Added 8-19-2024 Tech Tip (#305)

docs/source/tech_tips/tech-tips.rst

@@ -11,8 +11,79 @@ on this page chronologically, with the newest content at the top of the page.
 
 Just click to expand the Tech Tip you'd like to read.
 
+.. dropdown:: Week of 08/19/2024 "REV Driver Hub Batteries"
+   :open:
+
+   .. _driverhubbatteries:
+
+   **REV Driver Hub Batteries**
+
+   This week’s Tech Tip of the Week focuses on the REV Driver Hub. Sure, we
+   already did a pretty thorough deep dive on the REV Driver Hub in the
+   11/06/2023 Tech Tip "Driver Hub or Smartphone?", but we never really covered
+   the batteries used in the Driver Hub themselves - and, of course, this topic
+   was recently brought up in a team question. The question was, “Why aren’t
+   batteries for the REV Driver Hub interchangeable?”
+
+   Well, that wasn’t the actual question, as the team didn’t know the question
+   they SHOULD have been asking, but that was the root of the issue. The team
+   in question had purchased an extra REV Driver Hub battery, charged it, and
+   was using it as a spare. We’ve also heard anecdotes from teams who attended
+   events where FTAs would also purchase spare batteries (or pull batteries
+   from spare Driver Hubs)  and let teams with depleted batteries use their
+   charged batteries. However, in each case the teams noted that the spare
+   battery never lasted as long as their “regular” batteries, often
+   significantly shorter (half or less). The issue is actually not specific to
+   the REV Driver Hub, but in the batteries themselves.
+
+   I noticed the same thing a few years ago when I owned a smartphone that had
+   user-replaceable batteries. My phone battery stopped holding a charge, so I
+   bought a battery online to replace it. However, I noticed that the
+   replacement battery had a significantly lower “lifespan”, meaning it would
+   go from full charge to near-dead in a shorter period of time versus the
+   original battery. Over time the battery seemed to “last longer”, until after
+   about a dozen charge cycles it was very close to the original battery’s
+   performance. Did the battery get better, or did my phone adapt to the
+   battery?
+
+   What I didn’t know was that minor variances in how batteries are
+   manufactured, especially in lower-voltage Li-Ion batteries, can affect the
+   voltage stability of the battery as it depletes (how the voltage of a
+   battery changes as it’s used). In order to know how much battery power is
+   left, the device needs to know the “charged” voltage, the “depleted”
+   voltage, and generally needs to understand how the battery voltage changes
+   from one extreme to the other. Unfortunately this isn’t linear, and
+   differences in a battery’s specific internal resistance and other factors
+   will cause each battery to have different behavior (this occurs in all
+   batteries, but higher capacity batteries with low internal resistance tend
+   to show this difference less). The REV Driver Hub performs a
+   `calibration <https://docs.revrobotics.com/duo-control/troubleshooting-the-control-system/driver-hub-troubleshooting/driver-hub-battery-troubleshooting#battery-calibration>`__
+   phase as it charges a battery, and stores the battery charge characteristics
+   - that helps it know how the battery should behave when it’s being depleted.
+   In this way the Driver Hub “learns” how to interpret the battery it’s
+   charging so that it can create an accurate charge profile for the battery
+   as it’s used by the device.
+
+   When a team replaces the primary battery with a spare, the Driver Hub
+   doesn’t necessarily know that this has happened, and can only apply the
+   stored discharge characteristics for the primary battery to the new battery.
+   Unfortunately this often leads to the device misinterpreting the battery and
+   shutting down before the battery has fully depleted, or thinking there’s
+   more battery left when there really isn’t. If the new battery is then
+   charged, the Driver Hub will calibrate to the new battery, and changing the
+   battery again will cause the Driver Hub to mischaracterize the original
+   battery if replaced.
+
+   It is highly recommended that all teams use an 
+   `external USB Battery Pack <https://www.amazon.com/Portable-Charger-Anker-PowerCore-20100mAh/dp/B00X5RV14Y?th=1>`__
+   connected to the USB-C port on the Driver Hub to provide consistent power
+   (use USB-A to USB-C cables only). The battery pack will sustain your Driver
+   Hub and keep it from being additionally depleted by any high-power-drain
+   gamepads (such as the Sony DualShock and Sony DualSense gamepads) that your
+   team may be using.
+
+
 .. dropdown:: Week of 07/29/2024 "Servo Power Injectors"
-   :open: 
 
    .. _servopowerinjectors:
 
@@ -1544,6 +1615,8 @@ Just click to expand the Tech Tip you'd like to read.
 
    .. _huborphone:
 
+   **REV Driver Hub or Smartphone?**
+
    This week's Tech Tip of the Week briefly discusses the pros and cons of 
    Smartphones versus the Driver Hub. Which one should you use? Are there 
    hidden benefits or perils for using one over the other?