Hello. I am getting ready to install a Lynx BMS NG with Lithium NG batteries. I want to make sure I set it up correctly so that if the BMS opens the contactor and disconnects the battery bank (high cell voltage, low temp, etc), charging from the alternator (Balmar with MC618 regulator) is shut down beforehand. From reading the BMS manual, it looks like I can just run the brown regulator ignition wire first to one of the two ATC terminals on the Lynx BMS and then out the other terminal to the regulator. As long as there are no BMS error conditions, the contact is closed and the regulator sees ignition and activates the alternator field wire. Would this accomplish the job? I have seen schematics that use the BMS programable relay and sends its signal out to a separate relay device that then has the brown ignition wire run through it. Do I really need this if the ATC is available on the Lynx BMS?
TLDR:
The ATC contactor (pins 3 and 4) does not seem to be using the correct logic to prevent load dumps from alternators. It is probably just for MPPT chargers, etc.
From Page 18 of the Lynx BMS manual (not NG):
“When configured for Alternator ATC mode, the relay will only activate when the contactor is closed. Before the contactor is opened, Alternator ATC is opened first and 2 seconds later the contactor. These 2 seconds ensure that the alternator regulator is switched off before the battery is disconnected from the system.”
and from Page 2:
“The ATC contact is opened to stop charging at high cell voltage or low temperature…”
There seems to be no mention that the ATC contactor will open 2 seconds before the main BMS contactor is opened. Thus, your alternator could potentially load dump if the contactor opens either due to a fault, or if you turn the system off via the Remote (Pins 10 and 11 on the BMS).
You might consider a safer option to be running the ignition wire through the Lynx BMS’ built-in relay (COM+NC) with the relay configured to Alternator ATC mode. The diagram you mention having seen before has two relays. One in the BMS as well as another one. This seems redundant.
The Wakespeed 500 has a designated “Feature In” aka “Function In” wire that can force the system to Float, so the IGN wire can remain intact all the way from the regulator to the vehicle’s IGN source. You might learn from this wiring diagram: https://www.vanlifeoutfitters.com/wp-content/uploads/2025/04/Victron-48Volt-Secondary-Alternator-NG-v2-sm.pdf
Note: Don’t let the following from Pages 26 and 30 confuse you.
“ATC/ATD & Alternator ATC contact status are synchronised.”
“Q: How are the ATC, ATD and Alternator ATC contact statuses managed across multiple BMSes?
A: The ATC, ATD and Alternator ATC contact status are synchronised across all BMSes.”
This is saying that ATC is synchronized across BMSes. And ATD is synchronized across BMSes. But NOT that the ATC and the Alternator ATC are synchronized within the same BMS.