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Trying to understand stuff after a foolish mistake

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Jonathan123

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Joined
May 26, 2022
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Location
Arlington, MA & Chesterville, ME
Ok I made a mistake and almost fried my system. I’m not clear on exactly what the energy flow was or whether I did lasting damage to my equipment.

I had the positive cable that runs between my 12v battery and my solar controller come loose. I was in a rush and when I re-installed it I accidentally put that positive cable into the controller’s negative load port. It sparked green a bit so I didn’t actually attach it. Without noticing my mistake yet I tried again a few very short times and kept getting more sparking than what is normal when connecting, so I stopped and then noticed I had the cable in the wrong port. At that point I had everything disconnected and the wires were very hot. After letting everything cool down I put the positive cable from the battery into the correct port and set it all up as appropriate. I tightened down all connections and the system is working fine again.

My two questions are:
1. What exactly happened when I connected the positive cable from the battery to the negative load port? It was obviously a bad thing but what was happening there that the cable got so hot from just a few brief touches of these connections?

2. The positive cable end was very slightly melted. I could still see individual strands so it wasn’t a total meltdown. Have I done lasting damage to the battery? Since it’s working correctly can I move on or should I replace that positive cable with a new one?

Lesson learned of course. I need to slow down and pay attention in these situations.
 

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What happened is you were shorting the positive of the battery to the negative of the battery . Likely the terminals for Bat(- )and Load(-) on your SCC are directly wired together so that current did not flow backwards through your SCC. However damage could still have occurred depending on capacitors discharging from the dead short.

Hard to hurt a battery from momentary shorting. You can use them to weld with. Easily melt your wires though.
 
Mattb4 said:
What happened is you were shorting the positive of the battery to the negative of the battery . Likely the terminals for Bat(- )and Load(-) on your SCC are directly wired together so that current did not flow backwards through your SCC. However damage could still have occurred depending on capacitors discharging from the dead short.

Hard to hurt a battery from momentary shorting. You can use them to weld with. Easily melt your wires though.
Click to expand...
Should I leave it alone or do I need to change that wire do you think? It feels cool to the touch and seems fine. On the other hand there was some amount of melting.
 
Jonathan123 said:
Ok I made a mistake and almost fried my system. I’m not clear on exactly what the energy flow was or whether I did lasting damage to my equipment.

I had the positive cable that runs between my 12v battery and my solar controller come loose. I was in a rush and when I re-installed it I accidentally put that positive cable into the controller’s negative load port. It sparked green a bit so I didn’t actually attach it. Without noticing my mistake yet I tried again a few very short times and kept getting more sparking than what is normal when connecting, so I stopped and then noticed I had the cable in the wrong port. At that point I had everything disconnected and the wires were very hot. After letting everything cool down I put the positive cable from the battery into the correct port and set it all up as appropriate. I tightened down all connections and the system is working fine again.

My two questions are:
1. What exactly happened when I connected the positive cable from the battery to the negative load port?
Click to expand...

A short circuit. This is why fuses/breakers are important.
Jonathan123 said:
It was obviously a bad thing but what was happening there that the cable got so hot from just a few brief touches of these connections?
Click to expand...

It is full ampacity of the cable and wire with a short circuit. The amount of amps is only limited by the resistance in the cable and what the battery can discharge. In the case of LFP batteries, the battery can discharge more than the wire can handle.

You noticed how hot the cable did get. This is why fuses/breakers are important. High wire temps lead to fires and melted wire that can explode. You were very fortunate.

The BMS may have saved you. It should have attempted to stop discharge under high current. Each time you broke the arc, the BMS would resest itself.


Jonathan123 said:
2. The positive cable end was very slightly melted. I could still see individual strands so it wasn’t a total meltdown. Have I done lasting damage to the battery?
Click to expand...

Possibly, you need cell data access. Most likely the cells will be OK but if it is a FET based BMS, the FETS could have suffered damage.

Jonathan123 said:
Since it’s working correctly can I move on or should I replace that positive cable with a new one?
Click to expand...

If you hold it upright and it stands straight up, it was definitely overheated.

