Guru
Joined: 08/02/2015
Location: New ZealandPosts: 1204
| Posted: 03:03am 02 Mar 2026 |
Continuing with this project, have completed a layout for the high current inverter load switch.
The objective was to enable the BMS to disconnect any high power loads from the battery, should there be an over\under voltage event or excessive current draw.
The current BMS design doesn't have any facility to measure load current, I could have used a high power shunt of some sort or a pass through magnetic field detecting device connected up to a balanced input comparator; in the end I decided to make this more modular and incorporate the current detection into the load switch.
This version of the load switch uses 10 mosfets in parallel, one of which senses load current; theory being they will all somewhat current share as they are bolted to a 5mm thick piece of aluminum busbar and will be in thermal equilibrium with each other; the load current will be 10 x the sensed current... hopefully.
I was going to make the circuitry all analogue - No CPU - but that would have made things more complicated, so opted for a simple CPU to give following functionality.
Pre-charge the load (Inverter Caps) for a time and switching to full load when the load voltage comes up to the source battery voltage.
Some led's that show running state.
A comparator with setup pot to detect an over current, causing CPU interrupt to turn things off.
As the CPU knows the Load and Source Input voltages along with the sensed current, you could use this board as a very simple stand alone system, if individual cell voltages were not a concern.
The mosfets bolt onto 50mm wide busbars that extend off the LHS of the pcb, small solder lugs solder to the source pins for bolting to the "Source" busbar. The drain can use busbars bolted to top\bottom of the pcb, this presents extra current capacity and surface area for cooling; if max current is 40A in the sense mosfet, then 400 amps doesn't sound unreasonable for a current limitation.
I have pulled apart a few 200A commercial BMS units that had self destructed, catching themselves and the interior battery case on fire, was not pretty.... they used 30 or more surface mounted mosfets in parallel groups connected by several 5mm round copper rods soldered to the pcb tracks; approx 40mm^2, 200A no way. Hopefully this design might be more robust and repairable.
Schematics:
Main PCB 251 x 166mm, 2 layer 1oz copper:
Edit: Schematic
V300SW.pdf
Once I get this all working, will post some gerbers and software.
Then next mission is to purchase a 3D printer and have a go at making a small box to put it in, I have lots of projects on the go and can never get decent box's to fit them in, so its time to get Fusion 360 software and make a start.
Cheers
Mike
Edited 2026-03-02 16:48 by Solar Mike