View Single Post
  #18   Spotlight this post!  
Unread 18-03-2012, 08:48
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: 10,798
Al Skierkiewicz has a reputation beyond reputeAl Skierkiewicz has a reputation beyond reputeAl Skierkiewicz has a reputation beyond reputeAl Skierkiewicz has a reputation beyond reputeAl Skierkiewicz has a reputation beyond reputeAl Skierkiewicz has a reputation beyond reputeAl Skierkiewicz has a reputation beyond reputeAl Skierkiewicz has a reputation beyond reputeAl Skierkiewicz has a reputation beyond reputeAl Skierkiewicz has a reputation beyond reputeAl Skierkiewicz has a reputation beyond repute
Re: Battery Best Practices

Aaron,
The 7.2 amps is sufficient. set your cutoff voltage to 7-8 volts, which is the cutoff for most of the battery spec sheets. It will take more than a hour to run the test. After a few minutes the battery voltage should fall a slight amount. This is due to any remaining surface charge and the all of the components reaching a set temperature. After that the voltage should remain relatively constant until the end of the test when the curves start to drop off to the 8 volt limit. The program runs constant amp hour calculations and should match the battery curve very nicely on a good battery. What you are looking for is a drop of 2 volts during any part of the test. This is one of the cells giving up. I have a had a few batteries where one gives up and then a second gives up a little later. The cell loses capacity if the electrolyte is not full in that cell or if one or more plates have broken within the cell. On a rare occasion a battery will start off at 10 volts indicating a cell is shorted. Once the test is complete, save it with a unique ID that you also mark on the battery. That way you can track the same battery from year to year and watch for end of life when the battery no longer can ~18 amp hours. The program also allows you to overlay several curves so you can look at the same battery annual curves or you can compare batteries you want to use for competition. The 7 amp discharge is nowhere near what the robots will pull during a match but remember you are testing to manufacturer spec in this test and that is really the only valid test to determine if the battery meets spec. I also recommend that if you are testing a suspect battery, try moving the cables around and tap on the battery to see if you turn up any intermittent voltage dips. We added an Anderson connector to our CBA II to make it easy and all of our chargers are also so equipped with the APP connectors. We do not use alligator clips and neither should you.
If you search, I put up some curves a long time ago to demonstrate all the possibilities.
__________________
Good Luck All. Learn something new, everyday!
Al
WB9UVJ
www.wildstang.org
________________________
Storming the Tower since 1996.