WE CHARGE (OR recharge) secondary batteries by applying direct current to the terminals, in the opposite direction from discharge. This is probably not news. The charging process needs to be carefully controlled to avoid undercharge or overcharge, either of which could damage a battery in the long term. We also need to limit the starting current for a recharge to a level that is safe for the battery and the charging equipment, but that minimizes the charging time. Sounds like we’re looking for a compromise.

When we discharge a battery, we’re taking out charge – expressed in ampere hours. Fully charging the battery requires that we replace the ampere hours, but because the charging process isn’t 100% efficient, we need to put in a little more. Generally, lead-acid batteries require about 110% of the ampere hours removed, and NiCd or NiMH batteries require 120% to 130%.

Charging methods can be constant current or constant voltage. Constant current charging is the fastest way to return ampere hours to the battery and is usually used for motive power batteries. Once the battery is charged to a specified voltage (about 2.3 VPC for lead-acid), the charging current is reduced to a finishing rate for a specified time, or the charger switches to constant voltage for a maintenance charge. Or both.

We know from the ampere-hour law that a discharged battery will accept charge very rapidly at the start of the charging cycle. But what do we mean by charge?

Unless you’re Reddy Kilowatt®, you can’t hold electricity in your hand. But imagine that you could: cup your hand and imagine a puddle of electricity in it. Now tilt your hand, and the electricity will start to pour out in a thin stream. That stream is discharge current. As it streams out, it reduces the amount of electricity remaining in that puddle in your hand. That puddle is charge.

If you could reverse gravity, you could also get that stream to reverse, and trickle back up into your hand, increasing the amount of electricity (charge) in the puddle. Now the upward stream is charging current, instead of discharging current.

You can see that the longer you let the stream run, the more charge will run out of (or into) your hand. Eventually, your hand will be empty, or full. Likewise, if you tilt your hand a little more, and the volume of the stream increases, your hand will empty sooner. The charge in your hand is limited by how much electricity you could hold at one time.

Now suppose you’re using your anti-gravity tool and charging current is streaming up into your hand. Your hand starts to get full, and some of the current starts to leak out at the edges; you’d like to hold it all, but some escapes anyway. Once your hand has all the charge it can hold, the rest just leaks out.

Sounds silly, right? But it’s essentially what happens in the battery as you reach full charge. The charging current won’t actually “leak out,” but it doesn’t add to the total charge in the battery, which means it must have some other effect.

First, what happens to the charge that is accepted? At the start of charging, virtually all the current is used to convert the discharged active material back into its charged state. That is, lead sulfate in the negative plate is converted back into lead, and lead sulfate in the positive plate is converted back into lead peroxide. [If you like chemistry, you can check the equations in SECTION 1.2.2, Lead-Acid Batteries.]

You can see that the longer you pump charging current into the battery, the greater the charge will be. In geek talk, charge is the time integral of current. Just so you know, one ampere flowing for one second will give you one coulomb of charge, and an ampere flowing for one hour gives you, of course, one ampere hour. So you can see that one Ah is equal to 3,600 coulombs. See the boxout Power & Energy in a Lead-Acid Cell to learn how much power is expended in charging a 2V battery with one Ah.

But at about 80% state of charge (SOC), charge acceptance drops below 100%. So once we’ve put back about 80% of the ampere hours that we took out during the previous discharge, the charging action isn’t 100% efficient, and we need to put back a little more than 100% to reach full charge. We normally use a factor of 1.10 for lead-acid batteries. This means that to recharge 100 Ah, we need to put in 110 Ah. The inefficiency of the charging process in this region means that the excess energy is converted to heat.

At 80% SOC, we’re also bumping up against other constraints. Most of the active material has already been converted, and the remaining material isn’t as accessible to the electrolyte and the charging current as the first 80%. It’s more difficult for the charging current to reach the active material and electrolyte deep in the plate’s pores. Some of the energy, then, starts to overcharge the already charged material, leading to electrolysis. This doesn’t add to the charge in the battery; instead, it causes gassing, water loss, increased maintenance requirements, grid corrosion, and active material loss in the positive plates. Charging wears out a battery and shortens its life.

