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WHEN you shut down the internal combustion engine (ICE) on a hybrid or plug-in hybrid at every stop sign or red light how do you keep the alternator spinning? We still need a constant 12 volt source don’t we?

That little 12 volt battery has a larger brother that helps out, not only when the ICE is off but any time the big battery is hooked up to the DC-DC Converter (Converter for short).

It is the high voltage (HV) battery pack that supplies the 12 volt system with power, but at a much lower voltage (lv). In between the HV and lv is a step down converter known as the DC-DC converter (main image above).

The converter acts like the alternator in that it supplies the 12 volt system with power and charges the 12 volt battery.

A traditional 12v alternator is not fitted to a hybrid. The converter gets its power from the HV battery pack. The HV battery pack is not always connected to the converter, so there are times when the car is using the 12 volt battery, such as when the key is in the accessory position. 

Converters can produce any dc voltage supply needed, but hybrid vehicles must have one that makes 14 volts (Fig 2). 

Some hybrids such as a 2007 Toyota Highlander HEV (sold in Australia as the Kluger but without a hybrid option) have another converter that produces 42 volts dc for the electric power steering system. The Highlander also has a 12 volt DC-DC converter as well (Fig 3). 

In order to keep the tech safe the converter takes in HV DC and then inverts it to ac first and then the ac power in reduced in voltage through induction (much like an ignition coil in reverse) back to a much lower ac voltage (Fig 4).

Then the lower ac voltage is inverted into DC power at the lower voltage.

If something goes wrong, there is no direct connection from HV to the lower 12 – 14 volt DC. That can be done with coils of wire that transfer electrical energy without a direct connection. Even though some inversion is done, we call this device a DC-DC converter. 

Other than testing power, grounds, inputs and outputs this will be serviced and replaced much as you do an ECU. 

Honda and Ford (Fig 5) sell it separately but most Toyota and Lexus models sell it as part of a much larger assembly known as the inverter assembly. 

How are these DC-DC converters holding up? Think back to your last class on battery /starter/ alternator.

Remember the part where a bad 12 volt battery left in the car can cause a premature death to an alternator? Well guess what can kill a DC-DC converter?

You guessed it, a defective 12 volt battery will put a strain on the converter and it may fail. I provide hybrid tech support as part of my job and I have seen a problem with air-cooled converters that relate to older hybrids and chemically worn out 12 volt batteries.

Some converters are liquid cooled and these seem to be holding up better. The Belt / Alternator / Starter (BAS) hybrid system used on some GM vehicles sold in America have what looked like an alternator with three large blue cables attached.

These BAS systems runs on a 3 phase ac 36 volt system and a 36 volt dc battery. The BAS hybrids use a converter for 12 volt dc and they can be a problem. They are no longer produced. 

Testing is pretty simple. Use your existing equipment for 12 volt battery / starter / alternator and get the hybrid into READY mode. Then load test the 12 volt battery and watch the meter for output and ac volt (sometimes called ripple) that you would associate with the alternator.

You can read out the voltage output on your scan tool (Fig 6).

If you have a charging problem, test powers and grounds. Get high voltage safety training and the proper personal protection equipment and a CAT III 1000 volt meter.

Also remember the source of power used by the converter to produce the 14 volts and current is the high voltage battery pack and that any problems with the pack can negatively affect the DC-DC converter.

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