Quote:
Originally Posted by dcbrown
Since the specification for the battery from the manufacturer states "Max. Discharge Current 230A (5 SEC.)" - I'm not sure I'd be willing to assume the consequences of any discharge rate that exceeds the manufacturers claim as maximum. Just the safety risks alone give me pause.
The bussmann specification for the 120A breaker indicates that at room temperature, the breaker is nominally good with a 200% load for anything from 10 to 40 seconds before tripping. At 125% load, it may run for upwards of 500s or more before tripping or as little as up to 50 seconds depending on individual part. Either limit is within specification. Derating at 100F can be anywhere from -5% to +25% over rated current. Of course there is no graph given for the number of times it has been tripped vs derating. Now that would be interesting! The purpose is as a failsafe device, just design steady state current draw to be under the 120A rating and it won't be a problem -- any short high draw transients won't matter.
Bud
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The spec is for manufacturer recomendations to guarantee life cycle spec and stay within the terminal voltage spec, but the battery is capable of much more in the short term at reduced terminal voltage. A dead short will of course draw max current while at a terminal voltage of zero. Catastrophic failure of the battery is likely under these conditions.
The short term charachteristics of the breaker can withstand these loads. (as shown in your attached data sheet) I have not yet witnessed a main breaker trip during competition in a normally operating robot. Since a match is less than 200 seconds at operating current, the breaker will not trip even with four motors in stall for short periods of time.
I should mention that there are occasional manufacturing defects in the 120 amp breaker that will make it intermittant. All teams should check the breaker by tapping the red reset button when the breaker is turned to the "ON" position. A defective breaker will turn off (intermittantly) when the button is tapped. Replace any breaker that exhibits this defect.