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What is state of charge drift?

To understand your power system, it’s crucial to be aware of state of charge drift (SOC drift) and proactively check for errors. In this guide, we’ll explain what SOC drift is, what to watch out for, and how to correct errors.

What is SOC Drift?

State of Charge % (SOC) is a calculated estimate of the amount of capacity left in your batteries. Because it is an estimate, there can be small inaccuracies in the calculation that accumulate, causing the SOC estimate to be different from the true level of charge. This difference is called drift

SOC drift is a reality for every lithium battery system because there is no way to directly measure the level of a lithium battery outside of a laboratory setting. 

How do I know if my SOC has drifted?

Check the main voltage. It is a truth serum for confirming whether SOC is accurate. If voltage is less than 12.2v then you are almost out of power – regardless of what SOC says. You must ignore SOC, which has drifted, and charge right away.

A metaphor: you’re on a boat.

A sailboat, to be precise. And it is foggy: your visibility is only 1000 feet. Your navigation instruments have been calculating your speed and location, estimating that you are 5000 feet from the shoreline. But suddenly, you can see the shore. With such poor visibility, you KNOW that the shoreline is closer than 5000 feet. So, you ignore instruments and change your heading to sail away from the shoreline. 

In this example, the shoreline is our voltage and the navigation instruments are the SOC %. When you can see the shoreline (low voltage), then you need to take action and change your heading (charge your batteries) right away because you know that the navigation estimate (SOC %) is wrong. 


Why can’t I just use voltage to tell how full my batteries are all the time?

For some battery styles (lead acid, for example) voltage gives you an idea of the charge level. For Lithium Iron Phosphate batteries, when comparing 25% to 75% SOC, or even 10% to 90% SOC, voltage may not change more than a few thousandths of a volt! Voltage in lithium batteries is affected by load, temperature, and other factors, but not very much by the level of charge of the battery. This means we can’t tell much by voltage, and we must instead track how much energy is going into and out of the battery.

When the battery's charge level is close to empty or close to full, however, voltage starts to move dramatically as the charge level changes. In this sense, voltage is a very good indicator of full, empty, or somewhere in between. Returning to our boat metaphor, you cannot rely on that shoreline (voltage) to know your position at all times – because of the fog, you won't be able to see the shoreline at 2000 feet or 20,000 feet.

How do I fix drift? Can I avoid it?

To correct the drift inaccuracies, the batteries simply need to be charged to 100% by meeting these conditions (by driving or plugging into shore power). 

A full charge is crucial because when there is an excess of power available for charging and the batteries are not consuming it, it indicates they are full. This is known as a “synchronization event” because the full charge – indicated by an excess of available power for charging – synchronizes with the SOC also being 100%.

Large SOC drift inaccuracies happen most when there has not been a synchronization event recently. This is most common with shallow use and/or batteries that have not had a TRUE full charge in a long time.

The easiest way to avoid SOC drift errors is to fully charge regularly by plugging into shore power overnight or driving until the conditions for a full charge have been met.

For Revels, relying solely on solar power to top off your batteries can cause drift because the stock panels do not provide enough power to reliably fully charge the system. There are cases where solar can cause false synchronization events that contribute to drift instead of correcting it. 

A metaphor

Now you technically know what drift is, but let’s think about it in a setting that’s a little more familiar. 

Imagine an empty five gallon bucket.

  • You are filling it at the rate of 1 gallon/minute. 
  • After 2 minutes of filling, there will be 2 gallons.
  • Now, take out ½ gallon/minute.
  • After 2 minutes removing water at this slower rate, there will be 1 gallon left in the bucket. 

However, what happens if the timer wasn’t started at the right time, and water actually added for 1 minute 50 seconds? What if the measure of flow isn’t quite right. 

When our water measurements are not perfectly accurate, then we won’t know precisely how much water is in the bucket. The best we can do is estimate.

Now, imagine emptying and filling the bucket over and over. After 10 minutes, will you know exactly how much water is in the bucket? Probably not. That’s drift.

Let's take the metaphor a little further. 

When we don’t know the exact amount of water in the bucket, what’s the best way to know if it’s actually full? When it overflows. That’s a synchronization event: when we once again can be certain of the fill level in the bucket because water is overflowing.

With your batteries, there is a constant filling (charging) and emptying (consuming) of power. While this is happening, the battery monitor is estimating the charge level. Whenever the batteries are fully charged, that’s analogous to the bucket being so full it overflows – synchronizing the SOC % with the true level of charge in the batteries.


Got it! Now I want to know more.

Awesome.  Outside of a lab setting, SOC % for a lithium battery can only be estimated. To dig deeper into the estimates, learn more about dead reckoned estimates. The dead reckoned estimate for batteries is called coulomb counting, which is another great term to learn about.