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Unread 18-12-2010, 11:10
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: 10,811
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Re: Warming up the batteries

I know that this can be confusing so I am going to make a stab at this. The MK battery charts are generalized, guaranteed specifications for a new VLRA battery. The company has set these standards for the entire line and the graphs are plotted for Amp Hour vs discharge current rated in C where C = the rated discharge rate in amp hours. For our battery C=18AH. The MK Company charts are published on their website for VLRA batteries. The curve we have been looking at is a converted chart where the C values have been converted to discharge amps. .1C=1.8 amps, .5C=9 amps, etc.
For Team 111, the CBA tester is then set to it's maximum current (for this model) of 7 amps. Each battery, each year, is then tested at the beginning of the build season and the curves saved for each battery. If a battery tests out at 15 AH or above for the test, it is considered good enough for competition. Anything under is marked for "practice only". If there appears to be cell failures as shown previously, the battery is recycled. Failures of this type can be frustrating to team members and I fear can cause damage to chargers. I might be persuaded to run higher current testing if I were to purchase the higher current model CBA but I would not test at anything above 18 AH rates to maximize battery life (charge/discharge cycles). This testing does duplicate published company specs and curves leading us to group like batteries as healthy enough for competition.
As noted, even bad batteries can show normal terminal voltage before a cell(s) fail. Don's test, however, would also fail when testing a similar battery.
In Don's test, as pointed out by Alan, the current is much more than 12 amps. It is however within the realm of current drawn during match play. By performing this test, Don's team has been reasonably sure that the battery will survive a two minute match. I would agree, but point out that max battery life will suffer. The kind of current drawn during this test is beyond the current drawn by most FRC robots during a similar time period. An FRC robot might draw this current in a failed autonomous run where the robot is pushing against a wall or other barrier for the entire auto period. It is possible to draw this much current on an FRC robot as the 120 amp breaker will not trip. It is possible to draw up to 600% of the rated current for several seconds without a trip on this breaker.
Don and I are both amateur radio operators and a familiar term for us "key down". In our hobby we relate "key down" time to the average battery usage of portable operation. It is usually calculated by a percentage of actual transmit time (high current) and receive time (low current) operation. Many of us have discussed using this analogy for FRC robots but unfortunately, there is no standard robot for which to make predictions. In either case, both of our testing, although possibly flawed, can predict with some certainty that a battery so tested will survive a two minute match. Simple voltmeter testing, very high current load testing (car battery handheld load testers) for seconds, and short driving tests will not guarantee a battery will last a two minute match plus auto.
It is also a problem when people quote sections of the manual out of context. For instance, the maximum current specification on this battery is 720 amps for several seconds. Note that terminal voltage is not specified here. 720 amps would drop nearly 8 volts across the internal impedance of the battery. It is why the designers of the PD made two boost buck regulators to feed the Crio and wireless access point. High current demands would shut down these two critical systems without the additional power supplies rendering the robot useless for most 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.

Last edited by Al Skierkiewicz : 18-12-2010 at 11:13.