Battery Monitoring - State of Charge, Discharge floor, Time remaining etc. We clarify the concepts!

Digital Skipper |11/11, 2021

Learn why advanced battery monitoring is crucial for boats, RVs, and off-grid systems. We explain key concepts like State of Charge, Discharge Floor, and Time Remaining.

Batteries are used to store energy for later use, but how much energy remains cannot be determined with the naked eye. Battery life is affected by several factors, including:

Undercharging or overcharging
Deep discharges
Excessive charging or discharging currents
High ambient temperature

Whether you have a boat, RV, caravan, or an off-grid system, an advanced battery monitor is crucial. It helps you take corrective action in time, which extends battery life and makes the investment quickly pay for itself.

How does a battery monitor work?

Installation is simple and based on the same principle for all manufacturers: current passes through a shunt which, together with information about battery type and capacity, calculates:

Remaining capacity
Estimated time remaining
Charge status

Common terms in battery monitoring

Here are some of the most important terms and what they mean:

State of Charge (SOC): The battery's current charge level in percent.
Time Remaining: Estimated time before the battery is discharged.
Discharge Floor: The lowest permissible discharge level before the battery risks damage. Typically 50% for lead-acid batteries and 90–99% for lithium.
State of Charge Alarm: A preset value that triggers an alarm at low charge level.

Example: A 110 Ah lead-acid battery with a Discharge Floor of 50% provides 55 Ah of available capacity.

Differences between Victron Energy and Simarine

In the table below, you can see how these two popular manufacturers handle the concepts:

Measurement	Victron Energy	Simarine
State of Charge	Always shows 0–100%, regardless of Discharge Floor.	Takes Discharge Floor into account for a more realistic SOC.
Time Remaining	Calculated to Discharge Floor, but SOC always shows 0–100%.	Calculated to Discharge Floor and can be compared with SOC.
Discharge Floor	Set according to battery type.	Set according to battery type.
State of Charge Alarm	Alarm is based on SOC without regard to Discharge Floor.	Alarm is based on SOC with regard to Discharge Floor.

To further optimize your battery monitoring, read our guide on Peukert's exponent.