On the IFI website, the chicklet is specified to only allow 500mA, but the backup battery is 750mA. Can someone explain this? Is there an issue on the website? Thanks!
500mA is the maximum current draw of the Chicklet. 700mAh is the capacity of the backup battery. They are different units referring to different characteristics. The backup battery should do a fine job of powering the Chicklet.
700mAh refers to the fact that the backup battery should be able to provide 700mA for 1 hour. (700mA x 1 h = 700mAh). Typically a NiCd battery such as this can be discharged at between 10 - 20 “C”, where “C” represents the rated one hour charge capacity (700mAh, in this case). So in other words, the peak current draw that you could place on the backup battery would be around 7-14 amps, but at 7A it would only last for 6 minutes (7000mA x 0.1h = 700mAh). Actually it would last for less than that, as the quicker you drain a battery, the less efficient it is.
A similar thing occurs with the 18Ah pb-acid gel cell batteries used for the robot. Theoretically in a three minute match (I’m rounding up to simplify the math) you could draw 18Ah / .05h = 360amps for the duration of the match. Again, physics and chemistry intervene to reduce this discharge rate, but the estimate forms a good starting point. Considering that there is a 120A breaker on the robot, any fully-charged battery should be good for at least two full matches at FULL power for 100% of the match time… although the batteries are much happier if you can charge them after each match.
I believe the current specification is how much current can be delivered to a USB connected device. This relates to the current delivered by the USB host (Chicklet) and you will find similar specifications on USB cards for your computers.
As Jason pointed out, you do not want to draw that much current from the Backup battery so keep your USB devices to a minimum and be careful that each one does not require huge amounts of current. A NiCad battery of this type will maintain a 700maHr rate when discharged at 1/10 C. i.e. 70 ma for at least 10 hours. Between 1/10 C and 1 C that 700maHr rating will fall. At 1 C it is likely 500maHr or less.
the reason we were asking was because the rules state it can be powered by any 7.2 volt battery and I wanted to make sure that a 4200mah battery would fry a $120 chicklet because our mentor would kill me:ahh:
Nah, as long as the voltage is right, you won’t fry anything. Even if it couldn’t supply enough current, it wouldn’t break anything.
That’s a really big battery to have at the player station.
The 500 mA current rating on the Chicklet matches the USB spec. The actual current draw varies based on the device being connected. Of all the supported devices, the Microsoft mouse draws the most current at 100mA. You can use as large of a battery as you wish so long as the voltage does not exceed 15 volts DC. Voltages higher than 15VDC cause excessive heat and could damage the Chicklet.
I made this mistake the other day.
While you most certainly could use a 7.2-15v battery, it would not conform with the rules:
Originally Posted by GDC - The Robot
<R106> Devices connected to the joystick ports of the Operator Interface via a FIRST-approved USB adapter (the only approved USB adapter is IFI Part Number USB-CHICKLET) are excluded from Rule <R105>. If used, this USB adapter must be powered with a 7.2V battery functionally identical to the back-up battery. Power from the competition port or any other source shall not be used to power the USB adapter. The USB adapter must be positioned within the OPERATOR CONSOLE so that the indicator lights may be seen during inspection and operation in a MATCH.