Balancing a 12X6V battery bank for a 24V system

Started by dapago, October 23, 2017, 01:09:04 AM

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dapago

hello to the community,

I have a battery bank composed of 12x6V batts for a 24V solar system wired 4S3P.
I am a bit worried not to be able to balance properly the second serie of 4 batts (in the middle).

I have found 2 diargrams that allow to perfectly balance a serie of 4 + batts but do not know how to apply those diagrams to batts wired in parallel. Someone could help?

i have attached a picture to show the 2 options proposed. For the full article relating to those wiring, please refer to http://www.smartgauge.co.uk/batt_con.html
Off-grid. Midnight Classic 200. 2,480W array wired (8x310W/2P, 4S) .Magnum Inverter MS4448PAE. 8*225Ah @48V Trojan T105RE Smart Carbon. MN bkrs box.

dgd

Use the wiring shown in the first diagram with just 3 banks instead of 4.
You need to keep the three + cables the  exact same length and gauge.
Also the three - cables the exact same length and gauge.
The 3 battery interconnects within each bank must also be same length and gauge for all three banks.
The common terminal posts usually are one side of the Deltec shunt for the - cables and for + cables one side of the inverter 175A or 250A breaker for inverter. These are normally in an Epanel or an MNDC box.

dgd
Classic 250, 150,  20 140w, 6 250w PVs, 2Kw turbine, MN ac Clipper, Epanel/MNdc, Trace SW3024E (1997), Century 1050Ah 24V FLA (1999). Arduino power monitoring and web server.  Off grid since 4/2000
West Auckland, New Zealand

Ron Swanson

To avoid extraneous terminal posts



Each battery representing a string, 3 strings instead of 4.

The series resistance any electron sees will be the same no matter which of the 3 (or 4) choices it takes.

No rats nest of wires from a central post required.

mike90045

> The series resistance any electron sees will be the same no matter which of the 3 (or 4) choices it takes.

Only in a perfect world, where there is no Lug-Cable resistance, or Lug to terminal resistance ,  or different batteries have slightly different internal resistance.

Big post with math at  http://www.smartgauge.co.uk/batt_con.html 
http://tinyurl.com/LMR-Solar

Classic 200| 2Kw PV, 160Voc | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph )| Listeroid 6/1, st5 gen head | XW6048 inverter/chgr | midnight ePanel & 4 SPDs | 48V, 800A NiFe battery bank | MS-TS-MPPT60 w/3Kw PV

Ron Swanson

There is not much/any math on connection #2 as they refer to it.  I would have to either see the math or test a multi string set to believe it.  I could be wrong.

Also, we are talking about 3 strings here instead of 4.  I would bet that if there is an imbalance effect it is reduced again in a 3 string setup.

Finally, rather than putting a bunch of strings together, we usually just try to buy 1 string that does what we need and alleviate all these problems.

mike90045

When you deal with a couple of amps, it's not a big thing and can pretty well be ignored.

When you add an inverter and 50A of charging, and start pulling some serious amps, thats when the mill-ohm differences kick in and you get tenths of a volt differences across the bank,  which "exercises" some of the batteries harder than the others.
Then the exercised batteries age faster than the pack as a whole, and it starts a race to the bottom. as one battery goes soft, another one takes over the main load, till it goes soft, and so on.

The concept and math is the same, longer wires have more resistance, and if a battery has longer wires then it's neighbor, the milli-ohm difference has an effect, that's why the diagonal method works well.
http://tinyurl.com/LMR-Solar

Classic 200| 2Kw PV, 160Voc | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph )| Listeroid 6/1, st5 gen head | XW6048 inverter/chgr | midnight ePanel & 4 SPDs | 48V, 800A NiFe battery bank | MS-TS-MPPT60 w/3Kw PV