Hi ,

Here's some updates and news from FIRST^® Tech Challenge.

In this week's blast: 

💻 Tech Tip of the Week

🎉 Congrats FIRST Tech Challenge Teams Heading to FIRST Championship!

❓ Game Q&A Forum

Tech Tip of the Week  

This Tech Tip of the Week is a short one, Part 1 in a 3-part series surrounding a question that we are asked at events all the time - “What makes battery voltage sag?”.

As a battery is heavily used, teams will notice that the voltage of the battery temporarily decreases from its starting voltage during periods of heavy use, and then generally raises back up once the heavy use has subsided. So, what causes this? 

There are LOTS of reasons why battery voltage will sag during use. In this week’s Tech Tip, we will cover the most common two reasons - battery chemistry and heavy use. In subsequent Tech Tips, we will cover other reasons, such as battery cell health, battery temperature, internal resistance, and other factors to be aware of! 

The first thing to remember is that a battery is a chemical reaction factory and does not exactly work the same as the typical “gas tank” analogy makes it seem. The chemical reactions at the electrodes create a potential difference (voltage) between them. This voltage drives the flow of electrons generated by hydrogen and hydroxide ion creation and transfer. In NiMH batteries this reaction is reversible, but it takes time and energy.

What’s important to understand is that the chemical reaction can happen only at a specific rate (the rate is based on a number of factors which we’ll discuss later); if the demand exceeds the rate of reaction for the battery, the voltage and current will drop until the reactions can replenish the battery output (this temporary drop is known as “sag”). As the materials at the electrodes are gradually consumed, the overall battery charge will deplete and can no longer sustain the flow of electrons, and the battery will need to be recharged or replaced. 

So, what is the biggest reason batteries will sag? On a FIRST Tech Challenge robot, this reason is actuator (motor and servo) current draw. Motors and Servos can pull a considerable amount of current when they are used, especially when they are used in low-torque configurations. Motors that are geared closer to 1:1 gear ratio can spin faster - they can propel your robot’s drivetrain across the field much faster - but have less torque because of the lower gear ratio.

Motor configurations that have less torque consume significantly more current to operate (when driving the same load) than motor configurations with more torque. Systems driven by actuators that have more friction or less torque will cause the motors to consume larger amounts of current, and this can cause even healthy batteries to have their voltages “sag” during periods of high use.

Teams must consider their power consumption very carefully when optimizing their battery and motor utilization during a match, even though that is often an afterthought for most teams. 

Congratulations to our FIRST Tech Challenge Teams Heading to FIRST Championship! 

Congratulations to our awesome FIRST Tech Challenge teams that have earned a prestigious spot to our 2024 FIRST Championship presented by BAE Systems. Make sure to check out our FIRST Championship website, download our 2024 FIRST Championship App, and join us on FIRSTtv on Twitch to tune in to all the fun that's coming starting Wednesday, April 17! 

If your team is joining us in Houston, Texas, make sure to check out the FIRST Tech Challenge Event Guide Addendum for all our important program information.  

Game Q&A Forum

The Official Game Q&A Forum is closed for the season. A final PDF copy of the season forum will be available on Thursday, March 28 at 12:00 p.m. ET on our website. Good luck to teams advancing to the Houston Championship!