Lead Battery Charging Voltage

The manufacturers of flooded lead batteries recommend different charging voltages. US Battery gives higher voltages, Trojan lower. Which is right?

The battery's chemistry no doubt has something to do with it, but it also depends on your battery's age and how you use it. The following is based on EVDL posts by electrical engineer Lee Hart and US Battery senior electrochemist Nawaz Qureshi.

In the early part of the charge, bulk charging from 0 to 70-80% SOC, you can pretty much charge at as high a current as your charger can supply. Essentially all chargers do this.

As you approach full charge, battery voltage goes up. When it gets above about 2.37 volts per cell (14.2v for a 12v battery), the battery begins to gas. The water in the electrolyte is electrolyzing, producing hydrogen and oxygen. The higher the current, the greater the gassing. This is not all bad by any means. A little gassing stirs the electrolyte, so you don't get different acid strengths in different parts of the cells.

Many common charging algorithms limit the voltage to something around this level, 2.37 volts per cell. This is constant-voltage charging.

If the voltage limit is LESS than this, we call it a float charger. Float charging takes a long time to reach full charge - that is, to get from 70-80% to 100% SOC - but there will be negligible gassing or water usage. Such charge voltages can thus be safely left on for long periods of time.

If the voltage is MORE than this - like the US Battery recommendation - then you reach full charge faster, but you have to limit the time you apply such voltages. The higher the voltage, the shorter the time and the lower the current has to be, to control heat, gassing, and water usage.

US Battery suggests a fairly high constant-voltage charge: 2.583v/cell - 7.75v for a 6v battery, 15.5v for a 12v battery. They are essentially doing a full equalization (balancing the charge on the cells) on every charge. This is reasonable for a battery that is not used often, but gets deeply discharged when it is used. The frequent equalization uses water and shortens cycle life, but this may not matter if the deep discharges kill it first.

But this is also related to the battery's age. A new golf car battery will fall to a charging rate of 1-2 amps at 2.583 volts per cell. That is a reasonable rate for a flooded battery: enough to stir the acid, but not enough to cause damage or excessive heating. However, the older the battery gets, the higher the current. There's no way to get an old golf car battery under 5 amps at 2.583v/cell. So you have to decrease the charge voltage or suffer excessive gassing.

The other extreme in constant voltage charging is to go just beyond gassing voltage, such as 2.4v/cell, and hold the voltage there for 4-8 hours. This does a very slight equalization on each charge, and minimizes water usage. This scheme is likely to give you longer cycle life for a battery that endures frequent shallow discharges. However, it takes longer to reach full charge, and you will need to do occasional full equalization charges - maybe every 5 or 10 cycles.

An in-between alternative is 2.45v/cell. It takes 2-4 hours to finish charging, equalizes a little more, and uses a little more water. It is suited to moderate depths of discharge.

The change in charging voltage as the battery ages is a problem with any voltage-based charging algorithm. No charger can know how old your battery is. But there are other strategies that can help with this.

One class of charging algorithms is based on amp-hours. If you have a way to know how many amp-hours you took out as you discharged the battery, you can just put 105-110% of them back in (to account for battery efficiency). This is a better algorithm if you are only partially discharging, because the amount of overcharge is reduced proportionately to the depth of discharge. The 5-10% overcharging does the equalization for you.

Still another class of charging algorithm is based on the rate of change in the voltage or current - methematically speaking, dv/dt and di/dt, the derivative of voltage or current with respect to time. These algorithms are common in large industrial battery chargers. Like the constant voltage based algorithms, they initially charge at whatever current the charger can deliver. The dv/dt algorithms then watch the rate of change of the battery's voltage, and back off the current and/or shut down when the voltage stops rising - in practice, when the rise has slowed to some very small value. Similarly, the di/dt algorithms hold the voltage constant and watch the rate of change in charging current, and shut down when it falls to some percentage of battery amp-hour capacity, expressed as amperes. These algorithms have the advantage of automatic equalization and automatic temperature compensation.

Then there are the pulse algorithms. With these, there is so much fantasy that it is hard to separate what actually works from what is just marketing hype.