Russell, Was the sound coming from the battery or the relay? Was the relay initially open and then closed after the battery was connected? I had a coworker recently try to drive a 24V DC brushed motor from two 12V VRLA batteries through a beefy DC-rated relay. Sometimes it would make a loud whistling sound when the relay closed. Other times the relay would stick closed. We measured the starting current of the motor at about 200A DC. I believe that the mini arc blast that was happening when the relay contacts bounced was enough to prevent them from closing fully, leading to a sustained arc that made the whistling sound and melted the contacts so that they would stick together when the current was interrupted elsewhere in the circuit and they could finally come together. Relay was enclosed so we could not see the arc. Problem was eventually solved by using a DC solid-state relay instead of a mechanical one. Sean On Sun, Dec 21, 2014 at 5:12 PM, RussellMc wrote: > 40 Ah 25V LiFeYPO4 battery connected across 30A capable relay terminals > makes an impressive sound. But, not for long. > > > > R > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .