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Unread 23-09-2011, 07:57
Unsung FIRST Hero
Al Skierkiewicz Al Skierkiewicz is offline
Broadcast Eng/Chief Robot Inspector
AKA: Big Al WFFA 2005
FRC #0111 (WildStang)
Team Role: Engineer
 
Join Date: Jun 2001
Rookie Year: 1996
Location: Wheeling, IL
Posts: 10,763
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Re: Current Draw vs. Stall current

Adam,
As pointed out your calculation is based on pushing against an immovable object. Typically designing for something under 40 amps per motor while driving is ideal. Battery life factors in here as well. If you draw high amounts of current a great deal of the time you are driving, your battery might not last the match. Many teams do not take this into account. The result is usually intermittent driving near the end of the match when performance is needed most. Modify your design if your strategy requires you to drive while moving/lifting/scoring game objects. While a freshly charged battery is capable of producing 600 amps, it can only do that for seconds, not minutes. You may need to make a decision between lifting or driving at the end of the match. While some teams listen and check battery life, many do not. Our mechanical team has always managed to make fairly efficient designs. This allows us to be conservative on battery usage often running 10-15 minutes full out during practice without battery change. As a rule of thumb, a team should be able to run two competition matches on a single charge. You should change batteries between matches, this is just an indicator of the efficiency of your design. If you can only run one match, someday, a great opposing alliance will run your battery down before the end of the match.
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Good Luck All. Learn something new, everyday!
Al
WB9UVJ
www.wildstang.org
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Storming the Tower since 1996.