Scion xB EV – DC/DC converter

This page discusses the DC to DC converter (or DC/DC) in my Scion xB EV conversion.

Purpose

The DC to DC converter (DC/DC) takes the place of the alternator in a normal gas powered car.   It converts the traction battery voltage down to 12 volts to run the body electrical system and charge the SLI battery.  The body electrical system includes all the normal 12v systems in a car: lights, turn signals, wipers, heater fan, radio, and other automotive electrical systems.    Often, the EV specific components of the car such as the inverter or controller also requires a source of 12V electricity as well in order to function.

Without a DC to DC converter, just as if the alternator in a regular car was not working, the 12V battery would ultimately run down.

Sizing

Input Voltage

DC/DC converters whether commercially made or something you hack together will have to work over the entire voltage range of your traction battery, from peak charging voltage to nearly depleted and under load.   This can be a 20% to 30% difference in voltage.   For example,  the 320V nominal traction battery in the scion xB can range from 250V to 360V.   In my experience it is a good idea to select/or design for an input voltage range that puts the peak charging voltage as close to the maximum input voltage of the DC/DC as possible.

Output Wattage

The second factor to consider is output wattage.   Basically, the DC/DC needs to be able to put out enough power to keep the body electrical system running under the worst case conditions,  which is typically night driving in the cold and rain, which requires lights, wipers, and heater fan, which are the biggest loads on the system.   It should be able to maintain enough output voltage to not force the SLI battery to discharge under these circumstances.    A good starting point for determining this is the original alternator output wattage.   Most small cars have 40 amp, or 500 watt, alternators.   Larger cars and trucks have 60 or 100 amp alternators.

It is possible to reduce worst-case DC/DC loading (and thus wattage requirements) by a couple of amps by replacing the running and indicator lights with LED bulbs.  This can cumulatively reduce the worst case loading by several amps with no down side except the (minimal) cost.   I did this with good results.  The only gotcha if you replace the turn signal bulbs is you will need an LED compatible turn signal flasher.   These are available for modern cars.

The xB had a 40 amp alternator, and I sized its DC/DC to match.

Output Voltage

Technically speaking,  the DC/DC in an EV with a 12V body electrical system should actually output from 13.8 to 14.2 volts.   This is to allow proper charging of the 12V lead acid SLI battery that is expected to be there.   The exact voltage will vary depending on the kind of SLI battery you are using.  A normal automotive SLI battery should see about 14.2 volts.   An AGM or Gel SLI battery, such as from a computer UPS, should be a little less.   Also, many automotive bulbs and other electrical functions are actually designed to operate properly on slightly more than 12V for this reason.

I selected an output voltage of 13.8V for mine.

Installation

Size and Location

engine compartment largely completed.

DC to DC is black box at far end of engine compartment.

The DC/DC is relatively small.  Modern DC/DC units range from the size of a paperback novel up to the size of a webster’s dictionary.   As such, physical mounting is not generally a problem.

The DC/DC should be mounted outside the passenger cabin owing to the high voltage present, but near to the SLI battery and a source of traction battery voltage.   It should be someplace where it is relatively well protected from water spray and road dirt.   The trunk or the engine compartment firewall are common locations.

The Chassis of the DC/DC and its negative output terminal should be grounded to the vehicle chassis.

If the DC/DC does not have a built in heat sink, one should be provided.    It should be positioned, if possible, to allow air flow across it as the vehicle drives.

My DC/DC, which is somewhat large, is located in the engine compartment above the driver side wheel well.   This was the best fit location.

Cooling

Most modern DC/DC units are around 90% efficient.   They do not require really aggressive cooling, but depending on their location, size, and expected load, a cooling fan aimed at the heat sink may be a good idea.   Fortunately, worst case loading conditions (night, rain, heater fan running) generally occur in cooler weather (at least in Seattle), which will help keep the DC/DC from overheating.

In my case, the Solectria DC/DC housing has a very large heat sink that was designed for much more heat generation than the current electronics.  As such,  I did not provide a cooling fan, though I did mount it with the fins vertical to encourage convective air flow.   I have never observed the DC/DC to get more than slightly warm.

Electrical

There are several approaches done with conversions here.  In my opinion the best approach is for the DC/DC to be switched from the ignition so that it only turns on when the vehicle is turned on. (just as an alternator only runs when the engine does)    This requires a high voltage DC rated relay to allow the 12V ignition to switch the high voltage going to the DC/DC.    I used a 60 amp, 400VDC rated relay from ebay.   (identical to the one I used to switch high voltage to the heater core)   This is total overkill as my DC/DC running on 320V, will draw less than 2 amps and full load.

Some people do not bother to switch the DC/DC and let it run all the time.   This is dangerous as either the owner or someone else may not expect that circuit to be live when the car is parked with the ignition off.   This also puts an unnecessary parasitic load on the traction battery while the vehicle is not in operation.

