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.
- SLA charging basics
- Charging Lead Acid
- Lead Acid SoC vs Voltage
- Internal resistance vs Capacity
- Battery Performance Characteristics
- Charging VRLA
(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!