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In my off-grid solar system I am using 12V lead-carbon batteries capable of surviving 4000 cycles when discharged up to 50%. Capacity is 200Amp-Hours (100 Hour discharge) per battery.

There is no BMS or coulomb counting. I simply configure charging voltage (absorption 14.1V, float 13.6V) and cut-off voltage to limit discharging. The latter is the question: how do I find out the voltage at 50% discharge?

Here are some specs which I reckon should be useful to answer the question but I am not sure how to read them:

enter image description here

(one battery apparently consists of 6 cells in serial).

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  • \$\begingroup\$ You don't because there is no such voltage. Looking online suggests that lead carbon is one of the many chemistries that have a fairly flat voltage for most of their discharge, then rapidly decrease right at the end. You'll probably need to integrate what you put into and pull out of it, instead. \$\endgroup\$ – Chris Stratton Sep 17 '18 at 1:34
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    \$\begingroup\$ @ChrisStratton your comment could be accepted answer if it contained convincing prooflinks. \$\endgroup\$ – Greendrake Sep 17 '18 at 6:28
  • \$\begingroup\$ Nothing is absolutely flat, but with the flatter discharge chemistries it gets very hard to tell anything about state of charge as that variation may be small compared to changes due to load, temperature, the immediately recent history of both - and even for some, physical orientation. \$\endgroup\$ – Chris Stratton Sep 17 '18 at 23:14
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Terminal voltage will only give you a very rough estimate of state-of-charge, since it doesn't take into account capacity fade, temperature, chemical additives, and surface charge, all of which will affect the terminal voltage. However, if a rough estimate is good enough for you (say, +/- 20%) then the number you're looking for is roughly 12.2V.

Lead acid terminal voltage vs state-of-charge

Source: Battery University, which also gives you some alternate methods of measuring state-of-charge.

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