SOC Drift in LiFePO4 Batteries

Understanding SOC Drift in LiFePO₄ Batteries Over Winter

As we move deeper into winter and the days get shorter, we’re seeing reduced solar production across the board. That makes now the perfect time to talk about a common issue that can arise when using your LiFePO₄ (LFP) battery system in a motorhome, caravan, or off-grid setup: SOC drift.

So, what exactly is SOC drift?

In short, it’s an inaccuracy in the BMS (Battery Management System) State of Charge (SOC) reading — whether you're viewing it via a display or smartphone app. During winter, solar yield often drops significantly, and users naturally begin to conserve power, using fewer devices and drawing lower current. Logical, right?

Here’s the catch: most BMS units become less accurate at very low current levels — typically under 3A. And when your system is only drawing 1–2A consistently, the BMS may struggle to track SOC accurately. The result? Your battery may appear to have more charge than it actually does. In extreme cases, this can lead to an unexpected low-voltage shutdown, even when the display shows a decent SOC. Frustrating — and potentially disruptive.

Why does this happen?

BMS units are primarily designed to protect your battery cells, not to offer ultra-precise monitoring. Monitoring is a secondary feature, and low current readings simply aren’t their strong suit.

How to avoid SOC drift this winter:

 Fully charge your battery at least once per week.

Not just according to the app or SOC display (which may already be inaccurate), but by visually confirming that your charging system has completed its bulk, absorption, and returned to float. This may require running a generator or plugging into mains power if the sun isn’t cooperating.

Use a Victron SmartShunt.

If your system typically runs at low current, consider adding a SmartShunt. It’s designed specifically to monitor SOC with far greater precision than a built-in BMS — down to 0.01A resolution — and helps avoid false confidence in your battery level.

Use your Victron Solar Controller’s history feature.

If you're running a Victron solar charge controller, check the 30-day solar yield graph. You’ll be able to see if your system has recently completed bulk and absorption stages. If not, chances are your battery hasn’t been fully charged in a while — and that’s when drift can occur.

 As outlined in the battery manuals available on our website, we recommend fully charging your battery weekly. Avoiding SOC drift is one of the key reasons why.

Nothing in this world is perfect — but if you follow these simple steps, you can stay ahead of the curve and avoid any nasty surprises.