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Lead-acid batteries have -approximately- linear discharge curves and are therefore more easily monitored through their open-circuit voltage. Is there a way to monitor their SoC without disconnecting them from the circuit? (For example, when they are being charged/discharged.)

This article claims that you need to disconnect the batteries for at least a few hours in order to get proper measurements:

the battery needs to rest in the open circuit state for at least four hours

I am trying to build a monitoring system for solar batteries and this method wouldn't be usable if I need to disconnect the batteries every time I want to measure their SoC. Also, does this mean that monitoring through the output of a voltage divider will cause inaccuracies because of chemical imbalances caused by the current going through the voltage divider?

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  • \$\begingroup\$ With some difficulty. If you know the internal resistance, you can make a first order approximation from the voltage and current. How accurate SOC do you need? \$\endgroup\$
    – winny
    Commented Oct 23, 2021 at 22:16
  • \$\begingroup\$ (1) Can you keep charged spares, swap one of them for the battery you want to test, then test the swapped-out battery after a few hours? (2) Are the batteries in continual use, so that there is never a specific moment when you know they are fully charged? \$\endgroup\$
    – jonk
    Commented Oct 23, 2021 at 22:17
  • \$\begingroup\$ @jonk (1) No, that won't be possible unfortunately. (2) yes, the batteries are being used continuously since they are used for storing solar energy and providing it for home appliances. \$\endgroup\$
    – A.H.Z
    Commented Oct 23, 2021 at 22:22
  • \$\begingroup\$ @A.H.Z Then before you can consider any approach, this topic is going to require significant prior study of existing research. Lead-acid batteries aren't "a thing." Some are made far, far better than others -- back in the day we allowed a week's time for the curing phase before placing the plates into a battery for use. Today, that almost seems to be on the order of minutes to an hour (for car batteries, anyway.) If that. They are not the same today, as once. But you may be sourcing these from forklift systems. I don't know. \$\endgroup\$
    – jonk
    Commented Oct 23, 2021 at 22:28
  • \$\begingroup\$ @A.H.Z Also, charging causes the returning lead to form little bubbles around which the lead solidifies, forming a kind of honeycomb structure. So snap-reverse during charging can shake these from the surfaces leading to a better, more uniform return of lead to the plates. So the charging process itself also affects the long-term battery condition. I think a viable solution, workable over a long period of time, will need to include many, many details. So that's why the need for substantial study. Batteries age. That will need to be a factor in your equations. \$\endgroup\$
    – jonk
    Commented Oct 23, 2021 at 22:30

1 Answer 1

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Partially. But you will also need to monitor the current.

While there are many algorithms for SoC (State of Charge) estimation, the classic one uses two components: voltage translation and Coulomb counting.

  • Voltage Translation estimates the SoC from the OCV (Open Circuit Voltage), at zero current and after some rest time)
  • Coulomb Counting calculates the relative change is charge by integrating the measured current and adjusting for battery capacity; this works regardless of voltage sag during use

Each of these has limitations, but the two together can give an acceptable estimate of SoC.

A white paper I wrote goes into some detail.

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