If it bends easily and insulation is not melted, it will probably just need a new end.

Jonathan123 said:
Lesson learned of course. I need to slow down and pay attention in these situations.
Click to expand...
It's a good lesson, it shows why fuses and breakers are important. If there was a fuse, it would have blown.
 
Jonathan123 said:
Should I leave it alone or do I need to change that wire do you think? It feels cool to the touch and seems fine. On the other hand there was some amount of melting.
Click to expand...
Insulation melted, new wire. End melted, new end.

Was this a lead acid or LFP battery? Lead acid would not discharge as fast as LFP.
 
One more thing, do you have a fuse/breaker (properly sized) between the SCC and battery?
 
Must say the layout order of those ports is atrocious. Bad practice for the manufacturer to put labels at the top that make the ports seem like their counterpart is next to one another, then putting different labels on the bottom.
 
Easy error to make given the position of the terminations on the controller. Most controllers group the connections in pairs, just a poor design.
Every positive cable from the battery needs overcurrent protection.
 
Agreed. You actually have to look at the bottom of the unit to see those labels. Still that’s ultimately on me, clearly.
HighTechLab said:
Must say the layout order of those ports is atrocious. Bad practice for the manufacturer to put labels at the top that make the ports seem like their counterpart is next to one another, then putting different labels on the bottom.
Click to expand...
 
I would size it based on the amperage your SCC puts out to the battery (example 20a max charge rate put in a 25a breaker). Your wire should also be properly sized (example 20a max charge rate use 10 awg or 6mmsq).
 
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Mattb4 said:
I would size it based on the amperage your SCC puts out to the battery (example 20a max charge rate put in a 25a breaker). Your wire should also be properly sized (example 20a max charge rate use 10 awg or 6mmsq).
Click to expand...
The SCC puts out up to 60 amps according to its marking. Did you mean that or the amperage coming from the panels?
 
Jonathan123 said:
The SCC puts out up to 60 amps according to its marking. Did you mean that or the amperage coming from the panels?
Click to expand...
It is the 60a that you would need to provide circuit protection for going to the battery from your SCC if your Solar panels can put out that much wattage. W=VA

So it is a bit harder to find a close match in breakers and fuses. Likely have to go to a 80a or higher and 4awg wire if the full 60 amps charge current can be reached.
 
Mattb4 said:
It is the 60a that you would need to provide circuit protection for going to the battery from your SCC if your Solar panels can put out that much wattage. W=VA

So it is a bit harder to find a close match in breakers and fuses. Likely have to go to a 80a or higher and 4awg wire if the full 60 amps charge current can be reached.
Click to expand...
We have a very modest system with minimal draw. Some LED lighting, a solar fridge. The 4 panels are 200 watts each. I may install the inverter that came with my system but I’m never going to be using a ton of power. The battery is 12v 200 amp hour / 2500 watt hour.
 
Jonathan123 said:
We have a very modest system with minimal draw. Some LED lighting, a solar fridge. The 4 panels are 200 watts each. I may install the inverter that came with my system but I’m never going to be using a ton of power. The battery is 12v 200 amp hour / 2500 watt hour.
Click to expand...
My other circuit breaker is 30 amps. Wonder if that would do for the battery to scc as well. What’s the formula here for calculating this? I obviously have everything to learn.
 
800w/12vDC=66.67 amps Your SCC can only output 60 amps (720w at 12v but 840w at 14vDC) so you are around max paneled. Your battery at 200ah would be charging at a .3C rate with 60a incoming. This may or may not exceed your batteries recommended charge rate. Most seem to like .2C, you would want to research this.

Since you are dealing with the full 60a, and no way to adjust it with your SCC, you need to provide proper circuit protection and wire size.

BTW: Your wires/breakers/fuses going to your 12v/DC fuse panel from your batteries (I assume you are not running an inverter) should be sized and fused based on the fuse panels needs. Say it carries 30a worth of loads at any one time you would likely try to wire it for 50a service. It might have individual fuses that add up to more than 50a but it is the anticipated loading you need to provide for. Just like in a homes Main panel that has 200a service it might have breakers adding up to higher than 200a.
 
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