Battery Beak

[Mike Copioli]

Originally Posted by Siri

So just to make sure I understand, if I use the Beak on a battery I just finished charging in the pit, the Good ones will read "Fair" because of the higher Rint? (Do Fair batteries read "Bad"? Bad ones still read "Bad"?)

Not necessarily, the batteries Mark tested had a higher than normal internal resistance it just so happened that the last battery was right at the threshold between 'Good' and 'Fair'. I would reserve that battery for practice only. A good rule of thumb for surface charge in SLA like the ones used in FRC is 10%. What this means is; if you take a battery directly off of a charger and then test it you get a 10% increase in the internal resistance and SOC from the surface charge that has not had time to penetrate the lead plates. For a healthy, new FRC battery this should not cause a 'Good' battery to read 'Fair'

Originally Posted by Siri

What's the minimum percent Charge that still indicates a surface charge to you (somewhere between 130% and 113%, I take it)? Is surface charge ok in actual use of the battery?"

Surface charge is a result of the chemical reaction inside the battery being slower than the rate of charge provided by the charger. There is always some surface charge when a battery is first removed from the charger due to this reaction, it is perfectly normal. Again it accounts for apx 10% of the total charge in FRC. In other words the surface charge is deterministic and not unpredictable. So if you wanted to test your battery right after removing it from the charger just deduct 10% from the result. Or let it rest for about 5-15 minutes.

Originally Posted by Siri

Thanks! I suspect these are stupid questions to anyone with a modicum of practical EE knowledge, but I regret I'm not really one of those people. I do need to fix this.

These are valid questions that we asked ourselves during the time spent developing this device. As it turns out surface charge is not a large of a variable as we expected.