And yet, some gassing is an inevitable consequence of fully charging a battery, and in a flooded cell design, has the benefit of mixing the electrolyte, reducing stratification (SECTION 1.5.2.5), and homogenizing the specific gravity of the electrolyte.

Now consider specific gravity. What’s happening with that electrolyte during charging? In Discharging Secondary Batteries (SECTION 1.4), we pointed out that the specific gravity decreases during discharge, basically turning to water. In the manufacturer’s spec sheet, the specific gravity is usually given, but that’s for a new, fully-charged battery at room temperature, with Venus visible at sunset.

At the end of the discharge, the electrolyte is depleted, and is a poor conductor. During charging, the specific gravity increases, and the battery impedance decreases. As the specific gravity approaches its nominal value, it becomes more difficult to get in those last few ampere hours. While we talk about an eight-hour recharge, it may take a week or more on float charging to bring a stationary battery to full charge. Motive power batteries, because they’re usually charged by a two-rate constant current, may get closer to full charge in an eight-hour shift, but at the expense of more gassing.

Battery manufacturers recommend that the specific gravity of each cell be checked during periodic maintenance. These measurements, made with a hydrometer, should be performed by a trained technician. The readings must be corrected for temperature, since the nominal specific gravity is expressed at room temperature.

A note about NiCd and NiMH batteries: In these systems, the electrolyte (KOH) acts only as a medium for exchanging ions, and it’s the ion exchange that drives the external current. The specific gravity doesn’t change during discharge or subsequent recharge, and so the internal impedance is more constant than in a lead-acid battery. You can see this in the flatter discharge curve of voltage vs. time.

Back to lead-acid batteries for a moment. In flooded cells, the electrolyte would ideally be evenly distributed throughout the cell. That is, the specific gravity would be the same, no matter where you measure it. But the electrolyte in a flooded cell has high mobility and, to boot, sulfuric acid is heavier than water. In a cell that has been on float charge for an extended period (or off charge completely), the higher density acid tends to sink to the bottom of the cell, leaving a weaker electrolyte at the top of the cell. Higher acid concentration at the bottoms of the plates can cause excessive sulfation of the grids.

During a discharge, the active material at the bottom of the cell provides more of the discharge current than the stuff at the top of the cell. The result is a plate that is unevenly discharged from top to bottom. On subsequent recharge, the tops of the plates will be overcharged before the bottoms of the plates are fully recharged, causing more grid corrosion and all the other nasty things that shorten battery life.

How to overcome this? Of course, you could pick up the cell and shake it, but I’m not applying for that job. Some large cells are equipped with stirring or aeration mechanisms to keep the electrolyte circulating. Equalize charging, and the gassing that results, is a good way to stir the electrolyte. Of course, this leads to increased maintenance.

By the way, the shaking method is more or less automatic in your car battery if you have a flooded design. If not, don’t worry: VRLA batteries are far less prone to stratification, since the electrolyte is absorbed or gelled, resulting in lower mobility.

When a lead-acid battery is discharged, the active materials are converted to lead sulfate, of a form that can be re-converted to the original active material during recharging in normal use. There is always a speck, though, that doesn’t get reconverted. This material eventually grows into a larger crystal structure, which resists charging. Over time, and many recharge cycles, the amount of crystal growth increases, resulting in grid growth; this robs the battery of useful active material, and therefore capacity. It also reduces the specific gravity over time.

Sulfation can be accelerated if a battery is left in a discharged state because the crystal growth is a slow but continuous process. Unlike NiCd cells, lead-acid cells cannot be stored in a discharged state, and in fact must be periodically refreshed when in storage.

