Float-recharging: What does it mean?

[DigiTan]

I recently purchased a Lead-Acid Battery Charger IC from a company called Unitrode. In their documention, the datasheet repeatedly refers to a technique called "dual-level float charging." I understand that this process involves multiple recharging steps, but I was hoping someone could explain its meaning in a little more detail. Does it have something to do with the discharging step at the end of the recharge cycle?

 

[k7elp60]

Float charging for 12V lead-acid batteries means that the charge voltage is between 13.5 and 13.8 volts. When the battery is properly float charged it cannot become overcharged, as at this range of voltages the battery automatically only draws the current necessary from the charger to keep fully charged.
I don't believe the charger is doing any kind of discharge on the battery.

 

[DigiTan]

I see. 'Guess that makes sense. After the battery is float-charged, would it be necessary to discharge it across a load to make it fall back to 12 volts like in the sample circuit?

 

[k7elp60]

No. If you remove a charger from a fully charged 12V lead-acid battery it will automatically drop in voltage to approximately 12.6 or so in a few hours. Even thou a 12 volt battery says 12 volts it is beacause it consists of 6 ea 2 volt cells. The nominal voltage is 2 volts but in reality they are slightly higher than 2 volts when fully charged, and they do drop below 2 volts when discharging. In some applications a 12 volt lead acid battery my be discharged to 10.7 volts. I have even seen some discharged to near 0 volts and been recharged.

 

[Dialtone]

Tel Co's float their lead-acid batteries at 2.17 volts per cell. Nominal cell voltage is considered to be 2.08 volts per cell. At this float charge rate, they generally last for 10-15 years before needing to be replaced. Most of the time, new strings are charged (at 2.5 volts per cell) for a pre-determined amount of time (generally in the range of 1-1 1/2 weeks), then the plant voltage is dropped back to float voltage for the string. The most damaging thing for a lead acid cell is the discharge. If it goes too deep, they are prone to the cell reversing polarity. Once that happens, it is very difficult to undo, and almost always causes some permanent diminished capacity in the cell. Easiest way to determine if a cell is reversed, is to measure each cell individually (in a string of attached cells). A reversed cell will generally measure twice the expected voltage indicating it is really acting like a resistor.

Edit: These are large cells; 400-500 pounds each. I wouldn't recommend charging your car battery at that rate for that long. You would probably boil it dry in that amount of time.

Dialtone

 

[zevon8]

They may be refering to what some companies call "equalizing charge rate." This is a slighty higher charging voltage than a normal float charge voltage, and is designed to bring all the cells up to a nominal level, and help prevent sulfation, or at least reverse some if it has begun. It is the kind of thing you do on a maintenance cycle, not everytime you charge the batteries.

Pretty much what Dialtone was refering to, you see this setting on commercial battery chargers, a switch labeled "Float" and "Equalize"