QUOTE(pianotuna @ Nov 23 2009, 02:59 PM)

Hi Prof,

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Can you explain why balanced wiring is not important for charging? Resistance is still identical at even very low amperage, so the last battery in the chain may never fully charge, or if it does then the first one may over charge? It can't be both ways?

If the balanced load points are already in place on the load side--there is no reason on earth to not use them for charging. It costs very little extra to wire in a balanced manner--sometimes even zero cost--if for example there are only two 12 volt batteries.

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Don,

Try this formula:

Voltage drop = k x p x wire length in feet x current in amperes divided by the wire area in circular mills

For Copper, K=11

For Aluminum, K=18

P is Phase Constant. In this case P=2

Let’s use #2 AWG copper as our interconnect cable. The area in circular mills will = 66,400

Our interconnect cables will be 1 foot long each

Again, let’s make an assumption the amperage draw from the batteries is 120, typical for a 1,000 watt inverter load.

Run the numbers:

11 x 2 x 1 x 120 = 2,640 divided by 66,400 = .039759

So the voltage drop on that 12” piece of #2 copper is less than 4/100th of a volt.

Since my example assumes two batteries, the interconnect cable will only carry 50% of the current. Thus the voltage drop on the parallel cables will only be 2/100 of a volt – or better said 20 millivolts.

A 20 millivolt loss is no big deal in a battery bank set-up for an RV – even if we have more than 1 one foot interconnect cable.

Manufacturers of high power inverters often recomend two size “0” cables in parallel. (106,000 CM each, total 212,000 CM).

Run the above numbers again for a lowered charging current. The voltage differential amounts to nothing significant!

More specifically, if you have four batteries in a parallel string with no “balancing” and connect a charger, the maximum current will go to the lowest value battery first. Once it is equal to the next lowest they will “appear” as a single battery and combine with the remainder as they are charge equalized. Even 4 brand new batteries in parallel will not exhibit the same state of discharge. Manufacturing is far from being that perfect.

I’ll say what I did previously again – for RV use all this “balancing” stuff is unnecessary unless you are totally battery anal. There are other uncontrollable variables in the system that will have a greater impact.

As for intentionally wiring so that a bank is "balanced"; yes – sometimes it is just as easy and will not require an additional length of cable to configure it as such. But the question always remains when setting up batteries in an RV as to where they will be mounted. Often they will be some distance apart. Not every RV’er that wants to expand battery capacity has a big motor home with a huge pull-out battery tray (but it would be nice if we did

).

As for the 1% - We’ll just have to agree to disagree. My thirty years of association with RV’ers gives me different numbers than you project. Besides, what difference does it make if one of us is off a few percent? The fact remains that the majority of Rv’ers are NOT high power inverter users (I am in the minority – I have a 3,000/6,000 watt inverter.).

BTW - your solar set-up is extremely nice.