Sparks when trying to test golf cart batteries with a volt meter

I bought a used golf cart a few days ago which had their batteries replaced last year. They hooked up an am/fm radio to it and it worked fine when you flip a separate switch. There is a “Tow” switch by the batteries that I was told to switch it to when not using the cart so there wouldn’t be any drainage when it in use. So I charge it with the Modz Max 48 charger until it shows green, then take it off and it sits over night. The next afternoon, my wife tries to take it out. Flips the tow switch, put in the key, then hit the accelerator. Goes 3ft then stops. Won’t move after several attempts. Gets me and after I flip the tow switch a couple times it works. Wife takes it out and when she comes back, tells me the radio doesn’t turn on. Flipping that separate switch that turned it in the night before doesn’t work. So I charge it again until it turns green and let it sit for an hour or so. I then decide to test the batteries with a volt meter from a YouTube video I saw. Put the meter into the bar with three dots on the bottom and put the red probe on one of the positive + terminals and the black probe on the negative terminal. When I connected to both, there was small sparks and then the volt meter wires got real hot. Felt like the wires were going to melt so I pulled them off right away. I tested the golf cart and it worked fine, but still can’t get the radio to power on. Did I mess up something with the volt meter in simply trying to test the batteries? Thanks.