Now the cell modules have been removed from their original home, it’s time to get them repurposed! A custom mounting board has been constructed from timber, and the modules mounted on them. To say this assembly is heavy would be an understatement – it’s barely a two-man lift!
As assembled in the car, the pack as 96S1P, with every cell in series. As we need a low voltage bus, the modules have been reconfigured for 4S4P, in total this makes 4S24P with all 6 modules bussed together. As the cell interconnects are laser welded, some ingenuity was required here.
It turned out the best method (and the safest, to avoid any swarf shorting out cells!), was to use a grinder to cut off the top of the loop on the aluminium interconnects, separating them.
12 5-way bus bars have been installed on the board, and 25mm² cable links them together. To get the angry pixies from the cell modules, 8mm² flexible silicone cable has been used, 4 links to a bus bar. This setup should provide more than enough current capacity.
Here can be seen the cell interconnects – and the grinder cuts to separate them where required to break the module up into 4S strings. As the interconnects are Aluminium, special solder was required to get the copper cables soldered down, in my case I used Alusol 45D solder, which contains a very active flux capable of stripping the oxide from the Aluminium.
Finally, here is the new pack, all connected together. All that needs to be done now is the balance wiring loom, which will allow the BMS to sense each cell voltage individually, and connection of the BMS, Coloumeter & fuses, this will all be covered in a future post!
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.
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.