Quote:
Originally Posted by MAWilsonPE
So someone in the Montana Solar Facebook group posted a reply to my new setup concerned that I will quickly end up with very unbalanced batteries with such a long run to the two new ones compared to the very short run for the factory batteries reducing my actual A-Hr capacity. I appreciate his comments and concerns and he's got me thinking.
I selected 4/0 welding cable to connect the two new batteries because I calculated the voltage drop to be less than 0.20 volts at 100 amps, 0.10 volts at 50 amps, and 0.04 volts at 20 amps. I felt these were pretty low. Running one A/C unit is about a 100 amp draw. With my new panels, the max charge rate is 180 amps. Only half these amps should come from either pair of batteries if properly balanced.
I spoke with a tech at BattleBorn and he did not like this long 25' run at all and could not recommend it. It is likely the front set will draw down first before pulling from the back set. Looking at the BB voltage vs capacity chart a 0.20 volt difference between the two battery sets could be as much as 50% capacity. Hmmm...
Conversely, when the batteries are charging, the front set will charge first and then the back set. But, when the chargers switch to absorb mode the amps are so low there should be no preference to one set or the other in my opinion. I could not get the tech to agree with me on that. He stated he is not an electrical engineer and BattleBorn does not have an EE on staff, what?!? He could only tell me what they recommend, which is to balance the wire lengths so the voltage drop to each pair is the same.
I am not convinced... It would be very difficult to calculate the variation in amps to each set as the wire resistance (voltage drop) is a function of amps, sounds like some differential calculus would be needed (it's been a looong time)
So my plan now is to install a Victron SmartShunt on each pair located close to the batteries themselves. Then I can measure the amperages independently for each pair and see any differences. Then I can make an informed decision as whether I need to take further steps to balance the voltage drops.
If necessary, I could add 8 feet of #2 wire (which is rated for 200 amps) to the hot and ground of the front battery set. This would have the same voltage drop as the 25 ft of 4/0 wire pair. He didn't like that idea either and wants me to either run 25 more feet of 4/0 or relocate the inverter to the middle of the camper, not gonna happen...
I welcome any thoughts or suggestions you folks might have.
Thanks, Mike
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Mike,
I'm low voltage certified for commercial buildings in our area. So, while the concepts are familiar, the work our company does is to install based on plans engineered for the contractors. (and I've long since stopped working in the day to day, so have folks way smarter than me handling this anyway)
I say that to say this - I'm sure no expert, but I do know enough to agree you will have some imbalances. I think installing another shunt and measuring each set of batteries will give you some visibility into what is going on; that may well help to know how much of an imbalance you may have, and whether your idea of adding resistance to the first set of batteries will restore balance.
One other item that you didn't mention, but perhaps should be added to the mix - for those frequent "low usage" days, you will likely end up with far more cycles on the front set of batteries compared to the back. Whether that means just that they will reach their lifetime cycle limit sooner, or whether it means more than that, I don't know. But I do suspect it will make a material difference, as is.
As you know, ideally large battery banks are connected all together in one location, connected using bus bars, and with external connections on opposite ends; all this to achieve as close a balance between all cells/batteries as possible. So I do think your install introduces some variables that are significantly different and worth investigating.
My 2 cents, and all the best with sorting it out!
Brad