Re doing the Battery Box

Re: Re doing the Battery Box

Regarding over sized negative cables... It is the total resistance of the string (positive+negative+connections) that should be equal between all the strings.

If all strings have the same sized negative cables (awg and length), then it will not hurt the balance. If on string has short/heavy cable and the other are long/light cables--then the strings will not be in balance.

-Bill

Re: Re doing the Battery Box

Well, it is true that adding resistance does make the balance better... As long as the wire is still the correct gauge for the application.

You are correct that "over sized" wire can make the issue of balancing heavy currents in a bank worse... Because the resistance of the wire "drops out" of the equation and the resistance of the battery and the electrical connections (and fuses/breakers) becomes the major elements in the resistance.

A 3' length of wire is a pretty fixed resistance. Your connections and battery internal resistance may be more variable...

To see what kind of resistances we are looking at--Take a look at a typical Optima Lead Acid Battery spec... The internal resistance for a 50-60 AH battery may be 0.0025 to 0.003 Ohms.

Using this typical copper wire resistance chart--we see that large diameter wire has very low resistance per foot too:

0 awg = 0.0983 ohms per 1,000 ft
2 awg = 0.1563
4 awg = 0.2485
6 awg = 0.3951
8 awg = 0.6282
10 awg = 0.9989

If we assume a 60 AH battery is around 0.003 ohms, then a 600 AH battery/bank could be around 0.0003 ohms...

Or, about the same resistance as 1 foot of 4 awg wire...

So--it kind of gives you an idea of the values you are looking at here... And how the wiring (and electrical connections, over current protection devices) for a large bank actually set the current balance more than the internal resistance of the battery itself...

So, insuring that the electrical connections around the battery banks is very important to the ability of the batteries to properly share current.

-Bill

Re: Re doing the Battery Box

There is a difference between AC and DC current flow in a wire... DC flows through the entire cross section. AC flows in the "skin" of the wire...

Skin Effect:

Skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. That is, the electric current tends to flow at the "skin" of the conductor, at an average depth called the skin depth. The skin effect causes the effective resistance of the conductor to increase with the frequency of the current because much of the conductor does little. Skin effect is due to eddy currents set up by the AC current. At 60 Hz in copper, skin depth is about 8.5 mm. At high frequencies skin depth is much smaller.
Methods to minimise skin effect include using specially woven wire and using hollow pipe-shaped conductors.

This chart includes the skin depth frequency for various wire sizes for solid conductors.

If you were to look at the huge AC "bus bars" in a power substation--they are actually hollow.

For welding, very fine wire allows for a more flexible cable and reduces work hardening of the copper wire.

If you start to add high frequency to welding--then you probably have skin depth issues in the design of the power system.

Then there is PWM mode in solar charge controllers---They probably cycle around 10-20 kHz (guessing)--so you can have skin effects in your solar array/battery cabling too... However, with PWM, they only go into less than 100% on when reducing current/power flow because the battery is nearing full charge--so I would not be concerned design a "normal" system (other than Radio Frequency Interference--which is real and can be a real pain because of PWM/MPPT controllers--a few mfg. do offer FCC class B on their equipment--which I think, should be required for all equipment).

-Bill