Charge imbalance between strings

[plongson]

Hi All,
Been away for a while working on tons of other projects. The system at the house has been working great, so good in fact that now we're over two years off grid and all seems well.

I do have a question though. I do a minor check of the batteries once a month and a major every two months. This includes not only checking the water level, but checking lugs for tight, clean any corrosion and SG etc.

This month after watering I checked each cable loop between batteries with a clamp-on digital DC ammeter. Here is where I'm concerned, I found one string charging more than  the other.

For example the main cable was carrying about 23 amps but one string had 10 amps the other 13 amps.

Whats going on here?? In the past I believe I always saw like 11.5 all around. (at least that is what I remember seeing...) and it has performed great for 2 years. Did something suddenly change? One string with more resistance than the other? The cross over/inter-connect cable amiss? Looking for suggestions.

These are generalized numbers because the system does fluctuate, but you see what I worried about.

THANKS and I hope everyone is doing well!!

Paul

[Vic]

Hi Paul,

Glad that your system has generally performed well over the past two years.

With parallel strings,  charge imbalance is bound to occur,  even if all interconnections are perfect,  or have exactly the same resistance (and am not saying that this is the case with your system).

Batteries themselves change over time,  and have different characteristics.

Have you measured the SG of each cell recently?  Some of this could occur because the batteries need an EQ.

Trying to cycle the bank to about 50% SOC,  or just a bit above that every month or more often is also good for batteries.  Surrettes like to be 'worked',  is one of their sayings.

But,  is would be good to check the voltage drop across each of the interconnecting cables of the batteries,  and the end-to-end drop of the cables connecting the battery bank to the inverter(s).  This measurement is perhaps easier when in the Bulk stage.  But,  realize that as any charge stage progresses,  the current flowing in the cables changes.  This change in battery current will result in a change in the voltage drop across any of the cables.   Load changes can have the same effect.

You are not looking for absolutes,  necessarily (at this point),  but for these voltage drops to be small-ish and fairly consistent from one cable to the next.

More later,  good luck,     Vic

[plongson]

Vic!! Good to hear from you man! Hope all is well on your end. THANKS for the help.

I'm gonna check that volt drop out right now. I hope there is no issues with the system, but as you know, I'm only an expert on what I have been through on these thing and a damn rookie on everything that pops up...LOL

I'll keep you posted.

Paul

[TomW]

I'm gonna check that volt drop out right now. I hope there is no issues with the system, but as you know, I'm only an expert on what I have been through on these thing and a damn rookie on everything that pops up...LOL

I'll keep you posted.

Paul

Paul;

I will butt in here to mention it would be best to test the voltage drop while either under heavy charge or draw as that will give a better indication of any issues in cabling. I will butt out now.

Good luck with it.

Tom

[plongson]

No way Tom! Dive in...LOL

I see what you guys are saying though, It needs to be charging heavy or discharging heavy to see the voltage drop otherwise in a steady state all will be equal.

I'm going to wait until tomorrow morning now to perform this test, run'er down a ways tonight and kick on the genset in the morning with a large bulk charge...

What is an acceptable voltage drop in real world conditions? A few dozen millvolts delta between terminals?

[Vic]

Because the normal voltage drops are in the order ten or so  millivolts at 50 Amps.

It has been a while since I have looked.   IIRC,  the last time I measured these drops the highest that I saw on one cable was about 30-ish millivolts at 130 Amps charge current.

Tom is correct about higher current being better,  because the resistances are quite low,  and in these low volt ranges,  there are more errors due to very few "counts" at lower currents.

Also would suggest you measure the SG of each cell when the bank enters Float.  It could be time to try an EQ,  as well.    Opinions,   YMMV.         Vic

[plongson]

Charging at 77a, all connections were nearly identical. I did have the connection between two batteries that had .100 to .150 v drop. Looks like a place to take apart and clean/reassemble.

Gonna get it to float and check the SG next...

Paul

[Vic]

Hi Paul,

Thanks for the update.   And,  yes it does seem that one cable/connections has excessive voltage drop.

Obviously,  shut down the inverters,  CC,  open breakers  etc  and be very careful with tools and the free end of the battery cable when it is disconnected from the batteries.

After cleaning the Flag terminals on the batteries,  and the lugs of the cables,  use some petroleum jelly on the entire lug on each end of the battery cable and on the entire battery terminal.  This should help keep all of the contacting metal parts from corroding from any acid vapor/splatters,   and so on.

Know that you know about this.  And measuring the SGs is a very good idea as well.

Hope the cleaning helps reduce this drop to something more reasonable.

Did you measure the drops in the inverter to battery cables?   This drop will be higher,  of course due to the probable greater length of these,  but the ratio of drop to distance should be similar to that of the battery connections.

Good Luck,   Vic