Large spark when putting battery in series?

Mike7599

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Mar 16, 2022
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Hey guys I have a question. I’m getting a very large spark when trying to connect my batteries in series. So much so that it slightly damaged the battery terminal. Is this normal? I’m not connected to anything else, just the two batteries together. Back story: I have two 100ah 12v battle born batteries. I’ve been running a 12v system but want to convert to 24. So I charged them up in earlier in the week using solar. Once they were floating I totally disconnected them from my system and left them sitting in parallel all week just to make sure they were at the same voltage before trying to put them in series. This morning i disconnected them and measured them individually. Each read exactly 13.5 volts. So I started hooking up in series and pop! Huge spark. I’m like wtf? So I thought maybe this was some kind of quick discharge (sort of like when you discharge a capacitor) but sure enough it happened again the second time I tried and was just as strong a pop. The lug got black and the battery terminal got slightly messed up from the arc. I did not try again for fear I may short one of the bms’. I’ve heard of large sparks if your hooking to an inverter but that is not the case here. What do you guys think is going on? Large rush of current from one battery to the other? But why if they have been sitting in parallel for a week? Any help would be appreciated. I’m slightly nervous to try again.
 

Wjm1964

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Feb 4, 2022
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I'm assuming here you only connected + to - or visa versa with a single cable? The remaining + & - are your new 24v feed and they don't connect but again its just an assumption as i don't know what else would cause this. No doubt someone will know more
 

Mike7599

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Mar 16, 2022
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I connected the first battery cable from + to - with no problem. It was when I tried to hook up the second cable from - to + that it sparked
 

Supervstech

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I connected the first battery cable from + to - with no problem. It was when I tried to hook up the second cable from - to + that it sparked
Stop...

HOW MANY BATTERIES ARE YOU CONNECTING?

series with two batteries is ONLY ONE CABLE...

If you then connect the other two positive to negative you have a potential explosion...
 

Wjm1964

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Feb 4, 2022
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You don't connect the the remaining terminals together, the positve + is now main positive, the remaining negative terminal is now main negative terminal. Those wire go to plus and minus on your inverter
 

chrisski

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Aug 14, 2020
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I suspect a large inverter or somethinng is in the complete circuit. If so see precharging with a resistor.
 

Mike7599

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Mar 16, 2022
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Bingo. You are right on. Haha how embarrassing! Sorry for a newb mistake. Good thing I put this in the beginner section. Yes only 2 batteries I should have never tried the second wire. Thank you!
 

Wjm1964

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Feb 4, 2022
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Bingo. You are right on. Haha how embarrassing! Sorry for a newb mistake. Good thing I put this in the beginner section. Yes only 2 batteries I should have never tried the second wire. Thank you!
We all make mistakes, it could have been much worse, just hope your BMS isn't fried
 

Mike7599

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Mar 16, 2022
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Last question guys. So the batteries are still measuring 13.5v. Hoping this is a good sign that I did not fry either bms. Is there any way to test the bms is working or should I just install them back in the system and see if they charge and discharge normally?
 

time2roll

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Mar 20, 2021
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Last question guys. So the batteries are still measuring 13.5v. Hoping this is a good sign that I did not fry either bms. Is there any way to test the bms is working or should I just install them back in the system and see if they charge and discharge normally?
Best test is to connect a 24v load. I doubt there is an issue for doing this once.
 
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