Batteries and SB20 Power Meters

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Today's question:

How does battery level affect SB20 power meter measurement?

Let's do a deep dive and find out!



Replaceable Batteries

The Stages SB20 power meters have replaceable batteries. An advantage: No degradation of built-in batteries as the power meters age. A disadvantage: No ability to top up batteries to 100% whenever you wish.

Battery Life

CR2032 button cell batteries are used in the SB20 power meters. Stages says they should last 150-200 hours. Let's verify this claim with bench tests and torque measurements.

Estimating Remaining Capacity


The StagesPower app, and others like BT Inspector, report battery capacity. They use the Bluetooth Low Energy (BLE) wireless protocol to ask each power meter for a battery life estimate.  
🔬It doesn't matter which app you use.  All apps report the estimate returned by the power meters.

Why Battery Estimates are so Bad

Many people complain that battery life estimates from the power meters show 100% most of time. The estimate drops to a number like 50% just before the battery is exhausted and stops working altogether.  FWIW: That's my experience too.

There are a few reasons for these bad battery estimates:

  1. An accurate reference voltage is required. Precision resistors are necessary - but rare -  in integrated chip devices like a bike power meter.
  2. The drain on the battery is not continuous. It is a pulsed load. A power meter measures crank force, does some calculations and then sends wireless data in small bursts, a few times a second. As it drains, a CR2032 battery will have much lower voltage during these instances of activity and then return to normal a few moments later. If battery voltage is measured when the device is at rest, you can get overly optimistic reserve estimates. 
Summary: When reported battery level is near 100%, you are good!  Below 80%, you can't say for sure. 

How to Measure Actual Battery Drain

(I was hoping to avoid this step 🙈)  To accurately determine battery level, I dutifully removed and bench tested my power meter batteries weekly over a 12 week period.  

Bench Test Procedure

I followed the measurement criteria specified in the Panasonic, Energizer and related CR2032 data sheets.  

Two tests were performed each week using a multimeter to measure voltage across a 15kΩ load resistor:

1. CONTINUOUS LOAD TEST: When battery has been at rest for at least 10 minutes. 

2. PULSE LOAD TEST: Immediately after battery has been subjected to a 2 second pulse load of 100Ω.




🔬Unlike the Stages PM, I have a multimeter and oscilloscope that can precisely measure voltage under different loads. This means my battery life assessments will be significantly more accurate.

When is a CR2032 Battery Unusable?

Criteria for determining battery life comes from the vendor data sheets that describe continuous and pulse load performance:  








I deem a battery to be unusable when it is 75% depleted in either test.

What did Testing Show?

I typically ride my SB20 bike 6 days a week for a total of approximately 10 hours per week. 

Here's how things went over a 12 week period:


Summary:
  1. After 12 weeks or 120 hours of riding, the batteries never failed the continuous battery test. No surprise here. This is likely how the self-test estimate is performed by the Stages PMs.
  2. After 10 weeks and 100 hours of riding, the batteries started failing the 2 second pulse test.
This suggests that a typical CR2032 battery lasts 10 weeks and 100 hours (for me at least).

How Accurate were the Power Meters as the Batteries become Depleted?


This is the most important question isn't it? The batteries may last for many weeks - but how well do they measure power over that time period?  To find out, I did some objective testing...


Each week, I did a Torque Test to statically measure crank accuracy with a 25 lb weight. Every month, I also compared the Stages PMs with my Garmin Rally pedals using the ZwiftPower dual meter comparison tool.  





Here's a summary of the results:



Notes:
  • Torque Test power readings were surprisingly good for 10 weeks or 100 hours.
  • ZwiftPower comparisons were less consistent but generally followed the torque measurements.
  • I had issues with the Garmin pedals producing wireless dropouts as I repeatedly removed and replaced batteries over the test period. 🔬This reminded me: With both the Garmin and Stages power meters, you must carefully re-insert batteries, keeping contacts clean and aligned ðŸ¤“.

Power Testing Summary

  • I have a love / hate relationship with replaceable batteries. They last longer than rechargeable equivalents. Not being able to "top-up" batteries means that they potentially need to be replaced prematurely.
  • SB20 power meters work well with partially depleted batteries. Stages claims to use a voltage regulator to keep readings consistent over the useable life of its CR2032 battery. My tests confirm this assertion.
  • I got 10 weeks or 100 hours of reliable power measurements from the SB20 power meters. This aligned with the battery tests showing continuous and pulse voltage to be acceptable over that time period.  
  • At weeks 11 and 12, I had declines in power measurement of approximately 5% in Torque Tests and 8% in ZwiftPower comparison tests. Time to replace these batteries.

My Overall Takeaways

  1. I recommend battery change interval to be 100 hours. That's 10-12 for weeks me. 
  2. Be careful with battery level estimates. Battery levels do tell when the battery is healthy (when near 100%). Estimates don't tell you the optimal time to replace batteries. Consider a fixed time interval that you are comfortable with.
  3. If I had a really important indoor event, I would consider replacing batteries if I was near the 100 hour threshold. Otherwise, I would expect the Stages PMs to be accurate.
🔬Observation: Based on torque and comparison tests, I found the Stages PMs to be more consistent than my Garmin Rally pedals over the 12 week test period. Not what I expected but that's what the data showed.
 

Want to learn more about PM batteries? See: Stages Power Meter Battery Tips.

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Many people ask: How do I get battery levels for my power meters? Popular answers are the BT Inspector and StagesPower apps. Both work fine. You find each power meter individually and then ask for a battery estimate. Repeat for any other battery powered devices.  

This tedious process inspired me to write BattView: A single app that reads battery levels for all Bluetooth devices in range. One click for all. 

BattView for iPad and iPhone is available in the Apple App Store. 
Download BattView

 

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