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Originally Posted by JBotAlan
On a side note, how do you plan to power this power-sled? I thought about doing this, but when I thought it through I realized that our Exide match battery lasts about 2 minutes...then it's pretty dead. That isn't a very useful lifetime. And if you put the batteries in series, 6 of them would deliver 12 minutes. Still not very useful.
Good luck,
JBot
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I hate to pile on here but six batteries in series will output 72 volts. Can you say "WOW that was a huge fireball! Where did the top of the RC go?" You will not get any greater current out of the batteries in this configuration. You can get longer run time with robot batteries in parallel (as above) but these need some steering diodes to keep from self-discharging via the other batteries. Dual battery diodes can be purchased at RV centers but are not cheap. A diode in series with the positive lead of each battery can then be tied in parallel. A simpler solution is to install a larger battery. A car battery or boat battery would work great. If you keep the same breakers, you should have no problem running for a much longer time. Be careful, car batteries can only be used in the upright position!
Back to the death in a two minute match, a well designed robot should be able to run several matches without killing the battery. When teams test their designs, they should monitor individual currents in order to determine whether the design is efficient. Often times, tank style steering with sticky wheels produce the greatest demand on the electrical system. I have experienced near stall currents on robots I have been called to look at. On a four Chalupa drive, that is well over 400 amps. A simple calculation will show that 400 amps flowing through the .011 ohms of internal resistance on the battery (full charge) will drop the battery terminal voltage to 7.6 volts. That is 0.4 volts below the point at which the RC drops out and goes to backup battery. If the drop lasts long enough, the RC will stop controlling your robot. When that occurs, motor current stops, the terminal voltage rises to normal and the robot continues. Watch some matches and when you see a robot hesitate or pause, it is often going into protect mode and disabling the PWM outputs.