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In my case I am looking at the Hoppecke OPzV (product page) lead acid batteries. In the two page Operating Instructions (link) it says

2.3 Maintaining the full charge (float charging)

Devices complying with the provisions of DIN 41773 must be used. They are to be set so that the average cell voltage is 2.25 V ± 1%.

Ok, so to maintain the optimal full charge, the float charging voltage must be 2.25V per cell.

  • Let's say the charger is float charging the battery, and thus the voltage is 2.25V. Then I disconnect the charger (and nothing else is connected to the battery). Will the voltage still be exactly 2.25V?
  • Let's say I have a cell (or many) not connected to anything. I want to know the charge level. Is 2.25V the "optimal", ie the voltage meaning that they are fully charged and not overcharged? Or should that number be slightly below 2.25V?

(Bear with me, I am not an engineer)

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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.

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