Cross-Tying Batteries

Horsefly

My battery bank is a 2S2P array of four 12V batteries. I hooked them up in the standard way, with two strings of 24VDC, and the cables to the Inverter (and Charge Controller) connecting to opposite corners.

I've wondered to myself if cross-tying the batteries (connecting the mid-point of each of the 2 strings) would help them to maintain balance. Instinctively it seemed like it would. When I first thought about it I did a search here and found a ~4 year old thread where the consensus seemed to be that cross-tying was a bad idea: http://forum.solar-electric.com/discussion/19141/cross-tie-parallel-strings-of-batteries-yes-or-no

Then today @Marc Kurth posted a link to a Full River manual about using their batteries (http://forum.solar-electric.com/discussion/comment/375271#Comment_375271), which specifically recommends cross-tying to maintain balance and extend the life of batteries. In fact, they show an example with a 2S2P configuration just like mine.

What's the current thinking of you experts on cross-tying?

Steve

BB.

I do not like cross tying batteries. If everything is working well, cross ties should not carry any current. If something goes wrong cross tying can hide problems (shorted cell, is fed by paralleled battery. Open cell, current skips one battery and the second now carries all of the current--Stressing the good battery/limiting overall bank capacity).

Cross tying also makes it difficult to measure current/fuse/breaker a battery bank (do you fuse each cross tie?). Lots of extra cabling (cost, complexity).

Large Li Ion battery banks do cross tie a lot (multiple cells bolted in parallel, then a single series connection to next pack of cells). So, there is precedence for do this. But Li Ion batteries are generally matched cells and have low self discharge rates (batteries are not really "balancing" between paralleled cells. May make battery management easier (one BMS connection per parallel pack?).

I am not a battery expert, and it would not be my place to argue with AGM battery engineers that cross tying batteries does or does not increase their service life. But at a system level, not my choice.

-Bill

Estragon

I wouldn't do it for the reasons given in the first thread linked. Balance problems can be more easily identified and more safely dealt with by periodic EQing and/or separate charging IMHO.

Horsefly

Ok, thanks @BB. and @Estragon.  I guess I'll leave well enough alone. 

The reason I was thinking about it: When I first got my four batteries, two had exactly the same voltage (13.09V), one was slightly lower than that (12.94V), and one was higher (13.15V). I hooked them all up in parallel for about 24 hours in hopes it would get them all to a common starting point. I thought about putting the whole bunch on a "dumb" 12V car battery charger for a while, but decided against it.

After a couple of weeks I pulled them off and let them rest for about 5 hours. When I checked them, they were all relatively similar to where they were before I first hooked them up. I guess it isn't enough to be a big deal, so I'm OK with not doing the cross-tying. 

Estragon

13.x seems high for a resting voltage, and 12.14 low. I would probably put the 12.14 one on a charger by itself for a while and watch voltage and temp.

Horsefly

I should have said: These are AGMs, and the manufacturer says that about 13.1V is 100% SoC.

Horsefly

Ooops. The 12.14V was a typo. I meant 12.94V.  I fixed it in my OP.

Crosstie

I have used 6 cross tied GC2 flooded cells(~600amp hours) in an off grid cabin in northern Maine for 17 years now. I am finally purchasing new BATTERIES!
The old ones , with 1 cell exception, still function at diminished capacity. Time to freshen up! 
Proof of the pudding is in the eating. If maintained cross tie  it is the only way to go IMHO.

jonr

No doubt that cross-tying improves balance.   But if you don't want to, you can get some of the advantage by measuring all 4 batteries and arranging them to be better balanced (than the usual random).