One thing to consider if you do switch the high voltage input to the DC/DC as I did, is that if the 12V SLI battery goes dead,  you will not be able to engage the DC/DC and thus “start” the car even though the traction battery may be fully charged.    You can install a “self jump” switch to bridge the relay and start the DC/DC.   The only thing to be careful of if doing this is that the switch you use is rated for the high voltage and DC current.

If dealing with high voltage traction battery (greater than 72V) make sure you use wire with insulation rated for the voltage.    Wire bought at auto parts stores is rarely rated for more than 72V.   Better quality wire will be rated for 300V or 600V.    Even then,  It is not a bad idea to provide additional layers of insulation, such as corrugated sheathing or a plastic conduit to protect the supply wires.

The input and ouptut of the DC/DC should both have dedicated fuses or circuit breakers.

The output of the DC/DC should be routed straight to the SLI battery using wire of size appropriate for the maximum amp load.   This is 10 gauge wire for a 40 amp load.   All the vehicle loads should come off of the other side of the battery.   This is to ensure maximum DC/DC output voltage at the SLI battery.   If instead the wiring is done such that there are automotive loads between the DC/DC and the SLI battery, it may cause charging problems.

Options

There are several options to implement a DC/DC in an EV conversion.

EV-Specifc or OEM DC/DC

You can buy EV specific DC/DC converter modules from many EV parts retailers.   They come in various input voltage ranges and output wattages to fit your needs.   Pricing range is usually around a couple hundred dollars, with the bigger and higher voltage ones being more expensive of course.

This type of DC/DC while often the most expensive are designed for a vehicle environment and will likely be very robust and reliable if set up right.

RV Battery Charger

RV battery chargers are designed to run off of 120V and keep the 12V house battery and any 12 volt loads in the RV running.   Ones that are of a switching power supply design such as made by inteillipower, among others, are capable of running from a DC power supply as well.    Conversions that have a nominal traction battery voltage from about 144v up to about 192v are good candidates to use this approach for a somewhat lower cost.    I used one of these with my 126V Toyota MR2 EV conversion.   It worked adequately but did have problems making enough power when the traction battery voltage got too low.

Downside with these are they are typically open frame design and will let dirt and moisture into the electronics, so will require better protection.  Also,  you will need to find one that is known to run off of DC supply voltage, or experiment (and risk smoke) to test.    The thing to do if you have to test one yourself is open it up and inspect the input stage.   At a minimum, you need to ensure the large input stage capacitor(s) are rated for about 130% of your expected maximum traction battery voltage.   If they are,  you can then carefully try running it on a range of DC voltages and loads to see how it behaves.    I have observed switching power supplies do bad things such as overheat, even under no load,  when running on DC at certain voltages, so beware.

Computer Power Supply

Very similar to the RV battery charger.   May not be suitable for running the whole car, but may work for a dedicated load such as a computer.    I used this approach for that purpose in my MR2 with success.

Pros:  Cheap.   Many will be able to run on DC just as the RV battery chargers do but also may have similar issues such as burning up or otherwise failing when running under too low of an input voltage.   The downside is most have multiple output voltages, not all of which are useful,  and the 12V outputs are exactly 12V, not the 13.8 to 14.2 needed to charge a lead acid battery.    Also rather big for what you get.

Homebuilt

This is the route I chose, and is not as bad as it sounds.   You can buy DC to DC converter electrical components that are very compact and do the heavy lifting for you.   These are at the heart of many commercially made EV DC/DC converters.   For example,  Vicor makes multiple DC/DC modules in various voltage and wattage ratings.   These are extremely compact.  High voltage, High wattage ones are of course more expensive, but they are readily available.  Mine is a 400V input, 15V, 40A output unit.   Lower voltage or lower wattage ones (still potentially ideal for an EV conversion) are common on Ebay, while I ordered mine from Digi-key.   In my experience they are somewhat delicate (I fried one during in-car testing) but once installed with appropriate care and feeding   (filter capacitors, surge limiter, EMI filter, etc) should be reliable.   My second one has been.   See more notes further down on what I did with mine.

Alternator

It is not very elegant or efficient,  but it is possible to retain the original alternator in an EV conversion.   to do this you need to power it from the EV traction motor.   DC motors are often available with a tailshaft, and you can attach a belt pulley system to that to drive the alternator just as in a gas car.

The only advantage here, at least at first glance, is cost.   Usually there is already an alternator in the gas car.  However once you figure the extra cost of the mechanical engineering and materials to mount it and the limitations it puts on motor selection, it really is not an advantage.

The downside here is terrible efficiency compared to an electronic DC/DC, and it does not run when the car is stopped (unless you find a way to make the motor “idle” which is also wasteful, but has been done with conversions of automatic transmission cars)    The setup will also be heavier than an electronic DC and more complex and probably less reliable as it would involve designing a belt drive with tensioner, etc.

