[FTC]: Tips for Teams going to the Vex Championships

[jbbjjbt]

Originally Posted by gblake

Precision is important when using English. Let me see if I can untangle the causes and effects being discussed here.

4 Amps is max current the Vex microcontroller will supply through the sum of all of its PWM/Motor ports.

Attempting to draw more than 4 Amps through the PWM/Motor does not cause a Vex microcontroller to reset.

Drawing current out of the Vex batteries will result in a voltage drop at the battery's output terminals.

If the current drawn is enough to lower the Vex battery's voltage far enough below 5 Volts, the Vex microcontroller will shutdown and be reset when it starts back up.

Sometimes this happens during matches when the loads the motors are experiencing rise and then fall and when the robot's battery isn't up to the task of keeping the robot healthy. The current drawn rises and falls. The battery voltage falls and then rises. The microcontroller shuts down when the voltage drops, and restarts when the voltage rises. At start-up the microcontroller starts executing it's user code, beginning with the first instruction of that code.

Blake

Now, This all makes sense and I believe it.

And this reminds me, Our team, Twisted Bots, owes a big THANK YOU to Blake and his team for loaning us batteries to get us through the finals. Ours gave up.

Blake,
Could you please post details on the battery charger that discharges the batteries so they work better?

Thanks,
Jon T
Coach Team 177
Twisted Bots

[gblake]

Originally Posted by jbbjjbt

... Blake - Could you please post details on the battery charger that discharges the batteries so they work better? ...

Jon,

1) I didn't spot your question when you first posted it. This answer is wordy; but hopefully it is also worth reading.

2) You were very welcome!

3) For as long as I can remember NiCad batteries being around, I have heard about something called the memory effect. There are other similar names for it. Whatever it is called there is a real observable effect that can bite you in a competition.

The bottom line is that drawing just a little of a NiCad cell's stored charge out and then recharging that cell, tends to decrease the total amount of current you can draw out of the cell in later uses. Doing that over and over again (take out a little charge and then put a little charge back into the cell). will drastically reduce the amount of charge you can withdraw from the cell.

Using a multi-cell Vex 7.2V Battery in a single competition might hypothetically pull about (2.5/60)Hrs * 4000 mAmps = 167 mAmpHrs out of it (the actual amount drained will typically be significantly less).

The Vex 7.2V batteries are designed to hold 2000 mAmpHrs (mAH) of charge.

If after using a battery in a single match (or after a period of light use during a team meeting) you slap it into a charger and replace those 167 mAH; and if you do this a over and over again, after a while, the battery is not going to be able to supply 2000 mAH. In fact, I predict that it is going to fail at supplying even the full 167 mAH (or significantly less!) that you have been pulling out and putting back in.

The failure will be manifested in a very dramatic drop in the Voltage the battery can sustain across its output terminals. The result of that voltage drop under load is something that I/we covered in earlier messages in this thread.

It is very difficult (well nigh impossible without meticulous measuring and record keeping) to tell the difference between a battery (#1) that sustains 7.8V under no load, but that only has 200-300 mAH available and will suffer a dramatic voltage drop if asked to supply much those mAH quickly to satisfy a high load; and a battery (#2) that shows 7.9V under no load, but will be able to keep its voltage up pretty high while supplying the 167 mAH (max) needed in a match (because the battery is able to supply nearly 2000 mAH if necessary, and its voltage won't drop badly until the charge is nearly exhausted).

This "memory effect" is, at least partially, reversible.

You reverse it in essentially the same way that you avoid it in the first place; by draining a cell before you recharge it.

Always attempt to completely exhaust a battery before recharging it. It is exhausted when applying a load causes the battery's voltage to drop quickly (from >7V down toward 5V and lower). In a Vex robot, a sign of this will be seeing the microcontroller's Red LED (in the eye part of the indicator panel) blink on each time you tell a motor to do some substantial work (like move the bot around).

Another way to exhaust a battery will be to use a commercial charger/discharger. These should have circuitry that prevents you from draining any of the battery's cells all the way down to 0.0V. According to my charger's manual, draining them that low can damage the ones that reach 0V first by causing current to flow backwards through them.

Using a big resister or other "dumb" device to drain a battery of cells is another method but it is not recommended for the reason cited in the paragraph immediately above.

Discharging a battery that is in good condition, before recharging it will help keep it in good condition.

A battery that is in bad condition because of often being partially discharged and then recharged; can be brought back to some measure of its original health (capacity) by running it through many "deep" discharge, then recharge, cycles.

I have some batteries that I got from eBay that don't hold the full 2000mAH that they should. They have improved a bit as I have run them through many discharge/recharge cycles; but they are still far from holding the 2000mAH that they should. They might have permanently lost some of their storage capacity. Others have become dramatically better after running through several cycles.

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Related but separate topic.
I bought a SuperBrain 977 charger so that I could keep track of how much charge each of my batteries can hold. The 977 has a clumsy user interface; but that said, it lets me do the following:

• Charge my NiCad packs using either a typical 120VAC socket as a power source or use a 12VDC car battery.
• Keep track of how much charge gets stored in a battery as I charge it.
• Keep track of how much charge I can extract from a battery as I discharge it.
• Vary the charge/discharge rate (measured in Amps) the charger uses.
• Vary size of the voltage dip the charger looks for to tell that a battery is fully charged.
• Charge two batteries at a time using independent settings, or charge one and discharge another.

When I bought my 977 I paid about $130 for at a local hobby shop.

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Blake
PS: Some teams (including mine) bad-mouthed their batteries incorrectly - I think that we were suffereing from overheating motors, and the symptoms looked like exhausted batteries - When I checked the batteries on the charger/discharger, they were doing fine - They could supply lots of Amps for far longer than one match without a dramatic voltage drop.