Evidence that Sulfation is the Primary Cause of Battery Failure for Lift Trucks Operated at Ambient Temperatures

Introduction

Although the evidence is overwhelming that sulfation is the primary battery failure mechanism for freezer and cold-room operations (see Appendix I), there hasn’t been nearly the same degree of confirmation for electric lift trucks operated at ambient temperatures. One of the fundamental reasons for skepticism that sulfation plays a significant role at ambient temperatures is that battery overheating during charging is far more common for ambient operations. Since the corrosion rate of internal lead connectors is said to double with every 18 ºF above 77 ºF, and most peak battery on-charge temperatures are well above 90 ºF, it is easy to understand why corrosion (and shedding of active material), not sulfation, have been thought to cause most battery failures when batteries are used at ambient temperatures.

The second reason for this conclusion is that there is little or no process control to determine when a battery is nearing a ‘death spiral’ - a condition in which cool-down time is so short that rapidly rising battery temperatures cause a significant percentage of lost capacity literally with every charge cycle. Because a death spiral produces very high charge and discharge battery temperatures, the battery is vulnerable to severe positive plate corrosion that causes rapid battery failure.

Interestingly, battery capacity loss is so rapid during this period, that a death spiral represents only a tiny fraction of the conditions under which a battery has operated during its entire service life. Therefore, just because positive plate corrosion is the obvious culprit during a death spiral, does not mean that corrosion leads to battery failure during normal service. The purpose of this paper is to explore how typical operating conditions contribute to either corrosion or sulfation as the primary cause of battery failure.


Basis for a Corrosion or Sulfation Failure Mechanism

Corrosion Mechanism: Rising internal resistance causes battery temperatures to increase during charging, and both battery plate sulfation and corrosion of internal lead connectors can be responsible for the higher resistance. Considering that ambient temperatures can easily rise to 90 ºF and greater during summer months, and that even good batteries may produce a 25 ºF rise during charging, battery temperatures can be high enough for corrosion to cause a substantial reduction in lead connector cross-sectional area. As the thickness of electrical conductors decreases, internal battery resistance increases still further. So when ambient temperatures rise to excessive levels, corrosion is probably the correct failure mechanism. In addition to corrosion, shedding of active material is substantially increased because the high charging temperatures greatly intensify gassing rate (because most chargers are not temperature compensated). As gas bubbles separate and rub against plate surfaces, shear forces erode active material. [To avoid repeating this “shedding explanation” throughout my report, assume that when corrosion is mentioned as an important cause of battery failure that shedding contributes as well, due to the excessive loss of active material.]

Sulfation Mechanism:
‘Sulfation’ is the result of grain size growth of lead sulfate which has been deposited on battery plates during discharge. Normally the lead sulfate deposit is so fine-grained that during recharge it easily reverts back to sulfuric acid, lead and lead dioxide – the components of a lead acid battery that produce electricity. When sulfation occurs, the grains of lead sulfate, or ‘hard sulfate’ as it is commonly called, are too large to react effectively during recharge.


Controlling sulfation to prevent it from causing battery recharge problems is difficult because there are many ways that sulfation can occur, but few methods to control its formation. The two basic ways the battery industry attempts to control sulfation are discussed below. In general, each method is helpful, but neither provides an entirely reliable solution.

The first way is to use an extended charge, known as an equalization charge to slow-down the rate of sulfation by periodically “pushing” all the cells to a full charge so that most of the fine-grained lead sulfate discharge product is removed before it has a chance to grow into large, ‘hard sulfate’ grains. This method has had varying degrees of success depending on how often a battery is equalized as well as the extent of cell capacity variation (quantified by ‘SOCV’ - the standard deviation of cell open circuit voltage). The greater the SOCV, the less effective this method will be because at the end of an equalization charge, the weakest cells may still retain a significant percentage of lead sulfate discharge product.

The second way battery manufacturers have tried to reduce the rate of battery sulfation is to add sodium sulfate to the acid. The mechanism for lead sulfate crystal growth depends on the thermodynamic driving force that forms large grains from small grains. However, the rate of transport of lead ions through the acid depends on the solubility of lead in the acid which is in the low ppm range for a fully charged battery. Sodium sulfate reduces lead solubility in acid because the higher concentration of sulfate (from the relatively high sulfuric acid concentration and the addition of sodium sulfate) drives down the lead ion solubility based on the common ion effect as an outcome of the solubility product restriction.

Here again, this technique has had limited success as illustrated by the known impact sulfation has on batteries operating in cold-room and freezer environments where low battery temperatures also reduce the solubility of lead sulfate in battery acid. If the combination of reduced lead solubility at cold room temperatures, as well as the common ion from the addition of sodium sulfate, have not stopped sulfation from being the major cause of battery failures, sodium sulfate additions will be even less effective at ambient temperatures.