I do not recommend going this route except in cases of zombie apocalypse.

Nothing

Technically,  you can run your 12V system off of the SLI battery, and charge it separately as needed.   This may be practical for race cars or dragsters, or very lightweight experimental vehicles.   I do not recommend it for a reliable daily driver however.

Solectria DC/DC Rebuild

I rebuilt an inoperative original Solectria DC750 DC/DC converter module to use in my Scion xB.

Original Condition and Teardown

When I purchased my Motor and Inverter I also acquired two Solectria DC750  DC/DC converters.  These are about the size of a Webster’s dictionary and are sealed against weather and have a large integrated heat sink.

Unfortunately, neither unit that I purchased was operational as it turned out.   One was an open circuit, the other smoked.  When I disassembled one to inspect,  I found it consisted of four Vicor 300V input 200W DC/DC converter modules wired in parallel with a small amount of peripherial components.  I did some research on these and found that these 10 year old modules could be replaced with a single modern equivalent (Vicor V375A15C600BL) that made 600 watts.

The Solectria DC750 units initially appeared to be well engineered, but they had several design and construction problems.   First,  the 300V nominal input voltage was a bit low given the rest of the system has a 360V operational limit.   Second,  the modules were not well mounted against the heat sink as instead of being attached via their mounting pad with six screws each, the four modules were held down by two aluminum cover plate that each held a pair of the modules, which had only two screws to hold them, meaning the modules were not held tightly against the heat sink.   When being assembled, far too much heat sink compound was used which may have had the effect of insulating the modules instead of cooling them.   Finally,   there was not sufficient protection circuitry.   There was no output fuse though there was an input side fuse,  and the input side filter capacitor was an inappropriate electrolytic that had failed due to heat or high voltage, and there were no filter capacitors to ground as recommended by Vicor.    Due to some combination of these issues the units probably were never very reliable.

These units are still seen on ebay and other venues for many hundreds of dollars asking price.   I would not pay too much for one given these issues.   If known working,  maybe one or two hundred.   The housing is robust and makes a good box for building your own, but the original guts are not worth much in my opinion.

Redesign and Rebuild

To maintain the Solectria appearance, my approach was to reuse the original Solectria DC750 housing, but to update the internals.   I did some research on the original four 200W vicor modules, and found that these 10 year old modules could be replaced with a single modern equivalent (Vicor V375A15C600BL) that made 600 watts; the rough equivalent to the original alternator.  This vicor module also had a higher input voltage rating and higher output voltage rating, meaning it should be running with more safety/design margin on my 320V traction battery.    Owing to the amount of space freed up,  I also placed the following components in the Solectria housing:

  • 400V, 60A DC rated switching relay
  • Filter capacitors as recommended by Vicor, except bigger on the input side.
  • ATQ5 internal supply side fuse (rated for 5 amps and 600 volts)
  • 12 volt, 100 amp automotive style output side fuse and holder.
  • A CL-60 input surge limiter to limit input surge current.
  • Input side polarity protection / isolation diode
  • input side EMI filter from a computer power supply

The last 3 items I added after I burned up the first Vicor module during testing.  ($300 please)  I was running the car off of my “bad boy” variac from 120V line power, and the EMI noise from the inverter and/or pulsed DC from the variac destroyed the first module, though it had worked fine from that power source while testing on the bench.

The 15V output Vicor module can be tuned to output a more appropriate voltage by adding a single resistor across its output pins, and their datasheet includes a link to an online calculator.   I tuned mine to put out 13.8V as my SLI battery is an AGM battery for a computer UPS.

Secondary DC/DC

my Scion xB has a secondary DC/DC which is intended to run continuously.   It is smaller (by half. physically and in terms of wattage) than the main Solectria DC750 unit.   At its core is a Vicor V375B15C300AL2 DC/DC converter module.   It is mounted at the rear of the car with the rest of the charging equipment.   The intent is that this one will power the onboard computer, once installed,  and also provide power to the J1772 interface and BMS during charging.

This unit is basically a smaller version of the rebuilt Solectria unit, with a smaller Vicor DC/DC module and similar care and feeding circuitry.   I won’t go into detail on this one as it was very similar to the Solectria rebuild.   The only differences in the design are this one has a power switch and indicator light since under normal circumstances it will run even if ignition is off, and I did not include an EMI filter on the input side, owing to lack of space and further distance from the inverter.   Note the picture of the internals do not show some of the protection circuits I added later including the surge limiter, isolation diode, and a larger filter capacitor.

While I haven’t done it yet,   It is possible to set this one up as a “backup” DC/DC for the main one with the addition of some switches or jumpers.   I have not done this yet but I might in the future.

Next Topics

Heater

Lithium Batteries and BMS