|Lee Hart's Battery Charging Basics|
Volts and amps are related, by
Adjusting one automatically adjusts the other. So if you have a manual charger with
a knob, it doesn't matter whether the knob changes voltage or current. |
A manual charger thus needs at least two meters, battery voltage and battery amps. A third meter (AC line current) is nice to avoid tripping circuit breakers or fuses.
The most basic rule is that you can charge at any current you like until the battery reaches 2.4 volts per cell at 25 deg. C (77 deg. F). That's the familiar 7.2v per 6v battery, or 14.4v for a 12v battery. Below this voltage, essentially 100% of the current goes into charging the battery, so there is negligible heating or gassing.
The batteries won't reach 2.4v per cell until they are about 70-80% charged. Therefore, to charge the quickest, charge with as much current as your charger or AC line allows. If you keep adjusting to maintain maximum current, or your charger does, you're getting what's called "constant current" or "bulk" charging.
Above 2.4v/cell, the batteries start to gas and get hot. You have to limit the voltage, and the time spent above this voltage, or you will use water and shorten the life of your batteries.
So when the batteries reach 2.4v/cell, turn down the charger, and keep turning it down as necessary so the voltage does not exceed this 2.4v/cell. This is called "constant voltage" or "acceptance" mode charging. It puts in the last 20-30% of the energy needed to reach 100% charged.
The current will gradually fall as the battery reaches 100% charged. When the current falls to about 2% of the battery's 20-hour amp-hour capacity rating expressed as amps (i.e. 2% of 250 amp-hrs = 5 amps), the battery is 100% charged.
You can turn off the charger at this point. This is all you need to charge on a daily basis.
However, all the cells in a battery are not identical; they will have slightly different amp-hour capacities, and be at slightly different states of charge. These differences get larger as the batteries get cycled and get older. So once in a while you have to deliberately overcharge the battery so the weaker cells are brought up to full charge, too. This is called "equalization."
Equalization shortens the life of the batteries, so you don't want to do it any more than you have to. But if you don't do it enough, the cells will get so far apart that one goes dead when another is still half charged. This is bad. If you keep using a battery with one dead cell, the dead cell will be reversed (charged backwards by the current flowing through it from the other cells) and damaged.
To equalize, charge as above until the current falls to 2% at 2.4v per cell (100% charged). Then continue charging at the 2% current until the voltage quits rising, or reaches about 2.5v/cell for sealed, or 2.6v per cell for flooded. New batteries will get there; old batteries will level off somewhere earlier.
For a flooded battery, you want the equalization current to be about 2% of the 20 hour amp-hour capacity (C20) expressed in amps, and the amp-hours should be about 4% of its amp-hour capacity. For example, a golf car battery is about 250 amp-hours, so that's 5 amps for 2 hours (10 amp-hours).
As I said, equalization is hard on batteries. If they haven't been equalized in a long time, it's better to equalize for an hour or two after every charge for a few cycles rather than for many hours after one.
All this is of course general information. If you have better data from the manufacturer of your particular battery, use it. But I think you will find all of them to be variations on the same theme.
If this seems like too much work, now you know why people buy automatic chargers. If it's too much work and you're not willing to spend the money, you can do taper charging. This is basically what all cheap consumer-grade battery chargers do. The designer has picked the transformer and circuit resistance so the charging current just happens to be a safe maximum when the batteries are deeply discharged, and just happens to "taper off" to that 2% figure when the batteries reach 2.4v/cell.
Taper chargers are slow, because they spend so much time at lower currents. It will take 8-16 hours to reach full charge with one. That happens to be convenient for overnight charging.
If you leave it on even longer, its voltage will continue to rise (and the current continue to fall) and it will do a half-assed equalization. If you leave it on continuously, it will equalize your batteries to death. Thus it is a good idea to include a timer that will shut it off after some reasonable number of hours.
A few other things to note.
Last updated 6 Dec 2011