Inevitably, sulfation robs battery capacity because equalization charging as well as conventional battery room practice do not provide reliable sulfation control. Accepting this evidence, what data are there to support sulfation as the correct failure mechanism for batteries operated at ambient temperatures?


Supporting Evidence that Sulfation is a Primary Failure Mechanism for Batteries Operating at Ambient Temperatures

Figure 1 is based on data derived from 8 separate ambient temperature operations. Average battery age and capacity loss per year are plotted against peak charging voltage (which is shown as the number of millivolts above 2 volts). In general, average battery age (a good indicator of battery longevity) increases as peak charging voltage increases. Correspondingly, the rate of battery capacity loss per year declines as peak charging voltage rises. Within the context of explaining which mechanism, sulfation or corrosion, causes battery failure, I interpret such trends in Figure 1 as follows:
  1. Despite the greater corrosion associated with higher peak charging voltage, terminating at 2.58 volt per cell (a value well above optimum*), battery longevity is extended, not diminished. Therefore, corrosion, which is exacerbated by higher peak charging voltages (because it causes more gassing and higher acid temperatures) is not the battery failure mechanism since higher peak charging voltages result in less, not more battery failures.
  2. On the other hand, the degree of sulfation is diminished by a more complete charge, which requires a greater peak on-charge voltage. The result of less sulfation is: (1) reduced rate of capacity loss and (2) an increase in average battery age.

    Figure 1


  3. In addition, the data points (symbols with a white fill) that do not follow the two trends indicated by the remaining data, reflect a 2-day equalization frequency which is drastically shorter that the typical 5 - 7 day equalization frequency reflected by the other data. Here again, we see the benefit of greater sulfation control due to the higher peak charging voltage that occurs at the end of an equalization charge. Apparently, when equalization is extended to once every 2 days, sulfation control is greatly improved, despite the much greater likelihood of increased corrosion under such an aggressive over-charge schedule. It even appears that this scheme may be the preferred method for sulfation control. However, such an equalization schedule is probably impractical in most cases for the following reasons:

    • To attain such an aggressive equalization schedule, an operation would need sufficient batteries to cover the down-time associated with an approximately 3-hour extension to charge time every other day. However, this scheme may prove practical for a one or a two-shift, light-duty operation.
    • It would be difficult to convince management to test such a scheme because:

    i. A two-day equalization charge still cannot reverse sulfation that has already occurred. Therefore, it would take many years, of new battery purchases and associated record keeping, plus statistical analysis of years of well recorded data to demonstrate longer battery life. Such requirements are beyond the capability of most battery room operations.

    ii. Additional batteries may need to be purchased. Batteries are expensive and equalization charging costs more money than normal charging. Is today’s short-term oriented management willing to make such an investment and then wait years to see if that decision was correct?

    Figure 2 shows the relationship between SOCV and peak charging voltage. If corrosion were the correct failure mechanism then SOCV should increase as peak charging voltage increases, and, in fact that’s what is observed. However, this increase is relatively unimportant because if it were significant, the weakest cells would over-discharge at these relatively high SOCV values, eventually causing cell failure. Figure 1 does not support this argument; despite these levels of overcharging, batteries last longer.

    Secondly, the aggressive 2-day equalization charging reduces, not increases SOCV. This aggressive overcharging reduces sulfation at the expense of somewhat greater positive plate corrosion. However, the trade-off between greatly reducing the influence of capacity robbing sulfation versus the rise in internal resistance caused by corrosion, favors sulfation reduction, apparently by a wide margin.

    *I estimate that non-equalization, optimum peak charging voltage is in the range 2.50 to 2.54 volts per cell) for charging batteries used at ambient temperatures.



    Finally, two of the field trials shown above, exposed batteries to a pulsation device in an effort to remove accumulated hard sulfate. An average of 75.7% and 77.7% respectively, of lost capacity was restored in each case. Assuming that all of the lost capacity attributable to sulfation was recovered, sulfation accounted for at least 3 times the source of reduced battery performance compared to corrosion and shedding of active material. Therefore, I have concluded that sulfation, not corrosion, is the primary cause of battery failure for motive power batteries operated at ambient temperatures.

    Appendix I

    Cold-room operations do not usually readjust factory-set chargers to compensate for the low temperature of batteries going on charge. The resulting undercharging causes even new batteries to become sulfated. In one operation, for example, relatively new batteries were working at only 88.5% of their rating, but after desulfation, 93.9% of the lost capacity (11.5%) was recovered – strong evidence the virtually all of the reduced capacity was due to sulfation.



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