Electronic – Appropriate float voltage for SLA battery in 120°F+ environment

1) The voltage will depend on the charger. 2) The voltage will depend on the temperature.

The charger will have a float range, not a single point. It may try to keep the battery within this float range with a trickle charge, or with a load-dependant charge, or simply by letting the battery voltage fall, then switching to BOOST until the battery hits the boost return voltage.

The "optimal" voltage is slightly temperature dependant. Some chargers can adjust for the temperature dependance, in which case the float range will be adjusted. And since the battery may be at the top, or the bottom, of the float range when you disconnect it, its voltage may be higher/lower than it would be at a different temperature.

If the charger is charging when you disconnect it, either because it is trickle charging or because it is in the low part of its float range, and there is no load connected, the battery voltage will fall when you disconnect the charge current. But the battery is capacitive, and full of migrating ions, so the voltage continues to hold up for a while after you disconnect a charging current.

And if the battery is hot because you have been using it previously, the open circuit voltage will drift down as the battery cools: it could be at the correct float voltage when you disconnect it, but some time later you could find that it is lower.

(And some time much later, you will certainly find that the battery voltage is lower, because of self-discharge)

Finally, if the charger is not charging, and there is no load, and so there is no current going through the wires ... and the battery is at the specified temperature, and the environment is at the specified temperature ... then disconnecting the wires does not change the battery voltage, and the average cell voltage for cells in good condition should be the specified voltage.

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).