for them.

There are also more than two dozen “NEW” vehicle models, here in Australia, with a battery mounted inside the cab and you NEVER hear of problems, plus there were 25,000,000 old VW Beetles with their cranking battery mounted under the rear seat, inside the cab and again, no exploding Beetles.

Here are the full operation instructions.


After finishing the installation of the DT90-L322 kit, start and run your motor for a few minutes.

Depending on which version of the L322 you have, it will take anywhere from 30 seconds ( cold motor start ) to a couple of minutes before the alternator voltage rises above the Cut-In level of the DT90.

Once the DT90 is cut-in, the GREEN LED will glow constantly, and you will now have power at the rear power socket.

When the motor is turned off, the GREEN LED will still glow constantly until the surface voltage in the batteries drops below 13.0v. This could take just a few minutes or all night.

Once the common battery voltage drops below 13.0v, the GREEN LED will glow constantly but there will be a short RED flash about every 2 seconds. This is to indicate the common battery voltage is still up the the surface charge level. ( between 12,7v and 13.0v ).

Again this could take just a few minutes or all night.

Once the common voltage of the batteries drops below 12.7v, ( the full charged state of a lead acid battery ), the GREEN LED will glow constantly for two seconds then the RED LED will give 5 short pulses, then the GREEN LED will again glow for two seconds, then 5 RED pulses, and this is continually repeated.

Other brands of isolators shut down between 12.5 and 12.7V

When the voltage at the DT90 drops below 12.7, this is where the isolator goes into what is called the SHARED Mode, where the DT90 randomly shuts down two of the three 30 amp relays. This is done to both extend the operating life of all the relays but it also reduces the DT90’s power consumption from around 215 ma to 115 ma.

While being used to power accessories, like a fridge, lights, an inverter, or what ever, as the batteries common voltage drops, the number of RED pulses will decrease to match the amount of usable capacity remaining in the batteries.

This is a form of a battery fuel gauge.

When the common voltage gets to 12v, the number of RED pulses will go from 2 to 1 and at that point, the cranking battery will be isolated.

The 2 second GREEN with a single RED pulse will continue, the 1 RED pulse indicates you are now powering your accessories from 1 battery, the AUXILIARY battery.

When the Auxiliary battery’s voltage drops below either 11.8v or 11.6v ( depending, on what position you have Switch 2 set in ), the DT90 will turn off the ACCESSORIES output to protect the auxiliary battery from being over discharged and the GREEN LED will give short flashes, indicating the DT90 is now turned off.

The biggest single advantage with using any of my isolators is the SHARED mode operation. By allowing up to half of the cranking battery’s reserve capacity to be used to power your accessories, you increase the amount of your auxiliary battery capacity by at least 50%

In the specific case of the L322 Range Rover set up, with just a 55Ah auxiliary battery, there is a 100% increase in available capacity.

There is also another advantage to this SHARED mode. With continual short drive use of any vehicle, the cranking battery is never properly RECHARGED after starting the motor.

With the DT90 and an Optima battery, because the Optima is a genuine fast charging battery, after a short drive, the Optima will always be in a higher state of charge than the cranking battery.

When you turn your motor off, the two batteries will settle back to their respective states of charge, but because the Optima will always be in a higher state of charge and combined with the fact the DT90 keeps the batteries connect, the Optima slowly discharges back into the cranking battery, progressively raising the cranking battery’s settled voltage.

Over a period of one to three weeks after you install the DT90 kit, your cranking battery will eventually be continually maintained in a fully charged state at all times.

This means your motor will start easier, and as your cranking battery is not working as hard to start your motor, it will have a longer operating life.

This info is based of feedback from users of my isolators over the last two decades and the advantages are more relevant now than they were 20+ years ago, when we started making the SC40 and SC80.