My question:

I want to know how to determine the highest safe voltage for a given SoC (state-of-charge). Or for a given float charge voltage, determine the lowest safe SoC.

Helpful hints to the solution:

In the answer to this previous question, Olin succinctly states:

Unless the battery is quite low, it can be charged by holding its voltage at the "float charge" level. That is usually 13.6 to 14.0 V for a 12 V (6 cell) battery. [...] For a really depleted lead-acid battery, you have to be careful to limit its charge current.

The links below have been invaluable to me. I don't know if my question is dependent on any particular lead acid battery, or can be roughly determined by generic properties of lead acid batteries.

(No wonder, then, why my head is gassing)

Background (if necessary):

The problem I'm actually trying to solve is designing a generic charging circuit which connects between a solar cell and a battery connected to a vehicle. The current draw by the vehicle at any time is unknown. Ideally, the capacity of the battery will not be known. Therefore, Olin's suggestion of limiting the charge current is not really an option, as this would be battery-specific. Saying that, The solar cells will be modest - probably at most 5A at a working voltage around the battery's float charge voltage. And the battery's capacity would be in the expected range for vehicles; so perhaps between 30Ah and 100Ah.

Obviously the charging circuit is not to replace the vehicle's charging system. Nor is it even to keep the battery at full charge - but to stop the battery emptying, as far as possible, while the vehicle's engine is not running. So while current is being drawn by the radio, lights, or accessories, or while the battery is relatively empty from said current draw, the solar cells would kick in.

Sorry for the lengthy question!

  • \$\begingroup\$ Usually those numbers are provided in the batteries datasheets. These usually contain some information about the max charge current that you want to obey too. \$\endgroup\$
    – PlasmaHH
    Mar 23, 2015 at 13:33
  • \$\begingroup\$ Thanks - but I was hoping to come to conclusions for my question based on generic properties of lead acid batteries - e.g. using the graphs on the links provided \$\endgroup\$
    – Jodes
    Mar 23, 2015 at 13:36
  • \$\begingroup\$ Since there are no generic lead acid batteries, they can not have generic properties. \$\endgroup\$
    – PlasmaHH
    Mar 23, 2015 at 13:37
  • 1
    \$\begingroup\$ As I said, most are fine with 13.xV (as long as current and heat are not exceeded) but the exact value largely depends on how the battery is built (luquid,gel, solid etc.) and therefore a manufacturer gives a recommendation which (we all hope) is for a maximum lifetime. Another old rule of thumb: if it bubbles or melts, it was too much. \$\endgroup\$
    – PlasmaHH
    Mar 23, 2015 at 13:50
  • 1
    \$\begingroup\$ .... Battery university have other relevant references to go with the one you cited. Note that a battery that is floated at 13.7V will need an occasional boost charge to 14.x to keep it full charged. \$\endgroup\$
    – Russell McMahon
    Mar 23, 2015 at 14:05

1 Answer 1


You will probably need an intelligent charger (micro controller based with a custom program to handle inputs from V&A meters, solar charge controller, & a power regulator).

Common methods of determining the SOC of a 12V(nominal) battery aren't very accurate. Most often people use a generic table that shows V & SOC %. The problem with that data set is that it applies to a new/good battery and as a battery ages & deteriorates the internal battery resistance increases, charge capacity decreases, & charge rate decreases (i.e. the charge rate of an aged battery is slower at a specific voltage than a new battery)--so you can't rely on it as a reliable method to charge a battery with solar energy while the engine is off & power is being drawn from the battery.

If you use a micro controller based smart charger with a good program, you should be able to do the following to determine the maximum safe voltage to apply to the battery while the engine is off: 1. measure the battery V 2. measure the A draw of the system 3. To maintain the battery's current SOC, your power system must produce as much power as the system is consuming. Your program would calculate the optimal charge A & V to provide sufficient charge current to offset the power consumption rate. 4. To add a net charge increase in the battery, your computer program will have to provide a higher charge rate than the charge consumption rate (you can decide that rate or make it a user configurable parameter).

Note: The regulated charging system of a running vehicle provides sufficient power to charge the battery & maintain other electrical loads. Your smart charger will have to emulate the same power availability while the engine is off (which might mean you'll need quite a bit of PV surface area (depending on loads).


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