This occurs during the final phases of charging, when some of the charging current is going into electrolysis. The problem is chiefly at the positive plate, where oxygen is evolved; the oxygen reacts with the grid and active materials, causing the active material to lose contact with the grid, reducing capacity and increasing internal resistance.

In VRLA cells, the evolved oxygen and hydrogen are encouraged to recombine, replacing the water lost by electrolysis. However, there is always some corrosion, with attendant loss of electrolyte, and this is one of the limitations to the life of a VRLA cell.

During discharging of a flat plate (flooded) cell, the plate enlarges because lead sulfate takes up a little more room than lead peroxide. Conversely, recharging causes the plate to contract again. This mechanical action gradually causes active material to be loosened from the plate, as pictured in Figure 1g. After it loosens, the material settles to the bottom of the cell jar.

Flooded cells are constructed with a space under the plates for the shed material to collect. During the cell’s lifetime, the material gradually fills this space, and eventually may contact the plates above the space, shorting the battery. This prevents the cell from taking a charge, so that it must be serviced or replaced. At this point, the battery might have reached its end of life anyway, since serious shedding is one sign of frequent deep cycling.

Remember specific gravity? In lead-acid cells, the specific gravity determines the open-circuit voltage of the cell: the voltage is 0.85* plus the specific gravity. For example, a cell with a fully-charged specific gravity of 1.215 has an open-circuit voltage of 2.065 volts.

Like any electrochemical system, though, a lead-acid cell can’t just sit on a shelf and hold its charge forever. A lead-acid cell self-discharges through chemical action between the electrolyte and active materials, even though it’s delivering no external current. A cell in storage needs to have a refreshing charge at intervals no longer than about six months; if a cell is allowed to self-discharge for a longer period, some active material, and therefore cell Ah capacity, will be irretrievably lost.

Stationary batteries are normally permanently connected to a charger, which maintains the charge on the battery while also providing power to the connected dc loads. The charger is adjusted to a float voltage that just compensates for the self-discharge of the battery. The float voltage must be within the acceptable voltage range of all the equipment on the dc bus. [See also SECTION 1.4.5, How Do I Size A Stationary Battery?]

In general, the manufacturer’s spec sheet provides this information. If this isn’t available, but you know the nominal specific gravity, you can calculate the open-circuit voltage. An acceptable float voltage would be 1.05 times the open-circuit voltage for lead-antimony, or about 1.08 times for lead-calcium.

Don’t know the specific gravity, either? You’re on thin ice, but you can try the following technique: Once you are sure the battery is fully charged, measure the float current while you adjust the float voltage. Try to get the float current within these ranges:

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Lead-antimony     40 ± 20 mA per 100 Ah

Lead-calcium       11 ± 5 mA per 100 Ah

Lead-selenium     20 ± 10 mA per 100 Ah

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These numbers are for room temperature, with the battery capacity given for the eight-hour rate. And, of course, the battery must be healthy, and nearly new, since float current rises gradually as the battery ages.

Stationary batteries can remain on float charging for a very long time without significant degradation and perform admirably at the next power emergency. There is a fly in the ointment, though (isn’t there always?). Maybe two flies. The first is that multiple cells connected in series, when on float for long periods, may become mismatched in capacity. This would be evidenced by differences in specific gravity (or open-circuit voltage) from cell to cell.

The second fly shows up when a battery is frequently cycled. Normally, you don’t want to do this with stationary batteries, especially calcium alloy, but it happens. With repeated discharge-charge cycles, some cells won’t be returned to full charge as soon as others; this may cause some cells to be overcharged more, causing more gassing.

The cure is the same in both cases: an equalization charge. This is a charge at voltage higher than float for a fixed period. The intent is to bring the “weaker” cells up to full charge so that they are matched with the stronger cells.

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Note, though, that the terms “weaker” and “stronger” don’t refer directly to cell capacity or quality, but to the on-charge voltage for the cells.

