@smartkid What this is, is a quickfix for those teams that were tripping the microcontroller's internal breaker last season and maybe a response to the fact that the new TETRIX kit uses two 7.2V batteries.
We want new microcontroller (waaa!!!).
Folks - For better or worse it is time for one of my long essays. Hopefully slogging through it will be worthwhile. :)
My hunch is an emphatic "Neither". I doubt it is a response to Tetrix using two batteries in series in a single circuit to energize a fairly small and light-weight robot, and I don't think the Vex microcontroller's 4 Amp limit should be thought of as something to fix.
Notice that unless something changes from the Tetrix prototypes I/we used at the 07-08 FTC World Championship, the Tetrix kits used their two batteries in series to double the voltage you can supply to Tetrix devices.
This is good if you are continuously practicing for a few hours, or if you are using a single set of batteries to try to survive an entire tournament, because when you double the voltage you use to energize a DC circuit, you halve the current you must supply to it (and in a Tetrix bot you can pull that current from the charge stored in two batteries, not one double voltage battery); but the second battery is not the equivalent of the Vex Power Expander.
When you crunch the numbers, you find that one Vex battery that has been properly cared-for and has been charged, has enough stored charge in it to run a non-expanded Vex robot for many[/U] minutes.
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[*]2000 mAHr = 2 AHr = 4 Amps for 30 minutes (4 Amps is the most the microcontroller will supply to attached devices) (I am ignoring the small current consumed by the microcontroller itself).
[*]Let's say that the battery is old, that even a new battery's voltage drops too close to 5 V as the battery is supplying its last few coulombs of charge, and etc; and assume that all of those effects add up to cutting by 50% a battery's ability to successfully energize a robot.
[*]4 Amps for 30 minutes now becomes 4 Amps for 15 minutes.
[*]15 minutes = [U]SIX![/U] matches in which the robot's motors were consuming just [U]as much current as they could possibly suck through the microcontroller[/U] for the [U]entire[/U] duration of every match (plus some piddly amounts while sitting idle on the field).
[*]And, I doubt many VRC (or FTC) robots pull 4 Amps continuously...
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OK, so the info above makes it sound like even a half-charged old battery should be perfectly fine to use in a match. Well... don't stop reading yet. The real answer is a bit more nuanced.
Pulling a lot of current out of a battery (for example taking 4 amps out of the six NiCad cells in a Vex robot battery) generally lowers the voltage the battery can sustain while that current is flowing. In our Vex NiCads that becomes noticeable. There are at least two important effects that we care about:
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[*]According to a recent middle school science fair experiment, the robot's DC motors spin a bit more slowly as the voltage across their inputs drops. That voltage comes from the battery. Based on what I learned in Electrical Engineering school, I'm not surprised. Based on what the student showed in his award-winning results, we all need to think about this as we write our autonomous-software instructions.
[*]If the battery's output voltage drops far enough below 5 V, the Vex Microcontroller can stop working. When/if the current drain stops and the battery's voltage rises, the microcontroller circuitry will begin working again (but it will have forgotten that it was first turned on a while ago and is in the middle of a match now (can you say "re-run autonomous and then do nothing more"?)).
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It's time for initial conclusions:
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[*]A well-charged battery that has not been abused and can consequently hold a lot of coulombs (a lot of electricity), and that can supply that electricity at a relatively stable and high voltage, is a good thing.
[*]Even if a non-expanded Vex bot is often pulling the 4 Amp max through it's microcontroller, a good battery can energize that bot for more than one match.
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So, ... Why sell the Vex Expander????
A second battery (plus the expander) in a Vex bot doubles the amount of power (current) that at any one moment [U]can reach the devices in the bot (mostly the motors).
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[*]The expander and the heavy battery that you attach to it, lets you send a total of 8 Amps instantaneously to your motors (4 to one batch and 4 to another). That means that even if your drive train motors are working as hard as they can using one battery, you can still have plenty of current available from a second battery to operate an arm, a winch, a shooter, the microcontroller, etc.
[*]Or, you can put almost the entire 8 amps into a drive train that can out muscle drive trains that only have 4 amps at their disposal.
[*]Or you can ensure that no matter how current-hungry your motors become or how exhausted the battery energizing your motors becomes, your microcontroller will not get reset by a voltage drop because it and some sensors are only nibbling at the current available from a second battery.
[*]Or...
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Blake
PS: I want the Qwerk for VRC!!!!
PPS: Thanks to RWSMAY for correcting the mistake in my original version of this message.
