Victron battery monitor

On board I have a Victron BMV-702 Battery Monitor to monitor the energy management on my boat. I have done a stress test on the house batteries, since they suffer from the higher temepratures in the Pacific. It turns out they are on 75% of their capacity. Within a year that will probably be 50%. So I will have them replaced to be safe sailing the second half of my journey. Also I was curious how and what to read from the Battery Monitor. In this blog I will explain that.

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What can you monitor and what does it mean?

1. Voltage (V)
   • What you see: 12.xx volts.
   • Why it’s important: The voltage gives an indication of the charge state of your batteries.
     • Full battery: ±12.7V (resting voltage, not freshly charged).
     • 50% empty (for AGM): ±12.2V.
     • Empty (critical): ±11.8V or lower.
   Note: The voltage can temporarily be lower if many consumers are active. For an accurate indication, measure with no load.

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2. Current (A)
   • What you see: For example, -8A or +10A.
     • A negative number (e.g., -8A) means power is being drawn from your batteries (consumption).
     • A positive number (e.g., +10A) means your batteries are charging (e.g., via solar panels or alternator).
   How to use it:
   • Check how much current your consumers draw together. This helps you understand which devices are high-power consumers, like the fridge.
   • Monitor the charging current from your solar panels to see if they’re working efficiently.

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3. Wattage (W)
   • What you see: For example, 120W.
   • Why it’s important: The wattage shows how much power your system consumes or produces.
     • When charging, you might see that your solar panels deliver 80W.
     • When consuming, it shows the total load, which is useful to quickly see how much you are using.

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4. State of Charge (SOC) – [% charge of battery]
   • What you see: The percentage (e.g., 85%).
   • Why it’s important: This shows how much energy your batteries still have available.
     • AGM batteries should not be discharged below 50% SOC, as this drastically shortens their lifespan.
     • At 100% SOC, your batteries are fully charged.
   Important: The SOC is one of the most useful data points, as it tells you exactly how long you have left before you need to recharge.

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5. Time Remaining (T)
   • What you see: How many hours the batteries will last at the current consumption (e.g., 8:30 hours).
   • Why it’s important: This is helpful for determining how long you can continue with the current load before your batteries are drained.
   Tip: If your consumption changes (e.g., turning devices on/off), this value changes dynamically.

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Practical tips for monitoring

1. Always charge batteries above 80% SOC
   This prevents sulfation in AGM batteries, which shortens their lifespan. Use the BMV to check when they are fully charged.
2. Avoid SOC dropping below 50%
   When you see that the SOC is around 50%, it’s time to recharge (with solar panels, alternator, or shore power).
3. Monitor current consumption (A)
   • Measure how much current your fridge, lights, and other devices individually use. This helps you prioritize if you need to conserve energy.
4. Keep an eye on the voltage
   • If the voltage drops below 12.0V, you’re nearly out of the safe range. Turn off heavy consumers or recharge immediately.
5. Monitor solar panel charging current
   • Check if the panels are charging enough (e.g., around midday). If the current (A) is much lower than expected, there may be an issue with shading, dirt, or the installation.

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Daily checklist

• SOC (%): Ensure it never drops below 50% and recharge regularly.
• Current (A): Check what your consumers are drawing and whether your solar panels/alternator are charging properly.
• Voltage (V): Keep the voltage above 12.2V at rest (no load).
• Time Remaining (T): Use this as an indication of how long you have left before you need to recharge.