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Delta and Your Hybrid Battery: What It Tells Us About Battery Health.

Unraveling the 'delta' mystery. What your hybrid battery's 'shift' reveals about its health.

You've likely heard about 'delta' in the context of your hybrid vehicle's battery. But what does it really mean? Today, we dive deep into this often-misunderstood term and how it can clearly indicate your battery's health.

Understanding Delta in Simple Terms

Imagine a seesaw. When both sides are perfectly balanced, it's level with the ground. That's what we want with our battery's charge - a balance. In this analogy, 'delta' is like one side of the seesaw being higher or lower than the other. It represents a change or difference in what we expect versus what happens with our battery's charge.

The BMS and Delta

Your vehicle's Battery Management System (BMS) wishes this seesaw to be level, meaning a delta of 0%. This would indicate that the battery's performance is exactly as expected. However, as a battery ages, it might not hold charge like it used to, leading to a difference in expected and actual performance - our seesaw is no longer level. This imbalance is what the BMS represents as an increased delta.

Toyota Hybrid battery replacementWhat Does an Increased Delta Indicate?

The question everyone wants answered! An increasing delta can be a sign of several things:

  • Battery Imbalance: 

Like cells in our body, a battery comprises individual cells. When some age faster than others, it leads to an imbalance. But a balanced battery can still have a high delta (I like to think of it as the battery failing as a whole but still working together).

  • Reduced Capacity

As the battery's ability to hold a charge decreases, the delta can increase, indicating it isn't performing like it used to. This is 100% true. With a high delta, there is no "room" to store electrons. Hence, a percentage increase tells us the battery capacity has decreased in this specific example of a hybrid battery.

Toyota prius battery bus bar corrosion

Delta measures increase or decrease based on parameters set forth by the system. In our case, the battery management system or BMS says zero is our baseline. Once we go above zero, the BMS provides an increased Delta reading but relates back to a reduced capacity in the battery. 

  • Fault Codes & Efficiency

A high delta might lead to error codes (P0A80) on your dashboard. It can even reduce your vehicle's fuel efficiency, impacting miles per gallon.

It's essential to understand that a battery can still be balanced, showing a minimal voltage difference, yet have a high delta. This scenario paints a picture of a battery that, while still functioning, has lost some of its original capacity.


Delta is more than just a technical term; it's a window into your hybrid battery's health. An increasing delta is a clear sign of battery degradation. However, all hope is still there! Unlike the misconception that a degraded battery spells the end, many of these issues are repairable. And the good news? It can be done at a fraction of the cost of a brand-new battery from a dealer. So, next time you hear about 'delta,' remember it's your vehicle's way of letting you know how your battery is doing. You can ensure it keeps running efficiently for years with the proper care.


*For those keen on getting the most out of their hybrid vehicles, understanding the role and implications of delta is key. As always, you can consult a battery professional to get personalized insights and solutions for your vehicle. Please don't hesitate to ask any questions about your hybrid battery and its overall state of health.*


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Peter Neilson is an automotive consultant specializing in electric cars and hybrid battery technologies. He holds a Bachelor of Science in Automotive Service Technology from Weber State University. Peter can be reached on Linkedin and you can tweet him at The_hybrid_guy on Twitter. Find his page on Facebook at Certified Auto Consulting. Read more of Peter's stories at Toyota news coverage on Torque News. Search Toyota Prius Torque News for more in-depth Prius coverage from our reporter.