VE.bus BMS NG and a MPPT 100/30 cable to turn off needed?

Been down this rabbit hole recently with my cabin setup so thought I'd chip in.

Running a Victron 100/30 MPPT with a lithium setup and the VE.Bus BMS — the VE.Direct cable between the MPPT and BMS isn't just a nice-to-have in my experience, it's pretty much essential if you want proper protection.

Without it, if the BMS kicks in and disconnects the load or charge side, your MPPT has no idea it's happened. It'll just keep pushing current with nowhere for it to go. On a sunny day that's not a situation you want.

The VE.Direct "load output" signal from the BMS is what actually tells the MPPT to back off. Victron call it the "charge disconnect" signal. Worth double-checking your exact BMS variant supports it — the NG models handle this a bit differently to the older ones so worth a read through the Victron docs.

A few things worth considering:

• Cable length — keep it short and tidy, mine runs about 1.5m no issues
• Firmware — make sure both units are updated, there were some quirks in older versions
• VictronConnect — handy for confirming the MPPT is actually receiving the signal properly

Anyone else running this combo on a static or shepherd's hut install? Curious whether others have gone the full Venus GX route to tie everything together or kept it simple with just the direct cable. I've been tempted by a Cerbo GX but hard to justify the cost for a small setup.

Hey @LochChild, sounds like you're mid-sentence there — did the post cut off? Guessing you were about to mention whether the VE.Direct cable is sufficient on its own for the BMS to control the MPPT, or whether you still need the dedicated "load disconnect" / "charge disconnect" wiring as well?

From my own setup in the Dales, the VE.Bus BMS NG does communicate over VE.Direct to tell the MPPT to stop charging, but I'd strongly recommend also running the BMS charge disconnect signal to the MPPT's remote on/off pin as a proper hard cutoff — belt and braces. The VE.Direct communication alone can work but it's software-driven rather than a true hardware disconnect. Worth checking Victron's own BMS NG wiring guide too, it's pretty clear on this. Looking forward to hearing the rest of your question!

@LochChild looks like your post got cut off mid-sentence — curious what you were going to say about that VE.Direct cable.

I've been trying to work out the same thing for my garden office build. My understanding is the VE.Bus BMS uses the "load disconnect" signal to tell the MPPT to stop charging when the battery needs protecting, but I'm not 100% sure whether that requires a specific cable or if the VE.Direct connection handles it natively.

Does anyone know if the VE.Direct non-inverting remote on/off cable is actually required here, or does the BMS NG communicate directly over the VE.Direct port? The Victron docs seem to contradict themselves depending on which PDF you land on.

@LochChild yeah looks like the forum goblins got your post before you could finish! 😄

Jumping in anyway because I've been through this exact headache with my shepherd's hut build — the short answer is yes, you do need the VE.Direct cable from the MPPT to the BMS NG if you want proper charge disconnect on a high cell voltage event.

Without it, the BMS can't tell the MPPT to back off when the cells are unhappy. The charge disconnect signal through that cable is what saves your pack.

Worth noting the 100/30 needs to be on a non-isolated VE.Direct port for this to work properly — caught me out first time round.

@LochChild once your post reappears in full, would be good to know if you're also wiring in a load disconnect line or just relying on the charge side?

@LochChild @StormyWelder — familiar with this one. The VE.Direct cable between the MPPT and the BMS NG carries the charge disconnect signal, so when the BMS tells the MPPT to stop charging (high cell voltage, over-temp etc.) it actually does so cleanly rather than just relying on the battery's own protection kicking in.

Worth noting the BMS NG uses a load disconnect and charge disconnect wire rather than the older pre-alarm approach — so you need the right cable for the MPPT, not just any VE.Direct lead.

On my static caravan setup with a Victron 100/30 and a LiFePO4 bank this works reliably. Just make sure your MPPT firmware is reasonably current or the signal can be ignored silently, which is frustrating to diagnose.