07 Precedent 4 12volt system big spark on battery post

[grandpa1]

Hello BGW. I have an issue with a 2007 Precedent cart that has 4 12 volt batteries. Just helped a friend put new Trojan batteries in his cart. Cart was running some what before but kept running dead real quick. 2 batteries had bad cells. Anyways batteries are in and wiring is correct. When you go to hook up the main negative you get a big arc not just a lil arc but a welding type arc. Run/ tow switch is off. What is weird is you can put the meter on the main positive and then touch the main negative cable which is not hooked up and I get 50 volts. I am a lil confused at what is going on. Can I get some help on this please?