Polarity issue

dougahole

I recently purchased a digital amp meter in order to correctly determine health of my system. I have 2300 watts @24 volts wired in parallel, with the system split, two controllers (EPSolar MPPT 35A), 2 strings of  4 x 6v fla L-16 batteries in series and then in parallel. I should also add that the system is fused all the way and properly grounded.

It is not unusual in our northern climate to encounter those dog days of winter where it is necessary to run the generator (it runs power to the batteries through the inverter (AIMS 3kw) in order to supplement the system so I am not surprised to hear the inverter singing for lack of power. I also recognize the issue with power and cold batteries not conducting power that well.

This system has been in place for a few years but don't ask why it took me so long to purchase an amp meter.

But, when I decided to compare the amp reading from the controllers to the actual amps being delivered I got a surprise. The numbers were correct, but one string was (-) instead of (+). Needless to say, I could not believe it and scoured the system to make sure that everything was properly wired. So, just to be clear, I put the clamp on the cable going to the (+) side of one of the strings, it read (-). At the opposite end, it read (+)

BTW, the other string is fine. So, do I have a critical situation? I repeat, the battery strings are in series, then in parallel  I do not know if this has always existed, of if something has occurred to cause it. What kind of damage is this possibly doing to my other components and the batteries? Should I cut the one string loose until this is resolved?

Any help is appreciated. Thanks

mcgivor

Don't develop too much sweat, DC clamp on ammeters are polarity sensitive, usually provided with an arrow in the claw to indicate which way the current flows, which is negative to positive, turn the claw 180° and a positive reading will result, AC on the other hand has no destinstinction, it will read current in any position.

BB.

It is possible to have the opposite current flow in an otherwise correctly installed battery bank... For example, if you have a 48 VDC battery bank with two or more strings--And one string has a shorted cell and is really now a "46 volt" battery bank... That 46 volt string will take more current charging, and with the battery static at ~52 volts, may actually discharge the other parallel string(s).

But if you have a battery/string backwards--Generally it is really obvious (smoke/dead batteries/overheating/etc.).

Now--If you have reversed a solar panel string (hooked up one string backwards)... The solar panels are just giant diodes, so when wired "backwards", the are now pretty much a dead short. The one backwards string should short out (or pop a fuse/breaker in the combiner box for that string). If the solar panel(s) are not exposed to too much current--The can be OK. If you hook a string backwards to a lot of parallel strings (especially no breakers/fuses/combiner box) or backwards directly to the battery bank--It will generally fry the panel.

If you have a 2p x 5s array... And connect one panel backwards in one string--Basically you have 4 series panels for that string, and either low current flow (compared to the other strings) or no current flow--Not easy to see (at the charge controller current readings), but very quick to find with a clamp meter (under sun in operation, measure the current in each string--All the strings should be pretty close to matching).

But like McGivor says... DC Current Clamp meters do read current polarity (AC operation--there is no +/- current direction with clamp meter). It is easy to "reverse" the current clamp on the cable and mix up the +/- sign.

And it can really play with your mind which direction is +/- current flow... Is current flow + for charging a battery bank? Or is it + current flow for discharging a battery bank. Just depends on how you look the system.

-Bill

dougahole

Thank you both for your advice. You are right, it has been driving me crazy. But as a mater of fact I do have a dead cell in one of my Crown batteries and I notice that the controller associated with that string is delivering more power to that string. So that could also add to the issue since the strings are also in parallel.

This is the second Crown battery that has craped out, only 4 years old. My other string is US battery, older and in better shape. I guess it is what you pay for.

Thanks again for your advice