What should the voltage of a fully charged lead acid battery be?

Question

I don't have a proper lead acid battery charger... But I own a small Yuasa 7Ah battery. I am using a 13volt 1.5A wall wart to charge it. And I have a volt-meter to check the voltage.

At what voltage should I take the battery off the charger?

Answer 1

See my stack exchange answer to "Lead Acid Battery Charger Design Factors" which relates, and follow the link there to the Battery University site which will tell you far more than you knew there was to know about lead acid (and other) batteries.

From the above answer note the quotes from the above website. Especially in this context

• The correct setting of the charge voltage is critical and
  ranges from 2.30 to 2.45V per cell.
  Setting the voltage threshold is a compromise, and battery experts refer to this as “dancing on the head of a needle.” On one hand, the battery wants to be fully charged to get maximum capacity and avoid sulfation on the negative plate; on the other hand, an over-saturated condition causes grid corrosion on the positive plate and induces gassing.

  To make “dancing on the head of a needle” more difficult, the battery voltage shifts with temperature. Warmer surroundings require slightly lower voltage thresholds and a cold ambient prefers a higher level.

2.30 x 6 = 13.8V

2.45 x 6 = 14.7V

13.8V is the nominal voltage that many automobile systems operate at.
14.7V is higher than you'd usually meet but around 14.4V is common for "topping" mode charging which is used to equalise charge in series connected cells. See the Battery University site for MUCH more information.

If you charge to only 12.6V as several people have recommended your battery will not ever be at full capacity and will have a shortened life.

See also Safe operating area for different types of battery chemistry?

And also Can I charge a 12v sealed lead acid with an old wall-wart (not made for charging)?

And also Charging lead-acid batteries?

Answer 2

Deep Cycle Batteries charge on 3 stages: bulk charge, topping and float charge. During the bulk charge, the current density (A/cm² or A/dm²) is very important. If the current is smaller than needed for the surface of electrodes, crystalline structure will appear. Sponge material is reversible, but crystalline material is irreversible and this is very important for PbSO₄ too.

After the battery gets charged to 80%, then the current decreases and during float charge is less than 1 Ampere.

The best choice is using a Smart Charger. The way a battery is used and maintained can change the battery life from 6 months to 7 years. Never let a discharged battery rest uncharged; discharging the battery at 50% before recharging can prolong the battery life by 2 to 3 times.

Answer 3

When in the armed forces, and frequently abroad for between 2 weeks and 6 months, which ruined car batteries, I purchased a variable voltage transformer (5 amp). Ensured battery fully charged (negligible charge indicated on ammeter when running engine)Connected trickle charger and voltmeter to battery, plugged charger into output from variable transformer. Then wound transformer voltage down until voltmeter indicated approx. 12.6/12.9 volts( I.E. slightly lower than ideal?). Was away nearly 5 months. Quick check: power still on and voltage not changed; electrolyte level appeared unchanged. Unplugged everything, hand pumped fuel up to the carbs. Engine started , battery seemed not to have aged and ammeter was very quickly showing normal fully charged indication while engine was running. May not be theoretically sound or perfect but proved very effective. Never had a problem using that system: it paid for the transformer, and my car was always ready to go (after I had checked tyre pressures).