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Unread 05-11-2002, 08:09
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Al Skierkiewicz Al Skierkiewicz is offline
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AKA: Big Al WFFA 2005
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Unfortunately strictly intrepreting the battery sheet isn't a good indicator of what actually happens in real life. The reality is pulling enough current out of the battery in a two minute match to make it unusable for a second match means that you have either sucked enough current out of it to reduce the terminal voltage to 8 volts(Controller reset) or so raised the temperature that the internal resistance has gone through the roof. I worked with several teams who were able to trip the breaker. If you would like to see some of those discussions, check the archieves. It is not just the current that trips the breaker but the buildup of internal heat as well. Drawing 100 amps for 15 or 20 seconds will derate the breaker to under 60 amps trip.
Of particular note, if you have the battery data sheet, is the discharge curve vs. terminal voltage on page 2. C in that battery curve relates to the current drawn at the specified amp/hour rating or approx 17 amps. Note that the battery when at only 3 x 17 amps=51Amps draws down to 8 volts in 6 minutes. Now it is true that the battery can deliver 600 amps for a short period of time (and much lower than 8 volts at the terminal due to internal resistance) battery damage results and this is sometimes catastrophic in nature. That is why the 60 amp breaker was chosen. In any event, permanent, irreversable damage to the battery results when high current demands are placed on it.
As to instantaneous current draw, we measured currents of very short duration, in excess of 130 amps per motor on a four motor drive without tripping the breaker or dropping the terminal voltage. We were lucky, many teams we helped were drawing much more than that when we analyzed with our Stangsense current monitor.
Please remember these facts when you design and troubleshoot next season. (Oh My. it's only two months away!) Remember, they are a number of people you can turn to with questions, I am available through e-mail as well as a lot of other good engineers.
Good Luck All
__________________
Good Luck All. Learn something new, everyday!
Al
WB9UVJ
www.wildstang.org
________________________
Storming the Tower since 1996.
 


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