Battery Cable length/resistance?

SCVJeff wrote:

Jeff,

You could not have said it better.

For what it is worth, I don't think if you have one battery cable 10 feet long and the other 6" long, you will have any change in battery performance, and days it will last while dry camping.

My RV has a 'E-Meter' a special meter that measures amp hours going into and out of the batteries. I have broken most of the rules you have claimed should be important in my system. I have the factory pair of batteries in front of the engine supplied by a #4 wire from the alternator. The second pair is about 12' away with #00 wire between them for both ground and +12 volts. I have the #8 wire from my 50 amp solar controller only going to the rear pair of golf cart batteries, and it is fed with #10 UV rated wire from the roof. One run from a pair of 120 watt solar panels, one run from a pair of 45 watt panels and a single 75 watt panel. 3 manufactures all to one controller, three ages. For what it matters, the factory batteries where installed in November 1996, and the second pair 3 months later. All where batteries replaced and given to a friend (still working good) in 2006, at around 10 years old.

Wire length really does not matter very much. The charger is only connected to the rear pair of batteries, while the alternator is only connected to the forward pair of batteries, and those are interconnected with really large wires. I would say that 90 - 95% of all charging came from the solar system, while only about 5% comes from the alternator in my case. I rarely ever run the generator, even while full timing, and watching 8 hours a day of satellite TV!

The real loads in a RV include the small ones, such as the 35 amp hours used daily by three things. The CO Meter, propane detector and refrigerator.

Other loads that can be significant include the furnace and lights. You can get a Olympic catalytic heater to reduce furnace draw, and LED lights to reduce that load. Can't do much about the water pump, it will continue to draw 7 amp hours per hour while pumping 120 gallons per hour, or use only 7 AH to empty most water tanks.

Fred.

Last story I heard was that the amps getting to the battery from the charger or the draw from the batteries from the load depended on the Resistance in the wires.

The neg path has an R and the Pos path has its own R. The R that matters is the total R of the pos and neg paths. The two Rs do not have to be equal in any way. You can "improve" your set-up by reducing the R in either the neg or pos path or do some of each.

BUT, when discussing balancing battery bank linkages to try and get an even draw from each battery under a load or to get the batteries to share a charge equally, that is a whole different game.

And you need to ensure the links are short and fat enough to handle their share of the total amps the bank is either accepting or delivering.

If you need one link to be longer when you would like them to be equal gauge and length, you can make the longer one fatter in an attempt to even them up for R.

You can way over-do this. Say your microwave causes the inverter to draw 150 amps from the battery bank. Say your bank is two pairs of 6s in parallel and you have your inverter pos on one pair and the inverter neg on the other pair as is proper. So you would expect each pair to do about 75 amps of that load.

#4 AWG is good for 100 amps in short lengths. That 75 is less than 100 so you could use #4 for the battery bank links but need fatter for the inverter to battery bank wires, since they will be doing all 150 amps.

Clear as mud? :)

I believe that in addition to the parallel connections, the 2 wires that connect the 2 6v batteries in each series pair should be the same length and wire size.

Whew !! Its even worse than I thought !
Battery paranoia has really set in.

No. Series circuit does not need equal length. Even very unequal length has no effect as there is nothing to balance.

Just balance the battery with equal length parallel connections and pull power from opposite sides and you are good to go.

Splitting hairs