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.
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.
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.
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.
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.
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!
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.
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.
Not much on the backside of the assembly, apart from a few transistors & passives.
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.
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.
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.
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.
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!
Connections are made via big copper slugs, with M4 screws in the ends. There’s a barrier between them to protect against flashover.
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.
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.
Here’s something I didn’t think I’d be doing! Here’s a teardown of a BMW 5 Series G30 530E Hybrid Battery pack – a monster 351V, 9.2kWh Lithium pack, obtained for it’s cells to replace the boat’s aging lead acids.
This is something I didn’t have the safety gear to do right of the bat – opening one of these packs is a potentially lethal exercise, with 6 unfused battery modules in series, quite capable of blowing pieces off a nice conductive sack of salt water like a person. Cue the purchase of high-voltage rated gloves for protection, just while I got the pack split into something more manageable.
Needless to say, the combination of current capacity & voltage present in EV or Hybrid vehicle battery packs is nothing short of lethal, and these units should be treated with considerable respect.
Here’s the beast of a battery. Enclosed in an aluminium cast housing, it’s very heavy, and definitely not a one-man lift!
After removing the top cover, secured by combination Torx/10mm hex bolts, the internals of the pack are visible. There’s no sealant on the cover, just a large rubber gasket, so this came off easily. There are 6 individual modules in this pack, all wired in series with massive links. There’s also a cooling system for each battery module, supplied with refrigerant from the car’s AC system – there’s a TXV mounted on the side of the battery pack. I didn’t see any heaters present, but I don’t know if BMW have done any neat reverse-cycle magic to also heat the modules if required using the AC system on the car.
The modules are arranged 3 to a side, double-stacked at the back, then a single module at the front. The pack would normally sit under the rear seats of the vehicle, hence the unusual shape. The refrigerant lines going to the evaporators on this side of the pack can be seen in the bottom right corner.
The main contactor pack is on the left side, just behind the massive DC output connector. I’ll dig into this in another post later on.
The right side of the pack is arranged much the same as the left, the main difference here being the battery ECU is tucked in at the top here, along with the interface connector to the car, and the refrigerant lines to the TXV on the outside, which I’ve already removed. Each module has a cell balance control unit, in this case one is mounted on the top of a module, and on the side of the module in the lower right corner.
Once all the modules have been removed, the evaporator matrix is visible on the bottom, a series of very thin aluminium tubes, designed for the best contact with the aluminium frame of the battery modules.
Popping the plastic insulating cover off the battery module reveals the internal construction. I’ve not been able to find exact data on these cells, but I’m assuming them to be a similar chemistry to the ones used in the BMW i3 packs, so 4.15v Max, 3.68v nominal, 2.7v Minimum. The alloy frame itself is of laser welded construction, and there are 16 cells in series per module, giving about 58.8v per module. These will need to be reconfigured as 4 sets of 4 cells in series for 14.72v.
All the individual cell taps are nicely loomed down the middle of the module to each cell, and there are 3 temperature sensors per module (the red epoxy blobs).
The individual cell links are laser welded to the terminals of the cells, so this does make life a little more difficult when it comes to reconfiguring them. The links appear to be made from Aluminium, so soldering is going to be a bit more tricky than usual.