I’m fairly new to playing with batteries, ESCs, etc but have a pretty good grounding in electrical theory. I disassembled my batteries this morning and noted nothing in the way of overcurrent protection (fusing or circuit breakers). I also noted that the BMS is marked 24S 80A. From a quality Klein clamp ammeter that I trust, I measured approximately 120A at full throttle in “chill” mode and approximately 160A in “sport” mode. Based on the markings on the BMS, it looks like any overcurrent functions of the BMS may be disabled (otherwise I would expect a cut-off at 80A). Could anyone elaborate on how overcurrent is managed in this system? What would happen if the battery leads were shorted?
Sure, the markings on the BMS are the continuous output ratings, not the maximum current. The BMS MOSFETs can handle much more than 80A; it all just depends on the heat dissipation. We changed it to a higher current type, but the case is still rated for 80A continuous output. We have tested 150A continuously with no overheating issues ( you can see the MOSFET temps is the app also its always has been well below cutoff points). For the maximum current, it is set in the BMS software to shut off if the current exceeds 280A. Additionally, it has short circuit protection that can trip around the 1000A range.
You can set the overcurrent limits to whatever you like if you have the Bluetooth BMS version battery, but we suggest leaving everything at the factory settings.
Thanks for the insight, Paul. Glad you tested to 150A.One of my batteries has the older QS8 connector. I am looking to change all connectors in my system to genuine Anderson SB175s. I’m not sure what the temperature limit is for the QS8s, but the Andersons are spec’d to 105C. Does the new BMS you are using have larger gauge cables than 7 AWG? Per NEC ampacity guidelines [Table 310.15 (B) (17) http://www.lappusa.com/PDF/Page678_APPENDIX_National-Electrical-Code.pdf] we should be running 2AWG to be conservative, 3 AWG if you’re willing to push things a little. I realize the wires are 200C silicone insulated, but the plastic of the connector is the limiting factor. Heat tends to accumulate, even when high-current events are momentary.
A suggestion for a future re-design: Use an unpotted BMS and heat-sink the MOSFETs to the aluminum lid of the battery case.
Is there some sort of operator’s manual or part data that documents things like this? Because it seems like a reasonable question (is use above spec safe?), and a reasonable answer (mometary vs continuous), and something I might want to think about when considering whether/how long to use sport mode.