February 2016

April 2016

Charging system.

Whenever an EV is not driven it is normally connected to the mains. At least my EVs are. So the car is either bulk charging, or finishing charging or re-balancing, or done, but the energy to run electronics at that tome comes from the mains. The Audi charging system is more complicated than what you will find in an OEM EV such as Leaf or Think or Tesla. I don't do it more complicated for the sake of complexity, the architecture is pretty much dictated by the battery pack design and desired charging power.

As you can see from the battery page, it is made up of two paralleled strings each of which is broken in the middle by mid contactors for safety reasons, so essentially I have four independent (as far as charger connection goes) strings permanently attached to its individual charger. That is BRUSA NLG543 which consists of four physical 3.3kW BRUSA NLG513-U1-01A-A01 liquid cooled chargers permanently connected to their own 1/4 of the battery. This allows flexibility for managing all the cells, but is also challenging as far as the BMVCU algorithm. All four individual chargers are on the common CAN bus and controlled by BMVCU independently. They are paralleled on the input side and are fed either directly from the mains or from an EVSE if I ever need that. It is uncommon to have more than 7.2kW EVSE around. This means it is typically limited to 30 A total mains current. My charger is 13.2 kW on the output side and 14.4 kW on the input side, twice as powerful, so it will consume up to 64A from the mains running full bore. In fact, I will charge faster than I discharge driving, but the point I'm making is that any time I connect to a public EVSE, the charging system will receive info from it about its power handling capability and limit mains current to the value transmitted to the charger over its pilot signal. Typically this will be 30A, or 7.5A per charger. This means input charging for each unit will never exceed 7.5 A * 240 VAC = 1.8 kW. But if I have to charge directly from the limited mains, the NLG543 allows manual mains current limit by a simple potentiometer varying voltage on special analog input. The total mains current can be observed on the AC power meter installed right next to the mains receptacle and the pot to set desired current is right there as well, so I can just dial any value between 0 A to 64 A by observing digital amp meter to make sure the circuit breakers won't pop or wiring overheat. Useful for camp sites outlets, washer/dryer/welder outlets or similar power sources.

A concept diagram configuration of the Audi charging system. Fuses, service and emergency disconnects and other details are omitted for clarity.

Since the charger is liquid cooled, I had to come up with local cooling loop - a small radiator and cooling fan outside the vehicle. With typical 87% efficiency maximum amount of power wasted as heat is 13.2 kW*(1-0.87) = 1.72 kW. I can get away with a heat exchanger core for a vehicle heating system. I chose Think core because it has convenient inlet/outlet locations, very fine fins and the size of exactly two square fans which will cool the liquid. The coolant is moved by two Bosch brushless PAD pumps.

The charger fits inside spare wheel well. Two layers of NLG513 stacked up. Mains cables run through the rubber grommets on the side of well to the small junction box where power wiring from the 75A J1772 inlet also comes to. On top of this, fused connection to feed the power meter is made here as well. One 240 VAC phase wire is fed through the CT (which is part of the Murata power meter) for measuring phase current. Finally, 240 VAC connection also extends to the well where 4A 14VDC power supply is installed - in addition of 0.5 A supplied by each NLG513 this power it will run the pumps, entire BMVCU while on charge, support auxiliary 12V battery onboard and run anything else while main DC/DC converter is off. That's about it for brief description of what was done.

Below are the photos depicting main steps taken to implement this system.

Aluminum plate matching the size of the bottom of wheel well is ready to tie down four chargers.
Four chargers are mounted and ready for plumbing. Two Bosch pumps shown up front will move the coolant. One pump is enough but it is important to have the same temperature of all the chargers so one does not cut out sooner than the others. With slow moving coolant its temp. exiting charger 4 will be quite a bit higher than the temp. entering charger 1. With two pumps coolant has no chance to change its temp much by the time it runs through all the units.
Overview of completed pluming. The chargers are plumbed in series, first two are at the electrical bottom of each HV string, the last two are at the electrical top.
EV heater core (out of a Think City EV) will serve as the radiator for chargers. Two pancake fans side by side happen to be exact size of the core face. 
The cardboard mockup of the core enclosure. It will hug the wheel well from outside. The well protects the core opening from splashing water and road debris.
The core enclosure is made from 1mm thick sheet aluminum. Not too thin to be flimsy when folded and bolted together, and not too think to be easily cut, bent and shaped.
This is what the enclosure looks like before final fit and being permanently bolted together. It took a few iterations to get this right, but eventually it all fell in place and worked out very nicely.
The core is installed inside. Good news is the gaps between it and the enclosure don't have to be hermetically sealed. As long as fans blow through the core, it is all that matters.
Two fans installed in front of the core. V heater core (out of a Think City EV) will serve as the radiator for chargers. Two pancake fans side by side happen to be exact size of the core face. 
The enclosure is completed. There is a wire mesh placed in front of the fans to keep them in place. Inlet and outlet are also mounted to the core; the whole contraption is ready to be installed on the vehicle.
This is what it looks like from outside. The gap deliberately left between the core and the well wall allows air to circulate but is small enough to prevent any kind of debris to enter the cavity. Granted, a belly pan will offer additional protection.

The only remaining items to install are the junction box and the J1772 inlet plate with power meter and adjusting pot. The box is located between right rear wheel and spare tire well. It is as simple as two isolated posts for ring terminals from the mains and all four chargers. 
The water tight junction box.
The box is ready for wiring. The mains connection (ring terminals are visible) will come through the plastic gland.
Wiring completed - appearance before closing the box.
Aluminum bracket for the J1772 inlet is shaped to fit the spot where former fueling lines were located.
The inlet is installed, there is cutout for the power meter display and mains current adjustment potentiometer. All this hardware is sealed and water tight.
The pocket right behind fuel door is very convenient place to mount this hardware. View from inside right rear wheel well.
The mains wiring is running along the trunk wall (on the wheel side). An M6 rivnuts are installed in a few places - this is far more reliable way to attach wiring then using sheet metal screws.
Attaching mains wires to the wall. There is about 50 cm run from the inlet to the junction box.
Photo of completed installation of the inlet and power meter.
Partial installation - the plumbing is done and mains cables are connected. There are no battery cables nor low voltage interface wiring shown yet, but basically this is what the well hardware looks like before it is covered by the rear battery tray.

Next - traction inverters