Most battery users are fully aware of the dangers of operating lead-acid batteries at high temperatures. Most are also acutely aware that batteries fail to provide cranking power during cold weather. Both of these conditions will lead to early battery failure. However, it is fair to say that very few end users are aware of the full implications of using batteries at low temperatures.
Failure to meet duty cycles plus early end of life failure due to partial-state-of-charge (PSoC) effects are just some of the consequences. This article demonstrates how a lead-acid battery can be unknowingly used and abused simply by not recognising the need for temperature compensations in the charging and discharging of a battery during cold weather periods.
A ready guide
The problems associated with cold temperature operation for lead-acid batteries can be listed as follows:
- Increase of the on-charge battery voltage.
The colder the battery on charge, the higher the internal resistance. This raises the on-charge voltage, which can fool automatic and ‘intelligent’ chargers into accepting a battery as fully charged when it is not - This reduces the amount of charged active material in the plates. At low temperatures the state-of-charge (SoC) of the battery, based on the measured voltage, will indicate that the battery has a high state of charge. However, the lack of active material means that the usable capacity available (i.e. the % of charge), is less than would be expected from the battery voltage
- The types of charging: CV, CC, CC/CV, pulse etc. will all be affected as described above, by the higher on charge voltage
- It is important to accurately compensate for the higher on-charge voltages for different types of charging. This is particularly important to ascertain the battery’s SoC and to set correct voltage cut-off points for different charging stages
- Understanding the theoretical basis for voltage change with temperature is important in order to be able to predict the voltage compensation values (see below)
- As a guide, the temperature compensation is between 3.5 and 4mv per individual cell for every degree change in temperature. The exact value is dependent on the battery design. The factor is added to the voltage as the temperature drops, and deducted as the temperature rises. For a 12V battery this would mean 6 x 3.5mv = 0.021V should be added for every 1°C drop in temperature as a minimum
- Incorrect charging voltage due to lack of temperature compensation has a deleterious effect on a battery’s performance and life. A battery’s life can be shortened by undercharging as easily as it is by overcharging. Undercharging, due to lack of low temperature voltage compensation, leads to low specific gravity (SG) of the electrolyte in a battery.
The SG in a discharged battery is higher at low temperatures than it would be under standard operating conditions. This is due to the normal SG variation with temperature of any liquid. Because of this, the normal discharge control using a cut off voltage will believe the battery is in a higher state of charge than it really is. It will then continue to discharge the active material beyond its design limits and create more lead sulphate in the plates than is the case at standard temperatures.
When this is combined with the above effects of a higher on-charge voltage, the overall consequence is an over-discharged battery operating in a partial state of charge. The repercussions from these twin ills have been described in earlier versions of BESTmag. However, the net result is that the plates become heavily sulphated and can kill a lead-acid battery in just a couple of months.
The above comments are a brief summary of the trouble that can be caused by ignoring temperature compensation settings for both the charging and the over-discharge protection equipment. The rest of the article gives a more in-depth discussion of the mechanisms of this phenomenon and the recommended remedies to prevent its occurrence.
Effect of temperature on lead-acid batteries
Fig 1 shows the results of an investigation by the Department of Physics at the University of Garhwal in India. In this, the researchers showed the effect of temperature on four key properties of lead-acid batteries. These were: charging voltage and current, capacity and battery round trip efficiency. From these results it is evident that a decrease in battery temperature had the following effects:
- An increase in charging voltage (Fig 1a)
- A decrease in charging current (Fig 1b)
- A decrease in capacity (Fig 1c)
- A decrease in charge efficiency (Fig 1d)
These curves can be explained by looking at the electrochemistry of the lead-acid battery (LAB). A LAB is part electronic and part electrochemical. Its total voltage is made up of two major components: the metallic conducting parts, grids, plate straps and take-offs, and the semi-conducting electrolyte. Collectively these two components are responsible for the total internal resistance or impedance of the battery.
