May 13 2011

DC/DC converter.

It's been a while since this site was updated, but actually more work on the project was done than reflected here. Major delay happened because the drive systems were not functioning as they should and I had to find a way to fix them - that process by itself took me about two years and deserves whole another web site. The things, however, get sorted out and persistence pays off, it just was happening far slower than I had expected.

Mean time unrelated subsystems of the car were worked on, namely installation of a DC/DC converter and finalizing mechanical arrangement (motor attachment, custom half-shafts and differentials).

The role of the DC/DC converter in an EV is self evident - because you no longer have alternator supplying power for your lights, wipers, audio equipment and to whatever else needs 12V to run, you need a substitute for it. The DC-DC converter does just that converting traction battery DC power to 12V auxiliary DC power that will be supplied throughout existing 12V net in the vehicle. The main requirements for such converter are (in about the order of importance):

- Should handle entire input voltage span
- Should handle max continuous and peak power demand
- Should tolerate short circuit without damage
- Preferred to have output adjustment
- Preferred to have remote enable
- Be as small and light as possible (preferably no larger than stock 12V SLI battery so it could take its place
- Be reliable as there are no other power sources are planned (more on that later)
- Be as inexpensive as possible, which [as always] contradicts all the above. It is not practically possible to have it all, so I just have to prioritize and choose without compromising key requirements.

What will be the power demand from 12V source? No doubt, the best way to find out is to measure it with DC camp meter before you start your conversion (use someone else's car or one on the dealership lot if you already started conversion and yours is taken apart). The meter should be clamped around positive output cable from the alternator, but if this is not practical, clamping around positive cable from fully charged 12V aux battery will be acceptable. Just keep in mind that if you measure around the battery, it could be ~ 20% lower than actual. With parking lights on, fog lights on, high beams on, stop lights on, wipers running, CD exchanger or radio on and playing, power seat on the move + whatever known load you plan (like 12V portable refrigerator), + about 10A margin,  that will be about the current your DC-DC converter should be able to handle on average in order not to fail on you at the most inconvenient moment. That assumes all "hidden" subsystems are on as well (airbag and ABS controllers, climate control, BMS, dash electronics, and other subsystems that is always on by default). That current figure times 13.8V will be your average power requirement. Typically the peak power converters can output is 20%...50% higher, depending on the duration and manufacturer.

The rough estimation though can be made without the measurements. Just look at the spec for the light bulbs (or HID lights controller if you have HID lighting system) and other subsystems. In my case each stock headlight requires 60W of power, so 120W total for that. The lighting around the car (including stop lights) in my AUDI is replaced with LEDs, but the stock brake lights were 25W each, or 50W total.   typical wiper motor will consume 10-12A when running and very roughly any given motor moving things in your car such as motorized sunroof, power windows etc.). The big one will be the power steering pump if I choose to run it from the 12V power source - that is as much as 70...90A peak at start and 40...50A running - very heavy load. In my case I will be running power steering pump (as well as A/C compressor) from the main traction pack so this will not be the factored in for the DC/DC converter power requirement estimation.

So, for the lights alone I need 170W which is 170/13.8=~12A.  10A for the roof and windows, 10A for the seat (I got heated seats, that's 30A for both) figure 15A for all the stock subsystems + 10A margin, that's 90A. 100A would be good rating, that is about 1.4 kW of output power.

There is not that large selection of units capable of 700+ VDC input and meant for the job. If there is no suitable unit in existence, there is always an option to design and make one, but the one I found is close enough to the wish list above - this is BRUSA BNW415 model. Its main ratings are:

- Nominal input voltage range: 250...760 VDC (overvoltage protection limit is 780 VDC).
- Nominal output voltage: 13.8 VDC (user adjustable within 11...14.5VDC limits)
- Continuous output current: 80 A @ 25'C
- Peak output current: 125 A (adjustable within 10A...125A limits)
- Efficiency: 92% peaking at 15A output, dropping to 86% at 80A output
- Short circuit protected
- Isolated remote on/off function

Bonus: output preload is not required. This converter is designed to support 12V net without auxiliary 12V battery in the vehicle and the current consumption while vehicle is parked can be low. For this reason some converters require minimum load (usually charging 12V battery), but this one doesn't. However, it will work with 12V battery if I ever will want to put it in as an emergency backup. For now I will not install this battery but make provision (prepare wire harness and fuse) for it.

This is the cavity where stock 12V SLI battery was installed. Good spot for this DC/DC converter - high up (e.g. dry - away from the road) and main 12V cable to the vehicle 12V net is near by. As long as it fits and the hood can close...
Trial placement to check it out. Fits perfectly with room to spare. Since this is air cooled unit, there is advantage not to install it inside the cabin so the cooling fan will not be heard.
This existing battery post clamp will be cut off and a ring terminal lug crimped to conveniently attach converter's output and harness for auxiliary battery if required. Also allows easy measurement for diagnostics.
This tray at the corner was also the candidate for DC/DC converter placement, but it is best suitable for auxiliary battery and/or a control unit of some sort. Reserved for future expansion
Overview of the unit before installation.
The most convenient way to mount it is to attach to the stock battery tray. The tray meat to support ~20kg (~44 lb) battery so will handle converter's weight easily
Supporting aluminum bars underneath converter.
All brackets are completed I made the mounting adjustable.
The converter will be mounted at this position.
Another photo of final position.
As any connection, the lugs are subjected to dipping into the Noalox preventing oxidizing.
The unit is installed and tied down. Turn out simple and neat, but quite trivial and conventional installation. Nothing very exciting here.

Next - motor hub