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Surprise Mouse!

Well, while working on the boat’s engine, I was surprised by this little sod, who’d managed to crawl into the air intake skin fitting on the transom, and got very irritated at the engine being fired up! How the little dude avoided getting sucked into one of the cylinders, I have no idea! The wee mouse was recovered from the air intake & released on the towpath.

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Epever Tracer 4210AN MPPT Charge Controller MOSFET Repair

Failed FETs
Failed FETs

Here’s some damage to a 1-week old Epever Tracer 4210AN MPPT Charge Controller, where some of the power FETs have decided they’ve had enough of this world. These are Alpha & Omega AON6512 N-Channel Enhancement devices, rated at 30V 150A. From probing around, these seem to be on the battery bus for output protection – they’re just used as power switches in this application. The controller did work in this state, but charging from the solar input was accompanied by a very strong burning PCB smell.
I’m not sure what caused the failure, but as they’re all in parallel, if a single device failed, then it’s likely that the remaining parts having to then compensate for the extra load put them under enough stress to cause a failure.

Cleaned PCB
Cleaned PCB

The hot air gun was used to get the old parts off the board, which had got hot enough to fully oxidise the solder on the thermal pad, along with causing a bit of damage to the PCB itself. I scrubbed the board with a fibreglass pencil to try & get all the Magic Smoke residue off, along with any oxide on the copper. There has been some flaking of the soldermask, but luckily only between connected pads, and not around the gate pads. There was some unfortunate collateral damage to the main fuses, with minor melting of the plastic case, but they’re still electrically intact.

Reflowed Replacements
Reflowed Replacements

Replacement MOSFETs were sourced from Farnell, in this case ON Semi NVMFS5C628N parts, rated at 60V 150A. Since these parts are in a DFN package, solder paste & hot air was used to reflow them back onto the cleaned pads, and then everything checked for short circuits.
The replacement FETs have slightly higher RdsOn resistance, but this shouldn’t be an issue.

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BMW Series 5 Hybrid Battery Contactor Pack Teardown

Main Connectors
Main Connectors

Now it’s time to dig into the main contactor pack from the hybrid battery I tore down in a previous post. This unit contains the main output relays, precharge components, current measurement & protection. It’s pretty heavy, which isn’t surprising when you realise how much copper there is in this thing! Manufactured by Lear Corporation in the US, this is a seriously heavy duty piece of electrical engineering.

Cover Removed
Cover Removed

Once the cover is popped off, the first thing is a large PCB on the top, and some low current wiring. Not much to see yet.

Control PCB & Current Sensor
Control PCB & Current Sensor

The main control & current measurement PCB is on the top of the unit, in a plastic frame. This is a complex arrangement in itself. Unfortunately I’ve not been able to identify any of the main components on here, as everything is conformal coated, so the numbers are obscured!

Main Control PCB Top
Main Control PCB Top

Removing the assembly from it’s plastic frame reveals a flex-rigid assembly, which is normally folded in half. The main CPU is on the top layer, and most of the power supply & measurement electronics on the bottom. There’s some serious isolation here on the right as well.

Main Control PCB Bottom
Main Control PCB Bottom

The bottom has the connectors, and some power supply components. The main current shunt is on the left, this would be in the negative return side of the main battery bus.

Main Control PCB Reverse
Main Control PCB Reverse

Not much on the backside of the assembly, apart from a few transistors & passives.

Control PCB Removed
Control PCB Removed

Once the control PCB assembly is removed from the main frame, the high current bus bars become visible. There are 3 switching devices in here, two for the main battery bus, and a smaller one for the precharge function. There’s also a main fuse hiding in the middle.

Main Positive Contactor
Main Positive Contactor

The main battery positive contactor is tucked in on the left side, with the precharge leads across it’s contacts. This normally isolates the car from the batteries when open.

Precharge Components
Precharge Components

Precharging is dealt with by this collection of components. A smaller relay, and a large ceramic 15Ω resistor limit the current that can be drawn when the vehicle is enabled. Closing the main contactors first would potentially cause damage due to the enormous inrush currents caused by the large filter electrolytic capacitors in the traction inverters.

Main Battery Fuse
Main Battery Fuse

The main battery fuse, in the DC + line from the cell modules is a 350A rated unit, 450v DC. Being a HRC type, this is capable of breaking 6kA under fault conditions.

Panasonic AEV14012 Contactor
Panasonic AEV14012 Contactor

Here’s one of the pair of main contactors, Panasonic AEV14012 400v DC, 120A rated units. These are serious devices, having a hermetically sealed ceramic capsule around the contacts, and a Hydrogen filling! 

Main Contacts
Main Contacts

Connections are made via big copper slugs, with M4 screws in the ends. There’s a barrier between them to protect against flashover.

Ceramic Capsule
Ceramic Capsule

Pulling the top plastic cap off reveals the ceramic capsule containing the contacts. This is the Hydrogen filled space of the contactor. The reason for the hydrogen fill is arc quenching.

Arc Magnets
Arc Magnets

The contact capsule sits in a permanent magnetic field, provided by these small ceramic magnets. These assist in pulling any arc towards the ceramic walls of the contact capsule, helping to cool & extinguish it.