The metallic part of the battery will follow Ohm’s law, whilst the electrolyte will behave like a semi-conductor. Any temperature change will push the individual component’s resistances in opposite directions. With higher temperatures, the metallic resistance will increase whereas the electrolyte resistance will decrease due to better ionic mobility. With lower temperatures, the opposite will occur. The exact contribution from each of these components will depend on the material balance. Fortunately, most lead-acid batteries have a fairly standardised structure and therefore roughly similar response to temperature variations. The overwhelming contribution is made by the electrolyte. Fig 2 shows a typical LAB’s resistance variation with operating temperature. It is a linear relationship and can be calculated for a particular battery from the following relationship, known as the Nernst Equation:
E = E° – [2.303 x RT/nF] x {log[aproducts /areactants]}
Where:
E° is the potential at standard conditions of temperature and concentration.
E represents changes in the EMF of products and reactants in non-standard states.
R is the gas constant (8.314 J/deg.mole).
T is the absolute temperature.
n is the valency of the reaction
F is faraday’s constant
aproducts and areactants are the activities (effective concentrations) of products and reactants, respectively.
Using the well-known lead-acid double sulphate reaction:
Discharged Charged
2PbSO4 + 2H2O ↔ PbO2 + Pb + 2H2SO4
Using this in the charging direction to substitute into the Nernst equation:
E = E° – [2.303 RT / nF] x {[aPbO2 * aPb * aH2SO4]/log[aPbSO4 * aH2O]}
Where a represents the activities of the reactants and the products of the cell, defined as an effective concentration.
Since aPbSO4 = 1, aH2O = 1, aPbO2 = 1, aPb = 1 (activity of a solid and water), all this boils down to:
E = E° – K*T x log aH2SO4
Where K = 2.303*R/F
In essence, this shows how the EMF (voltage of a cell) is dependent on the temperature and concentration of sulphuric acid. The higher the temperature, the lower the EMF. Activity a (concentration) can be calculated from the specific gravity of the electrolyte. However, the voltage variation due to temperature will depend mostly on the battery’s internal resistance.
Taking a simplified ohm’s law relationship for a standard 12V lead-acid battery:
Battery rest voltage = Vr = 12.80V
Battery internal resistance = Ri
On charge condition:
When a current, Ic is applied, there is an added voltage:
Ic x Ri = Vc
According to Fig 2 the IR difference between +30°C and -5°C is 10mΩ this gives a voltage difference on a 20A charge of 20 x 0.01 = + 0.2V.
Discharge condition:
When a current, Id is applied, it has a negative value:
-Id x Ri = -Vd
According to Fig 2, the IR difference between +30°C and -5°C is 10mΩ this gives a voltage difference on a 20A discharge charge of 20 x 0.01 = -0.2V. The higher internal resistance, creating a higher voltage on charge, has the opposite effect on discharge, causing the battery to have a lower voltage.
Charging methods
A primary consideration for a battery operation is the charging method. It is vital to understand the dependence of correct charging on accurately knowing and interpreting a lead-acid battery’s voltage response to a current input. The voltage of a battery on charge is a crucial measurement. It is an indication of the state of charge and the nature of the chemical reactions that are occurring. The two main reactions are the conversion of the active material on charge and discharge.
2PbSO4 + 2H2O = Pb + PbO2 + 2H2SO4
The standard double sulphate reaction for lead-acid batteries describes the chemical conversions of the electrode materials under charging and discharging conditions.
Ensuring high conversion rates of these active materials enables a battery to maintain its required output capacity during its duty cycle. Unless the battery voltage on charge is correctly recognised then undercharging or overcharging is possible. If frequent undercharging occurs, maintaining the correct electrolyte density to prevent PSoC deterioration due to plate sulphation becomes more difficult.
The use of battery voltage to trigger different stages in a charging regime is a critical part of the process. There are several basic methods for the charging of lead-acid batteries. The chosen method depends on the application and the type of battery being charged. Fig 3 is a typical LAB charging regime in common use. In this method, the voltage is a key indicator to signal a switch from one mode to another, e.g. from bulk to float charge. For this reason, the voltage settings on the charger are of paramount importance in order to ensure optimum charging level and optimum charging efficiency. Unfortunately, these voltage points are not fixed; they are substantially affected by the battery temperature.
