ATC/ATD Failure

Been having a bit of a head-scratch with my motorhome setup lately and wondering if anyone else has run into this.

Running a fairly standard Victron-based system — SmartSolar MPPT, a lithium bank, and the BMS handling charge/discharge protection. Everything was ticking along fine until I noticed the BMS was intermittently cutting the charge circuit via the ATC (Allow To Charge) signal. No obvious reason — battery temps were fine, voltage looked normal on the app.

Few questions for those more experienced than me:

• Is ATC/ATD failure a known gremlin with certain BMS setups? I've seen a few threads hinting at wiring issues causing ghost signals but nothing concrete
• Could a loose connection on the two-wire BMS signal cable cause the MPPT to see a false "stop charging" command?
• Is there a reliable way to log when ATC/ATD events are triggered through VictronConnect or VRM, or do I need to be watching it in real time?

I'm still fairly new to all this — first proper lithium install after years of AGM — so I might be missing something obvious. The Victron gear itself seems solid, just not sure if the BMS communication side is where I should be digging.

Anyone had similar intermittent issues and managed to track down the cause? Wondering whether it's worth going over every terminal in the BMS wiring loom before assuming there's a component fault.

@MuddyMaker you've cut your post off mid-sentence there, which is somewhat ironic given you're troubleshooting a fault 😄

But ATC/ATD errors on the SmartSolar are worth knowing about regardless — ATC = Allow to Charge and ATD = Allow to Discharge signals being blocked, usually from

@MuddyMaker @CotswoldNomad — post seems to have been truncated, so hard to diagnose properly. That said, ATC/ATD errors on Victron SmartSolar are almost always one of three things:

1. Temperature sensor — either the built-in or an external one reading out of range
2. Communication fault — typically a dodgy VE.Direct cable or loose connector
3. Battery voltage mismatch — controller seeing something unexpected and refusing to charge

Worth pulling the full error log via VictronConnect before touching anything. The app will show you exactly when the fault triggered and what the surrounding voltage/temp data looked like, which narrows it down considerably.

Had a similar intermittent on my garden office setup last winter — turned out to be a VE.Direct cable that had corroded slightly at the MPPT end. Reseating it fixed it, but I replaced the cable anyway.

Finish your original post if you can — the specifics matter here.

@MuddyMaker yeah need the full details really — @CarlBaker's right that it's hard to call without knowing your full setup.

That said, ATC/ATD faults on Victron kit usually come down to a few usual suspects:

• BMS comms issue — is your lithium bank talking properly to the MPPT? Which BMS are you running?
• Wiring to the load/charger output terminals — worth checking for loose connections
• Firmware mismatch — seen this cause weird behaviour, especially after updates

Had a similar head-scratcher with my garden office setup last year. Turned out my Fogstar battery's BMS was dropping the VE.Direct signal intermittently. Swapped the cable and it cleared up.

Stick the full post up when you can and we'll get stuck in properly. Victron Connect logs would help too if you've got them.

Haha the post cutting off mid-sentence while asking about ATC/ATD faults is genuinely top-tier irony 😄

@MuddyMaker while you sort out your reply, just to throw something useful in — ATC is "Allow to Charge" and ATD is "Allow to Discharge", both controlled by your BMS talking to the Victron kit via VE.Bus or a BMS assistant. When those trip it's usually the BMS pulling the pin for a reason — overvoltage, undervoltage, over-temp, that sort of thing.

Had similar head-scratching with my own setup before I realised my Fogstar cells were reading a false high voltage due to a dodgy balance lead.

What BMS are you running? That's probably the key bit missing from your truncated post. Check the BMS logs if it has them — they'll usually tell you exactly why it pulled the trigger.

@LK_Solar beat me to the irony but while we wait for the full post — ATC (Allow To Charge) and ATD (Allow To Discharge) signals on Victron kit are your BMS talking to the MPPT via VE.Direct or the BMS connection, basically saying "oi, stop what you're doing" when the battery hits a protection threshold.

Nine times out of ten on a motorhome it's either:

• Temperature — lithium BMS cuts charging below ~5°C
• Cell imbalance — one rogue cell crying wolf
• Dodgy VE.Direct cable — had this exact nonsense in my van, £4 cable from Amazon sorted it

Check your Victron Connect app history tab — it logs when the signals triggered, which narrows it down massively. Fogstar Drift cells also have their own BMS quirks worth checking if that's what you're running.