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Originally Posted by JBotAlan
"WOW that was a huge fireball! Where did the top of the RC go?"
Oopsie.  
I really need to put a disclaimer in my sig...
I don't understand why you need extra circuitry when using them in parallel. Why would they self-discharge if you tie the positive of each battery together and the negative and draw from that? Time for me to learn something...it's a good reminder that I do not know everything. 
"Often times, tank style steering with sticky wheels produce the greatest demand on the electrical system."
Yes, that's out 'bot. I know it won't last more than one match, but then again, I know we are really heavy and do a lot of pushing. I guess it totally depends on how efficient whatever it is you are powering is. A great, heavy pusher = lots of drain on the battery (right? I was pretty confident 'series' was the right word, and now I don't know how much I really know...).
Thanks,
JBot |
Batteries in parallel will naturally not all be at the same terminal voltage. Variables in production will make the output voltage vary due to the internal resistance, the absolute surface area of individual plates, the concentrations of the acid within each cell, even the resistance of the bars that connect each cell internally. That being said, some of the batteries will have slightly higher or lower values. The batteries with the higher terminal voltage will attempt to pass current to the lower battery to keep it at the same voltage. After it has done that for while, it's terminal voltage will drop and the other battery will try to pass current to it. Think domino effect and eventually, the batteries run themselves down to zero volts. All of the current having gone into heat along the way. By placing a diode in series with each battery, it can only supply current it cannot accept current. All current will only flow out of each battery and into the common (all cathodes tied together) of all the diodes. Regular power diodes will exhibit a voltage drop of at least 0.6 volts and increasing as more current is drawn out of the battery pack. Although that works, it is not ideal when you are already taking the terminal voltage down with a high current demand. The diodes at the RV shops are Schottky ( or should be) and they have a much lower voltage drop.
Heavy pushers need not be high current robots. However, a motor in stall is still a motor in stall is still a motor that is not moving. Many designers will take this into account and choose wheels that will eventually break the friction with the floor so as not to stall the drivetrain. In tank steering, a turn will produce the same high currents unless there is modification. Omni wheels are an ideal solution.
Note to all: When a designer sees a such a need in this competition to go into manufacture to make parts that teams need, that is a red flag. Take a look at the Andymark website or the IFI website and see parts designed to solve the particular problems we face in robot design. These people are not going to design and market items that no one will buy. Like selling refrigerators to Eskimos. Native people in Alaska and Canada do not need refrigeration when the ground outside their door is permanently frozen.
Keep asking questions. A person who recognizes they do not know it all will continue to progress and be successful.