Is + and - always + and -?

[Matthew Sokalski]

I'm quite certain this is a stupid question, but I'd rather look stupid than destroy a battery bank.

I'm currently wiring up a 3 string, 48V battery bank. Each battery is 6V. Now, I understand that three strings is not overly desirable, but this is the situation that I find myself in nonetheless.

The bank is located on a 3 tier shelf, with one string per tier. I'm running two inverters, and therefore require a total of 4 leads from the bank. In order to keep inefficiencies as low as possible, I will be taking the positive off of the nearest battery from the top string (roughly 4' cables), and the negative off of the farthest battery on the bottom string. Currently, the batteries are oriented as below:

+/- +/- +/- +/- +/- +/- +/- +/-
+/- +/- +/- +/- +/- +/- +/- +/-
+/- +/- +/- +/- +/- +/- +/- +/-

I'm unable to find a busbar heavy enough for my bank. In the way the batteries are currently oriented, the negative inverter leads would have 3 cable to one post (two inverter leads plus the parallel lead).

If I reverse one string's orientation (to -/+ -/+ etc) allowing me to maintain a maximum of 2 leads per post (while still maintaining 3 parallel strings, of course), will the polarity be effectively reversed on that string, compared to the other two?

[Rob Beckers]

Hi Matthew,

Have a look at the attached PDF showing batteries and interconnects. The take-home message I wanted to make clear is that you come off of the bank with the inverter wires from opposite 'corners' so that the path the current takes will have the same number and length of battery interconnects regardless if if flows through the top, middle, or lower string of batteries. That will help keep all three strings doing the same thing.

If both inverter cables came off the top string then that one would see the most charging and discharging (because the two lower strings have extra wires and therefore more resistance). The batteries would get out of sync with each other and you would be equalizing a lot more often.

Three strings is not too bad, it is the maximum we recommend people use for a battery bank. It's a lot of cells to water and take specific gravity readings from though!

The big fuse and battery monitor is something we do with our simple systems to keep it safe, and to provide some means for people to keep track of how much is left 'in the tank'. It's of course just a suggestion.

You cannot flip the positive and negatives in a string and combine them. That would be a massive short circuit! Effectively you would be shorting a 96V battery bank! Results would certainly get 'interesting' and raise your blood pressure...

There's no particular issue having 3 (or more) lugs on the same battery post. Lugs are made of thick copper and conduct very well. The "rule" of not putting anything between an inverter cable/lug and the battery post has to do with people using stainless washers, crappy small-gauge terminals and other things between the high-current inverter lug and battery post. Stainless is a pretty poor conductor.

Hope this helps!

-RoB-

[Matthew Sokalski]

Thanks for your response Rob.

Just to (hopefully) clarify for any other "newbies" who might stumble across this, I wasn't suggesting that I would swap the negative and positive connections, but rather swap the physical orientation of the battery, so that I would be connecting negative to positive (from left to right), rather than positive to negative. I know it seems like an absurd question, but I thought that in that manner, I would be able to keep two cables to one lug, and also take my negative inverter lead off of the close side of the battery bank (since the negative lug on the closest battery in that string would now be free).

I can see my error now though. Being that all three strings are interconnected on each end, such an arrangement would essentially only be properly charging and discharging the one battery with the positive inverter leads. The energy would flow through the negative inverter lead, down the negative parallel interconnects, and through the battery with the positive leads.

I'll just have to get some longer negative inverter cables, and do it as your picture shows.

Thanks again.