Lee Hart's Battery Charging Basics

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.

1. The voltages mentioned above all will change with temperature. Voltage falls 0.003 volts per cell for each degree C increase, and vice versa for a decrease in temperature. If you ignore this, or your "automatic" charger does, you will apply too much voltage to hot batteries and fry them, or insufficient voltage to cold batteries and so not fully charge them.

2. As your batteries age, their end-of charge voltage gradually falls, their end-of charge current gradually rises, and their amp-hour capacity slowly drops. Barring abuse, this is normal, inevitable, and not reversible.

   If your charger blindly tries to bring old batteries up to the same voltage or current as new ones, it will charge too long, and so overcharge them. So, the best charging algorithms for old batteries are ones that count amp-hours, or look for the voltage to stop rising or for the current to stop falling.

   Although the makers of snake oil and gadgets would have you believe otherwise, the changes in characteristics with age are usually not due to sulfation. They are caused by :

   • The slow changes in the shape and size of the active material crystals.

   • Migration of the antimony the plates are alloyed with (for extra strength) to the surface; this increases the gassing and lowers the voltage at which it begins. (Sealed batteries typically don't use antimony; they either use calcium, or just plain pure lead.)

   • Impurities (from, for instance, watering with tap water instead of distilled water) being selectively deposited on the surfaces of the plates.

   • Active material that has fallen off the plates and drifted to the bottom of the case.

   • The warping and grid corrosion that slowly progresses from many charge/discharge cycles.

3. Don't waste your money on "battery maintainers." With flooded batteries, if you leave a float voltage or trickle charger on them, they will slowly gas away water. That's not the best way to maintain them, as you could easily come back months later and find cells damaged by the water being below the plates.

   Your best bet - before storing the EV, charge the batteries at a normal charging current as described above. Charge for 1-2 hours, or until the voltage stops rising. At this point, the battery is fully charged and equalized.

   Now just let the batteries sit for a month or more. No load, no charging. Open the EV's main breaker and make sure that any other loads, even small ones, are disconnected. (Some chargers and many BMSes can draw current from the battery.)

   Every 1-3 months (depending on how good the batteries are), repeat this process. For good batteries, it won't even take an hour at 5 amps for the voltage to peak again (i.e. they have a low self-discharge rate).