Well, it might seem to work.
It might even work in practice for you for a while.
But it's fraught with dangers! I won't try to talk you out of it, but I will point out a few of the reasons I wouldn't do it on anything of my own!
Firstly, no two batteries can be assumed to be identical. As they age, the differences becomes more pronounced and the effects they will cause in this application becomes more difficult to predict.
In a series string such as you suggest, with 6 x 12V batteries in series (or 12 x 6V in series) it is inevitable that one battery will fail before the others. It's a 17% chance that the one you are monitoring will be the one to fail. (ergo, a 83% chance it will be one of the others that fails!)
Lets take two "failure" scenarios.
1. A cell fails open-circuit
2. A cell fails short-circuit.
In a 12V battery, situation (2) will mean you get a nominal 10V from the battery.
Each of these two failures could occur in one of two places:
A. Your refernce battery.
B. The rest of your string.
We also have a 3rd condition to consider:
i. Charging
ii. Discharging
So we have 2*2*2 = 8 possible conditions. Some are safe, some are not.
- 1Ai - if your reference bat goes open, and you are charging, you will measure full voltage (72+ volts) and will immediately go into dump mode (and/or blow up your controller!)
- 1Aii - open circuit discharging, you will apply *REVERSE* bias to your controller to the tune of approx (72-12) = 60V (probably enough to blow up the controller)
- 2Ai - short circuit cell while charging, you will never see a high enough voltage across the reference battery to consider the whole string "charged" and will keep pumping amps in and cook the rest of your string!
- 2Aii - short under discharge - probably the least dramatic. Slightly low output volts.
- 1Bi - open somewhere else under charging - you have no load for your generator, turbine overspeeding, dumpload won't help you because it won't see the high voltage.
- 1Bii - open elsewhere while discharging - no output.
- 2Bi - short elsewhere while charging - you see slightly higher voltage across your reference battery, ending charge and dumping load a little early. No serious damage, but chronic under-charging of the whole string.
- 2Bii - short elsewhere while discharging, same as 2Aii.
There are a number of "intermediate" situations of course - a cell may start to go high resistance before it goes open, so you can get "partial" variants of the above.
RossW