[RE-wrenches] Strings and series of batteries

[Michael Welch]

I wish I understood this bus bar use better. Electrically, these seem to be the same thing. But by using the bus bar, there has to be more cables, cable ends, and connection points. And cost.

I just drew a battery bank (see below link to graphic), three series strings in parallel. On the negative side I drew a bus bar. On the positive side I drew normal parallel cable interconnects.

The bus bar side requires 3 cables with 6 cable ends and 6 interconnection points.

The cable side requires 2 cables with 4 cable ends and 3 interconnection points.

How can a bus bar possibly be better? There will always be one more cable, and 2 more cable ends to connect.

I do not see how either way could cause the current for one battery pass through another. It is merely using the terminal of the battery as a connector between two cables. Ditto for any difference in how internal resistance reacts, they both seem the same to me.

Related question #2:

It also has been noticed that some installers use "cross tie" interconnects for paralleling batteries in the middle of the series strings where the positive of one battery connects to the negative of the other -- not just at the final pos. and neg. outer ends of the strings. In fact, some even recommend two cables between:
http://www.green-trust.org/wordpress/wp-content/uploads/2008/09/cross-tied-battery-bank-300x187.jpg

What's up with that? If it helps make charging equal, is it worth the extra expenses and connections involved?

Here is a little jpg that illustrates both of these questions:
ftp://ftp.re-wrenches.org/pub/bbvscablewcrossties.jpg



Tom Elliot wrote at 02:19 PM 12/2/2009:
 
>Darryl, The process of paralleling through buss bars means attaching each serial string to a pair of buss bars rather than to neighboring series pairs so batteries aren't passing current through each other and aren't affected by each others internal resistance.  The buss bars then feed the inverter breaker.  It's standard practice in large telco installations which is where I got clued into the process.  I got some batteries from a wholesaler who did those installations and he was aghast at the idea of series/parallel installations the way off-grid systems have been done traditionally.