Interesting Siren Test

I think the last one we had was in February. Others were cancelled due to clouds or potential bad weather. April's test was scheduled for the day after the Arlington Tornado. All the sirens worked fine the day before and no need to further terrify the public. July's test would have been on the 4th. August 1st: We had a good turn out of observers, both amateur radio and those reporting in by phone. A/C test: everything okay. Battery backup test: Test starts. I can hear a siren coming through an open phone line that was supposed to be put on "mute" by the observer.  A couple of seconds into the test. Observer: "Siren xx." That's unusual.  "Go ahead Siren xx." "Smoke is pouring from the box." Never had THAT happen before. I look at Irish (head of OEM) sitting a few feet away taking the phone reports today. Me to observer: "Do  you see flames?" "No." Irish says they will have someone in route immediately. I pass that info on. One of the OEM guys is out observing a different siren a few minutes away and immediately heads to check it out. I find out later that Dispatch was also monitoring and could have immediately dispatched a fire unit but don't know whether they did. One of the batteries apparently exploded due to heat and lack of water so that when the signal went out to start the battery test, it worked a second or 2 then went Pop, spraying battery acid and smoke throughout and out of the battery box. Hopefully my observer wasn't close by. If they are smart they have returned to their cars some distance away still within line-of-sight with windows UP. Sirens are REALLY loud.