Troubleshooting Guide: How to Fine-Tune Auto Sync Settings on Your Victron BMV or SmartShunt for Large Battery Banks

The Auto Sync to 100% function in Victron’s BMV and SmartShunt battery monitors is designed to keep the State of Charge (SOC) accurate over time. It works by automatically resetting the SOC to 100% once it detects that the battery is fully charged. While this works well in many systems, large battery banks can trigger a false sync — showing 100% even when the batteries are only ~70% charged.

Let’s dive into what’s happening, and how you can adjust your settings for a much more reliable reading.

? How Auto Sync Works

Victron battery monitors use 3 key conditions to determine when to sync the SOC to 100%:

1. Charged Voltage – Usually set around 13.2V (for 12V systems).
   
   

2. Tail Current – A percentage of your battery capacity (default: 4%).
   
   

3. Charged Detection Time – Time all above conditions must be true (default: 3 minutes).
   
   

If all three are met, the monitor assumes the battery is full and resets SOC to 100%.

⚠️ The Problem With Large Battery Banks

Let’s say you have a 1000Ah battery bank.

• Tail Current at 4% = 40A
  
  

• Your solar charger brings the battery up to 13.8V
  
  

• Current drops below 40A as solar tapers off in the afternoon
  
  

• These conditions last for a few minutes…
  
  

? The monitor thinks the battery is full — and auto syncs to 100%.

But in reality, the battery might only be ~70% full. Why? Because 13.8V is not absorption or float voltage, and 40A is still a lot of current — especially for a battery that isn’t yet topped off.

✅ The Solution: Adjust These Three Settings

To prevent false syncs and get accurate SOC readings, here’s what to tweak:

1. Lower the Tail Current (Most Important)

• For large banks, the default 4% is too high.
  
  

• Try 1% or even 0.5% for better accuracy.
  
  

• For example, a 1000Ah bank with 0.5% tail current = 5A, which is much more realistic for full charge.
  
  

2. Increase the Detection Time

• Default is 3 minutes — too short for solar tapering scenarios.
  
  

• Try increasing to 10–30 minutes, so it only syncs after sustained full charge conditions.
  
  

3. Raise the Charged Voltage

• Default is around 13.2V.
  
  

• Set to 13.6V–14.0V depending on your battery type and charger.
  
  

• Be careful: If you set it too high (e.g. 14.8V), the system might never auto-sync.
  
  

? Bonus Tips

• Manual Sync: You can always manually sync to 100% in the VictronConnect app if you know the battery is truly full.
  
  

• First-time setup: Always do a manual sync after first installing the unit or after a power interruption.
  
  

• Keep it charging: If you rarely get a full charge (e.g. solar-only systems in winter), consider syncing manually or using regular maintenance charges to trigger accurate auto sync.
  
  

? Example Settings for a 1000Ah 12V Battery

These will help ensure that auto sync only happens when the battery is actually full — not just close enough.

? Where to Adjust These

Open the VictronConnect app:

• Go to Device > Settings > Battery Settings
  
  

• Adjust: Charged Voltage, Tail Current, and Charged Detection Time
  
  

Need more help? Reach out to our support team or browse our knowledge base for detailed battery tuning guides.