Thread: Variable Voltage Alternators

Originally Posted by winaje

I'd be interested to know this, particularly as I have a 3 or so year old SC-80

I should hope so, as that's what Tim sold me for my MY13.

The D4-2S is listed as being the kit for the D4.

Hi again folks and if you fit any other VSR ( Voltage sensing Relay ) type isolator in any of these Low Operating Voltage vehicles, you will NOT charge auxiliary/house batteries anywhere near as well ( if at all ) as what my isolators achieve.

The reason for this is that most new vehicles with Low Operating Voltage, carry out some form of cranking battery analysis immediately after you start the motor.

The Battery Management System ( BMS ) can do this in a number of way but basically the BMS keeps the alternator’s output voltage at a low specific level and monitors the amount of current the cranking battery draws.

From the data gained, the BMS will then set both the initial BOOST charge cycle, where the voltage is run at a high level for a given time, and then the NORMAL operating voltage will also be set to optimise the cranking battery’s ability to get a decent charge while the motor is running.

Now for the difference between what my isolators achieve and what other isolator can not.

When you have been driving around town, with no accessories drawing power from your auxiliary battery, the two batteries will be ( should be ) near fully charged.

When you turn the motor off, with any other isolator, the isolator cuts out when or shortly after the motor is turned off.

My isolators remain cut-in ( ON ).

When you next start your motor, no matter which isolator you have, the BMS will see the cranking battery is near fully charged and will run a minimal short BOOST charging cycle.

No problems so far for any type of VSR type isolator.

Now you go on a trip and the same thing, you turn the motor off and then or a short time later, your ordinary isolator cuts out and you run your fridge, camp lights, camera and phone chargers off the auxiliary battery.

A few days later, when you are ready to go home or move to another camp site, you start your motor and because the cranking battery is still in a near fully charged state, the BMS, has no reason to set anything but a short BOOST charge cycle and then it lowers the voltage down to the normal low operating voltage.

So any low auxiliary/house batteries don’t get anywhere near the charge they need.

Now the same scenario using one of my isolators.

When you arrive at your campsite, your turn off your motor but this time, my isolator remains on ( cut-in ) and your accessories run off ALL your batteries, including your cranking battery.
If the power used, while you were camped, does not discharge your batteries down to 12.0v my isolator will remain on.

If the total power used discharges the batteries down below 12.0v, at 12.0v my isolator cuts out, leaving your cranking battery with 50% of it’s total capacity available to start your motor when needed.

When you start your motor to move to the next site, or go home, the BMS does it’s job.

But this time, after analysing the cranking battery, the BMS determines the cranking battery is in need of a charge, and sets up a different BOOST charge cycle, where the voltage is higher and/or longer during the BOOST cycle and it also may not take the voltage as low as it usually does in the normal low operating voltage level.

Because the BOOST cycle is higher and long, ALL batteries in the system receive the BOOST charging.

SIDE NOTE, there is a misunderstand that people have, that an alternator stops charge or lowers the charge CURRENT, once the cranking battery is full.

This is not so and an alternator, even with an advanced form of BMS, only alters it’s voltage. The amount of current being produced is based on the load being applied to the alternator at different voltage levels.

EG, if at a voltage of X the cranking battery is applying a 20 amp load on the alternator, the alternator will produce 20 amps a voltage X.

If you add another battery load which also places a 20 amp current load on the alternator when it’s at voltage X, the the alternator will now produce 40 amps.

This means all batteries connected to the alternator will receive a decent charge voltage and charge current.

With the way my isolators work, there is an additional charging advantage.

Because the ACCESSORIES load applied to the auxiliary battery is SHEARED with the cranking battery, if you use the same amount of power as you would with an ordinary isolator, with my isolator, your auxiliary battery will not be discharged anywhere near as low as it would have to be, when using an ordinary isolator.

This means, with my isolators, you batteries are not only going to charge properly, they are going to charge in roughly one third less driving time.

Hope this explanation helps to show why my isolators work so well with these new low operating voltage type vehicles, when other types of VSR isolators do not.