State of Charge

stmar

Most 24 volt systems have a "Float" voltage usually set at 26.6 - 26.8 volts. What is a normal resting full charge on a 24 volt battery bank? 4 X 400ah 6 volt Lifeline AGMs. I assume it is less than the Float so that brings me to another question. Why is Float higher that full SOC?

BB.

Float is just high enough voltage to keep the battery bank charged and will (roughly) pump somewhere between 0.01% to 2.0% "charging current" into the battery bank (AGMs have lower self discharge and will be lower float current, Flooded Cell Forklift batteries will have a higher float charging current). Higher temperatures increase self discharge, older batteries have higher self discharge, batteries near failing (age/cycle life) will have higher self discharge.

Float voltage is set to prevent (or reduce) charging related issues. Higher charging voltage (and current) will cause batteries to gas (use more water, more heat, waste energy, increase internal corrosion, cause plates to shed) and reduce battery life. For lead acid batteries, there are different "aging mechanisms" for different voltages (between loaded, resting, float, absorb, equalizing operations). Float voltage is usually chosen to be the "best option" voltage (longest battery life, least amount of "damage").

Resting voltage is the battery's actual voltage that is "generated" when there is no load. For lead acid batteries, is it a combination of specific gravity (higher SG, higher resting voltage) and temperature (higher temperatures, lower resting voltage). AGM can have higher resting voltage (~12.8 to 13.2 volts when fully charged, FLA closer to 12.7 volts). And remember since lead acid batteries have (moderate) levels of self discharging, they are actually very "lightly loaded" when resting.

AGM batteries typically have a catalyst in each cell that helps to combine hydrogen and oxygen gases back into water. And although the definition of a catalyst is that it is not "consumed" in a chemical reaction, in practice AGM catalysts do have a life. Everytime the battery gases, the catalyst "wears out" a little bit more. High charging voltages generate more gasses, catalysts get hot (and age faster). So, for AGM batteries there is a reason that float voltages are lower than other FLA batteries--Extending catalyst life (and AGM batteries have a lower self discharge, so they need less float current too).

Anyway, the "non-chemist" hand waving explanation of float voltage vs rest voltage... And I am not a chemist.

-Bill

mcgivor

The float voltage is to maintain the battery in an equilibrium of neither being charged nor discharged, the value of which is determined by the manufacturer, based on the electrolyte concentration. The resting voltage will be less because the maintenance charge is removed. When healthy the drop will be slightly lower, however as the batteries age the self discharge along with loss of capacity will increase,  thus lowering the resting voltage.