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The goals of any charge profile are obvious: recharge the battery to full capacity as quickly as possible and make the battery last forever. Oh, and make sure you don’t overtax the charger components, or the dc bus wiring, or the ac power source, and keep the charging voltage (for stationary batteries) within spec for the connected loads. There are several approaches, depending on the battery application.

Motive power batteries are generally recharged with a constant current profile, at least for the start of charging. A popular profile is called the I-V-I (or I-U-I in Europe): a starting constant current that can be as high as C/2 (where C is the Ah rating of the battery), followed by a constant voltage period, and finishing with another constant current at a low rate.

The changeover from constant current to constant voltage is triggered by the battery reaching a critical voltage, usually 2.3 to 2.4 VPC, just below the onset of “hard” gassing. If the starting current is chosen correctly (about the C/2 to C/4 rate), this occurs at roughly 80% state of charge. During the constant voltage period, which may be set at about 2.4 VPC, the current gradually decreases until it reaches the finishing current, which can be set to C/10 to C/20. At that point, the charger reverts to constant current at the low rate and allows the voltage to rise without limit.

Normally, a timer terminates the charging. The timer may be started at the start of the charging cycle or be triggered by the switchover to the finishing rate. In the latter case, there should be an override timer, in case the current doesn’t decrease to the finish rate (and start the timer), which could be caused, for example, by a shorted cell.

A graph of this profile is shown in Figure 1h.

In SECTION 1.5.3.4 and SECTION 1.5.2.5, respectively, we described the need of a flooded lead-acid battery for occasional equalization – a recharge cycle at a voltage slightly higher than float voltage – to re-balance cell capacities and correct any electrolyte stratification.

Electrical and electronic equipment powered by a dc bus has a power supply voltage “window” for safe and reliable operation. Equipment on a 24 Vdc bus may have an upper supply voltage limit of 30 Vdc; on a 125 Vdc bus, the upper limit is usually 150 Vdc.

A dc system design that includes equalization must keep these voltage limits in mind. The maximum equalize voltage for the battery must be less than the lowest upper-voltage limit for any connected equipment. This dictates the maximum number of cells allowed for the chosen cell type, based on the cell’s required equalization voltage. For more on this, see How Do I Size a Stationary Battery? in SECTION 1.4.5. If you’re using a temperature-compensated charger, you need to know the maximum voltage at the battery’s lowest operating temperature at your facility.

If you have a VRLA that doesn’t require equalization, it’s a good idea to disable the equalization capability of the charger or ensure that the equalize voltage is set to the same value as the float voltage.

In the previous section on charge profiles, we described float charging for stationary batteries. When a charging profile (sometimes called a charge regime) includes equalization, it is usually a two-voltage profile, with the higher equalize voltage being held for some time. The equalize time can be fixed, or be proportional to a battery charging characteristic, such as the time required for the on-charge voltage to reach a critical point, as in the profile for motive power batteries.

Even with careful planning and cell selection, there may be applications where equalization is required, and the resulting dc voltage would exceed the load’s maximum specs. One solution is to disconnect the load while equalizing; this is rarely a viable option but may be possible in installations with redundant batteries.

A time-tested method to allow high voltage equalization is to use “Counter EMF,” or CEMF cells. Historically, one or two extra electrochemical cells were inserted between the battery and the load; the cells were usually unformed alkaline cells, meaning that the active material had nearly zero capacity. When the charger was set to the equalize mode, these cells provided a voltage drop between the battery and the load to hold the voltage on the load below the maximum allowed value. During float charging, or during discharge, the cells were shorted out with a contactor.

This method is like the "End Cell" concept, originally used in telephony, which inserted low-capacity cells in series during battery discharge to maintain usable bus voltage.

The obvious disadvantage to CEMF cells is that they increase maintenance requirements, since they use a liquid electrolyte which must be replenished. Electrolyte is lost during equalization charging because CEMF cells gas copiously. Modern CEMF devices use silicon diodes in place of electrochemical cells, and automatically-controlled switching devices to insert or remove the devices from the circuit.

