Wiring battery in series gets a spark!

Farmerboyz1721

I have two 6 volt 210AH batteries that i was trying to wire in series. (Positive to Negative).

I wire up the first set of terminals just fine. 

Then, when i wire up the second set, i get good sparks and i can't leave it wired.  

Any idea what is going on??

BB.

What do you mean by "good sparks"? Arc welding/near explosion type sparks--Or just enough sparking to startle you (smaller amount of noise)?

Measure the voltage and polarity of the two strings before you connect them. You can get sparks if the batteries are at different levels of charge (100% vs 50% state of charge). Also, if there is a "bad cell" in one string, can make a problem too. Obviously, if one string is discharging into the second string--you have to figure out why (different state of charge--Charge up the "low string". Bad/shorted cell--replace battery. etc.).

If you had your charging system setup and running to the first string--Connecting the second string could give you a spark too.

As well as a spark connecting things like AC inverter DC inputs (charging the capacitors).

-Bill

Farmerboyz1721

The sparks are good enough to startle me and starts to burn divots into my battery terminals. 
The voltage on the batteries are 6.24 each. 
I will look at the specific gravity tonight and see if the SOC is different. 
How do you identify a bad cell?

BB.

If the batteries are all of the same voltage--Then bad cell/charge differences should not be the reason.

Take a DC meter and measure the voltage difference between the two cables you are connecting. Current only flows if there is a voltage difference. If it is 1-2 volt difference, a shorted cell may be the cause. If it is 6-12 volt difference, a series/parallel connection is miss-wired. If it is a 24 volt difference, then you have +/- mixed up on the two 12 volt strings.

Pay attention to battery polarity. In theory, if using automotive terminals, the negative terminal is large diameter. It is possible that a battery has been miss-assembled or miss-wired to mix the wrong +/- connections. If batteries have "flag" terminals, then you have to look closely at the battery for +/- and confirm with a meter.

It is also possible to charge a battery "backwards" polarity--And if done to lead acid, the battery should be tossed. It will never be "right" again.

-Bill

Johann

If you connect the battery cables to the battery and your inverter is connected already to the same battery cable/system then you will get a strong big spark since the capacitors in the inverter will take a big in-rush power to charge the capacitors inside that inverter.  Even if the inverter  on-off-switch is turned off it still may spark.
Maybe there is a load attached and turned on.

pdh

For the record, what exactly are you connecting to what, when you talk about the first and second set of terminals? Presumably the first set is just one cable from the first battery's positive terminal to the second battery's negative terminal, and the second set is a pair of cables, from the remaining two battery terminals to some load (probably an inverter) -- is that correct?

Johann

This minus cable goes to the '' - minus battery terminal'' at the charge controller.
                                                                         And is also used for the minus of your loads.
This plus cable goes
to the '' + plus battery terminal''
at the charge controller and also
is used for the plus of your loads.
Don't forget your fuses and or breakers.
 

Farmerboyz1721

Ok, I'm an idiot!!  IT's been so long since i have done only two batteries. I tried to connect as showen above. No wonder i got a spark.

Thanks @Johann. 

BB.

Oops... We learn more from doing things wrong vs right (as long as we survive our mistakes).

-Bill

mike95490

Farmerboyz1721 said:

Ok, I'm an idiot!!  IT's been so long since i have done only two batteries. I tried to connect as showen above. No wonder i got a spark.

Thanks @Johann. 

Something is wrong, if you added that wire and all you got was a spark.   I would expect a huge spark and ball of plasma, removing your hair.
https://www.youtube.com/watch?v=j25dh7w48Xs

Johann

Farmerboyz1721 said:

Ok, I'm an idiot!!  IT's been so long since i have done only two batteries. I tried to connect as showen above. No wonder i got a spark.

Thanks @Johann. 

Farmerboyz1721
Your welcome, glad you got the issue solved.

slim_coddy

Johann said:

This minus cable goes to the '' - minus battery terminal'' at the charge controller.
                                                                         And is also used for the minus of your loads.
This plus cable goes
to the '' + plus battery terminal''
at the charge controller and also
is used for the plus of your loads.
Don't forget your fuses and or breakers.
 

I’m actually doing this and it doesn’t spark until it’s time to connect any one as the last terminal.
yes , I’m connecting it to an inverter

BB.

Welcome to the forum Slim_Coddy,

It is pretty common to get a good size spark when making connections to the AC inverter (and some other DC loads). There are usually capacitors on the DC input of the typical AC inverter. When you make the first connection, these capacitors take a lot of current to charge. If the inverter is turned on, you can get sparkes too.

Do you have a heavy fuse or circuit breaker on the positive lead from the battery to the AC inverter input? Fuses and breakers are there to protect the wiring against short circuit (which can cause the cable to glow red and start a fire--in the worst cases).

Many times, using a circuit breaker is a good idea. It not only protects against short circuits, it also provides a handy on/off switch. Depending on the specific brand and model of AC inverter, they may need an external power switch (inverter is always on and does not have an on/off switch; or even when "off", some may still draw a bit of current).

When you turn your system "off" (summer cabin, RV, portable system), you want to make sure everything is turned off so the small loads do not take your battery dead over the following days/weeks.

-Bill