24V Battery bank connection questions

rmcewen

I am going to install a new battery bank for my 24V solar system.  The batteries consist of 6 SLR125 12V VMaxTanks batteries. So I will have three sets of two batteries each wired in series to get 24V and then wired in parallel which should produce 375AH.

My first question concerns creating a system as balanced as possible. As a reference we can use the methods described on this page but hopefully my descriptions will suffice:

http://www.smartgauge.co.uk/batt_con.html 

1. The first and most obvious method is to wire the six batteries in a serial-parallel arrangement and use the opposite corners for the connection to the rest of the system.  This is method 2 shown on the referenced web page.  According to the web page this results in a good but not perfectly balanced system.  I believe this is the most common method of connection.

2. The second possibility is to wire each individual serial wired two battery (24V) bank to common bus bars with equal length wires.  This is shown in "method 3" of the referenced web page.  In this case, the bank should be perfectly balanced as each pair of batteries is connected individually to the bus bars which provide the connection points to the rest of the system.  The only downside I see to this approach is the extra length of wire needed to connect each pair to the bus bars.  In my particular case, this would be about 30" of 1/0 wire.  I would have six 30" lengths connecting to the bus bars vs. two 30" lengths.

If it matters, the maximum charge and load amperage would be about 35A.

So is the second method worth it to achieve perfect balancing given the extra lengths of wire required?  Disregard any cost issue as I already have the wire for either method.

My second question concerns low SOC disconnect.  The reason I'm installing a new battery bank is because I destroyed my old bank by letting the batteries fully discharge.  This happened due to a loose connection on an MNEPV breaker resulting in no charge getting to the batteries for several days.  I assume the inverter shut down properly at low voltage but I also have several DC loads which continued to drain the batteries until they were dead.  How do most people protect against this type of occurrence?  How is the low SOC detected and then how are the DC loads disconnected to prevent the batteries from losing any more charge?  In my situation the system is in an off-grid cabin which I don't reside in permanently so am not onsite to monitor this type of thing manually.  Several of the DC loads are things like a 3G modem for internet connection, security cameras, refrigerator, etc. so need to remain running full time so just turning off all loads when I am not present is not an option.  I'm just wondering what most people do to prevent this kind of thing.  

Thanks.

westbranch

well ,  How do most people protect against this type of occurrence?  you plan for it, say 3 days of no incoming power ( in your case more...) and still have 50% of your capacity remaining and then the gen starts automatically and bobs your uncle or you shut off all the loads.....  so the other alternative is to have a  huge battery bank, 2v cells and more than one charge controller.... or  a friendly neighbour to check.

rmcewen

All your suggestions are spot on and I am increasing the size of the battery bank to allow for 5 days with no incoming power vs. the 3 days the old bank would support so that will help.  I would love to have the generator autostart capability but my research so far hasn't discovered any type of system like that at an affordable price point.  Currently if I have to charge the batteries manually I'm limited to my Yamaha 2K or my Champion 4K and manual operation.

So that leaves shutting off all loads when SOC reaches a certain minimum which is my plan.  I was just wondering how others implement this type of set up.  My idea is to monitor the SOC via modbus from the WhizBangJr and have each DC circuit on its own relay that my controller (currently an Odroid XU4) can start shutting things down as SOC gets to critical levels and finally shut itself down to reach zero power consumption.

Thanks for the suggestions.