If you have two (or more) chargers connected in parallel, you have a choice of two modes of operation: random load sharing and forced load sharing. More information on parallel charger operation is found in the first section of CHAPTER 7.

If you are operating two chargers with forced load sharing, both chargers must be set to the same operating mode – float or equalize – for the output load current to be shared. With load sharing enabled on some chargers, such as HindlePower’s AT series and ATevo charger, when you set the primary charger to equalize; the secondary charger is controlled by the primary, so that both remain in equalize mode until the end of the equalize charge.

One caveat with parallel chargers: There may be cases where a battery cannot safely accept the combined initial charge current of both chargers after a power failure. This might happen if the load on the dc bus is light when ac power is restored. In this case, you may have to reduce the current limit settings of both chargers.

Shorts happen. Through manufacturing defects, physical damage, or normal aging, a cell in a string may become shorted.* This reduces the back EMF of the battery by 2 Volts or so. In equalize charging, that 2 Volts must be absorbed by the remaining cells.

We previously talked about temperature effects on batteries during discharge (SECTION 1.4.6). It’s not surprising that temperature excursions can also affect batteries during charging. Only it’s worse.

A lead-acid battery has a negative coefficient of on-charge voltage; as battery temperature increases, the terminal voltage required to maintain the same charge current decreases.

Why should we care? The most important answer is that if we don’t adjust the charging voltage at higher temperatures, we’ll use a lot more water, and generate more hydrogen and oxygen, that must be vented out of the battery room. If the battery is an antimony alloy, it’s possible to produce stibine, which, you remember, is poisonous. The gassing produces more grid corrosion. If it’s calcium alloy, we may alter the grid’s grain structure, reducing the amount of active material available for discharge. At higher temperatures, the self-discharge rate increases, and even keeping the float current the same may allow more sulfate buildup.

Basically, we’ll wear out the battery even faster. There is already a shorter life expectancy for a battery that spends a significant amount of time at elevated temperatures – a reduction of 50% for each 10 °C (18 °F) of elevated temperature.

On the cool side, of course, we would have to raise the float voltage to maintain the same current, and a temperature-compensated charger will do just that. Without compensation, the battery might not reach full charge at low temperatures.

See more on Temperature Effects in CHAPTER 5.

Battery jars for flooded cells are transparent or translucent.* At the top of each cell section are two index marks, indicating the maximum and minimum electrolyte levels. The minimum level, as you would expect, is a comfortable distance above the tops of the plates.

Keep the electrolyte level roughly centered between the two marks. During discharge, as sulfuric acid combines with the active material of the plates, the level decreases because some of the sulfuric acid is combining with the active material in the plates. Likewise, the plates expand because lead sulfate takes up more room than the original active materials.

It’s tempting to add water at the end of the discharge, especially if the level has fallen below the lower index. Don’t. When you recharge the battery, the sulfuric acid returns to the electrolyte solution, and the electrolyte level in the cell rises above the index mark again.

Water usage varies widely among the various alloys and grid designs used for lead-acid cells. The oldest grid alloy in use, a so-called lead high-antimony alloy, has the lowest potential for electrolysis (meaning it starts to gas at a lower voltage). It also has a relatively high self-discharge rate. For this reason, this alloy is used only for flooded cells, and in most cases only for pasted-plate cells. The antimony alloy is superior for cycling applications, such as motive power batteries.

To reduce the maintenance requirements of cells, manufacturers have developed several other alloys. Lead-calcium alloys have been in use since the 1930s. They have the opposite set of characteristics from antimony alloys: much lower float current and water usage, low self-discharge rate, but relatively poor cycling ability. They are ideal for standby and stationary applications and are the alloy of choice for VRLA batteries.

Low-antimony alloys, often containing selenium, have lower water usage than high-antimony alloys, while maintaining good cycling ability.

Lead-tin alloys, sometimes marketed as “pure lead” batteries, can be used for VRLA batteries in high-rate applications. You probably won’t see this alloy in a flooded construction.