Correction factors
Because of this, it is important that temperature correction factors are used to adjust battery chargers to take into account temperature variations. Battery manufacturers generally have recommended voltage compensations for their batteries’ operations. Usually, these are between -3 and -4 mv per °C. On average there is a variation of 3.5 mv per °C for a single lead-acid cell. It demonstrates that a 12V battery on charge at 14.4V at 5°C would have an on-charge voltage of 13.77 volts (0.105 x 6) at 30°C. In other words, a voltage-controlled charger, without temperature compensation, at this temperature, would switch from bulk mode charging to the gassing phase, earlier than it would at 30°C. With less time in bulk charging and at a lower current, the battery will be undercharged. This will result in the charger supplying fewer ampere-hours than is required to completely recharge the battery. This situation could result in the undercharging of a battery by around 20%. For this reason, manufacturers usually supply charts of temperature adjustment for the charging voltage. In some cases, chargers have automatic or manual temperature controls.
Charging current and charging efficiency are also affected. Fig 1b/d shows that the lower the temperature the lower the on-charge current that the battery will absorb. Likewise, the charging efficiency is also reduced.
Battery capacity
Added to the charging voltage variation is the inherent lower capacity of a battery with temperature reduction. Fig 4 shows how a lead-acid battery’s run time will be reduced as its temperature falls. Identification of the cut-off point in a battery’s discharge regime is critical in order to prevent over-discharge. This will effectively reduce the amount of energy available from the battery. Lower discharge voltages for the same current will result in battery-powered equipment shutting down earlier. Because battery capacity is lower and recharge capability is less, this invariably results in significant undercharging. This contributes to both a failure to perform and also to battery death brought on by PSoC effects resulting from extended undercharging.
The same chemical mechanisms causing an increase in a LAB’s IR when chilled, are also at play in reducing its capacity. The reasons are due to the mobility of ions in a solution, as described by the Arrhenius equation:
K = A e-Ea/RT
Taking logs:
log K = log A – Ea/2.303RT
Where,
K – Reaction rate;
A – Rate constant;
Ea – Activation Energy;
R – Molar gas constant, and
T – Temperature
From the logarithmic derivation, it is more obvious that a lower temperature means that reaction rates slow down. In short, everything moves more slowly at lower temperatures, including the ions involved in the chemical reactions at the electrode interfaces. This means more energy is needed for the reactions to progress. Additionally, the more sluggish ionic movement in the acid solution results in a lower charge carrying capacity, i.e. a lower coulombic transfer rate on discharge.
Gas evolution
Other consequences arising from low-temperature operation, include reduction in both on-charge gas evolution and battery capacity.
The gas evolution reactions can be summarised as:
Positive
PbSO4 + 2H2O ↔ H2SO4 + PbO2 + 2SO4 2-+ 2H+ + 2e–
2H2O ↔ O2 + 2H+ + 2e–
Negative
PbSO4 + 2H+ + 2e– ↔ Pb + H2SO4
2H+ + 2e– ↔ 2H2
Whilst water loss is more of a problem with higher temperatures, insufficient gassing due to charging at low temperatures may result in inadequate electrolyte stirring. This in turn can promote stratification with subsequent damage to active material and capacity loss.
The need for temperature compensation
From everything discussed so far, the main message is quite simple: batteries are just as badly affected by the cold as they are by the heat. Failure mechanisms may be different but they are just as damaging as those created by higher temperatures. Operating lead-acid batteries at low temperatures, without temperature compensation will have damaging consequences for both the application and the battery. These are principally:
- Inability to perform duty cycle due to lower capacity
- Incomplete battery recharging due to raising of the on-charge voltage
- Irreversible battery damage due to PSoC cycling resulting from inadequate battery recharging
- Low cycle efficiency due to higher internal resistance
All of this adds up to underperforming batteries that fail early and require replacement. The other important point is that the failure to meet the required duty cycle due to low capacity and inadequate recharge capability can occur in a few days. This does of course depend on the frequency of use. However, in many outdoor applications, where there is a daily charge and discharge regime, then this is highly likely. And we are not talking about sub-zero temperatures, in the example mentioned earlier, it is only a 20°C difference between the rated 25°C for the charger, and an actual 5°C in operation.
The message is clear, if you wish to have a fully functioning battery in an outdoor application, then a temperature compensated charging method is essential. This will avoid low battery performance and early battery failure. In almost every case it would be a false economy not to have this facility.
