Understanding SOC Drift

Why Your Battery Percentage Jumps & How to Fix It

Is your battery monitor showing 30%, but then suddenly drops to 5%? Or perhaps it jumps from 90% straight to 100% in seconds?

This is a common phenomenon known as "SOC (State of Charge) Drift." It is not a fault with your battery; it is a characteristic of how lithium batteries work and how battery monitors calculate their charge.

This guide explains exactly what is happening inside your battery, why it occurs, and the simple fix to correct it.

The Problem: The "Flat Voltage Curve"

To understand SOC drift, you first need to understand the difference between a Lead-Acid battery and a Lithium (LiFePO4) battery.

1. The Lead-Acid "Fuel Gauge"

A traditional Lead-Acid battery behaves like a car's fuel tank. As you use the energy, the voltage drops in a straight, predictable line. A monitor can simply look at the voltage (e.g., 12.2V) and know almost exactly how much capacity is left (e.g., 50%). It is easy to measure.

Lead Acid Voltage Curve Graph
Figure 1: Lead-Acid voltage drops linearly, making it easy to read.

2. The Lithium "Cliff Edge"

A LiFePO4 battery is different. It is designed to provide stable power for a very long time. Its voltage remains almost exactly the same whether it is 80% full or 30% full.

This is great for running your appliances, but it is a nightmare for a battery monitor. The monitor cannot use voltage to guess the percentage because the voltage barely changes.

Lithium Flat Voltage Curve Graph
Figure 2: LiFePO4 maintains a flat voltage until the very end, making it hard to estimate remaining capacity.

How Your Monitor Counts Energy (Coulomb Counting)

Because it cannot rely on voltage, the BMS inside the battery uses a method called "Coulomb Counting" (same as external shunts).

Imagine standing at a door with a clicker, counting people entering and leaving a building.

  • You know the building started with 100 people (Full).
  • You count 50 people leaving.
  • You assume there are 50 people left.

This is what your BMS does. It measures the current (Amps) going in and out over time to calculate the remaining capacity (Amp Hours).

TITAN LITHIUM
Ah
BMS SENSOR



→ IN (Charging)


← OUT (Usage)
Figure 3: Animation showing the BMS "Counting" energy as it enters and leaves.

Why the "Jump" Happens (The Drift)

Coulomb counting is incredibly accurate over a day or a week. But over months, tiny measurement errors begin to stack up.

  • A 0.1% measurement error doesn't matter today.
  • But after 3 months of constant charging and discharging, that 0.1% error has accumulated into a 10%, 20%, or even 30% discrepancy.

Your monitor thinks the battery is at 40% based on its count. But physically, the battery cells are actually empty.

The "Ghost Load" Effect

To ensure safety and handle high-power loads, TITAN Lithium BMS units are calibrated to ignore extremely small currents which can look like electrical noise - if we were to increase the sensitivity to 0.2A; this opens a window for the BMS to 'see' eletrical noise as current - if the BMS thinks current is flowing when it isn't, it might confuse the safety logic. For example, if the battery is "Full" (Over-voltage protection active), the BMS waits to see Discharge Current before it lets you charge again. If "Noise" looks like discharge current, the BMS might reopen the charge port while the battery is already full, leading to Overcharging/Fire risk.

The 0.6A Threshold:
Our BMS has a sensitivity threshold of approximately 0.6 Amps (roughly 8 Watts at 12V). If you are running very small loads - like a single LED light, a USB phone charger, or the standby light on a TV - that draw less than 0.6A, the BMS may register this as 0 Amps.

The Result:
If you run a 0.4A load all night (10 hours), you have physically removed 4Ah of energy. However, the BMS thinks you have removed 0Ah. The screen will still show 100%, but the battery is actually at 96%.



REALITY
(Physical Energy)








BMS SCREEN
⚠️


"Ghost Loads" (<0.6A) sneak past sensor...Real energy drains, but Screen stays 40%...Voltage Crash! BMS jumps to 0%.
Figure 4: How Ghost Loads create SOC Drift and eventual Jumps.

Automatic Corrections (The Jumps)

To try and fix this automatically, the BMS monitors the resting voltage. If it detects a mismatch, it will "Jump" the percentage to match the voltage:

  • The 90% Correction: If the battery is resting above 13.6V, the BMS will correct the reading to 90%.
  • The Low Voltage Correction: If a cell drops below 3.0V, the BMS will instantly drop the reading to low capacity (approx 10-15%) to warn you to charge immediately.

The Fix: Re-Synchronising the System

To fix this, we need to force the BMS to "reset its counter" back to a known 100% point. In TITAN Lithium batteries, we have engineered a specific reset point into our protection logic.

The Solution: The Full 100% Charge

You must charge the battery fully until one of two things happens:

  1. Pack Over-Voltage Protection Triggers (approx 14.4V)
  2. Cell Over-Voltage Protection Triggers (approx 3.60V on any single cell)

When the charger pushes the battery to this voltage limit, the BMS protection briefly kicks in to stop the charge. At this exact moment, the BMS knows for an absolute fact that the battery is physically 100% full.

It instantly resets its internal counter to 100%, eliminating all accumulated drift errors. Your monitor will now be perfectly synchronized with the actual energy inside the battery.

BMS Reset Logic Illustration
Figure 5: Reaching 14.4V forces the BMS to reset the SOC to 100%.

Preventive Maintenance

We recommend fully charging your battery until it hits 100% (and triggers the BMS reset) at least once every 3 months. This regular "synchronisation" ensures your SOC reading remains accurate year-round.