Open-circuit voltages (OCV) of various type cells:

Transcription

1 Open-circuit voltages (OCV) of various type cells: Re-Chargeable cells: Lead Acid: 2.10V/cell to 1.95 NiMH and NiCd: 1.20 V/cell Li Ion: 3.60 V/cell Non-re-chargeable (primary) cells: Alkaline: 1.50 V/cell Silver-oxide: 1.60 V/cell Lead Acid Batteries: These are general voltage ranges per cell (for 6 cells) Open-circuit (quiescent) at full charge: 2.10 V (12.6V) (12 hr. after charging) Open-circuit at full discharge: 1.95 V (11.7V) Loaded at full discharge: 1.75 V (10.5V) Continuous-preservation (float) charging: 2.23 V (13.38V) for gelled electrolyte; 2.25 V (13.5V) for AGM (absorbed glass mat) and 2.32 V (13.92) for flooded cells 1. All voltages are at 20 C (68 F), and must be adjusted for temperature changes. The opencircuit voltage cannot be adjusted with a simple temperature coefficient because it is non-linear (coefficient varies with temperature). 2. Float voltage recommendations vary among manufacturers. 3. Precise float voltage (±0.05 V) is critical to longevity; insufficient voltage (causes sulfation) is almost as detrimental as excessive voltage (causing corrosion and electrolyte loss) Equalization charging (for flooded lead acids): 2.5 V (15V) for no more than 2 hours. Battery temperature must be absolutely monitored. Gassing threshold: 2.4 V (14.4V). References: A fully charged lead acid battery will be at 12.6v at 70ᵒ F, this is the open circuit voltage (OCV), measured after charging and after sitting unused for about 12 hours. If the battery is not fully charged, then the voltage measured after resting 12 hours will be less than the OCV. If the battery is being charged, the voltage will float higher than the OCV. If there is a load on the battery the voltage will go below OCV. The OCV is an important measure of the health and state of the battery. To get an accurate reading of the battery, it must be disconnected and left to sit for about a day. This also applies if you are using a hydrometer to measure the specific gravity. If you need to get a quick reading while charging, you can disconnect the battery and wait a few seconds before attaching the volt meter. If the battery is above 13.2V then you can assume it is near a full charge.

2 Charging A Lead Acid Battery A lead acid battery should be charged in three stages: Stage 1 constant-current charge, Stage 2 topping charge and Stage 3 float charge. Stage 1 applies the bulk of the charge and takes up roughly half of the required charge time. Stage 2 continues at a lower charge current and provides saturation. Stage 3 (float charge) compensates for the loss caused by self-discharge. The figure shown below illustrates these three stages. During Stage 1, the charger is set to the stage 2 voltage, but the charger is also set to a constant current of 1.0 amps. This means the voltage cannot go over the set stage 2 voltage, but it drops below that limit to maintain the current at a constant 1.0 amps. The voltage should slowly increase during stage 1 until it reaches the set voltage limit. At this point the battery is about 70% charged. This takes 5-8 hours. The switch to stage 2 happens automatically simply by the voltage reaching the voltage limit. The remaining 30 percent charging happens during the slower stage 2 charging. It takes another 7 10 hours. During this time the current decreases as the battery charges. This topping charge is essential for the well-being of the battery and can be compared to a little rest after a good meal. If deprived, the battery will eventually lose the ability to accept a full charge and the performance will decrease due to sulfation (vide infra). The correct setting of the voltage limit (stage 2 charge voltage) is critical and ranges from 2.30 to 2.45V per cell (13.8 to 14.7 V). Setting the voltage limit is a compromise, and battery experts refer to this as dancing on the head of a needle. On one hand, the battery wants to be fully charged to get maximum capacity and avoid sulfation. On the other hand, an oversaturated condition causes grid corrosion on the positive plate and induces gassing (vide infra). To make dancing on the head of a needle more difficult, the battery voltage shifts with temperature. Warmer surroundings require slightly

3 lower voltage thresholds and a cold ambient prefers a higher level. Chargers exposed to temperature fluctuations should include temperature sensors to adjust the charge voltage for optimum charge efficiency. If this is not possible, it is better to choose a lower voltage for safety reasons. The following Table compares the advantages and limitations of various set Stage 2 voltage settings. Stage 2 Voltages Whereas the voltage settings in the Stage 2 Table above apply to most typical lead acid batteries, some types of batteries require higher voltage settings. The Stage 2 voltage limit should be set according to the manufacturer s specifications. Formation of gas bubbles in a flooded lead acid indicates that the battery is reaching full stateof-charge (hydrogen on negative plate and oxygen on positive plate). Once fully charged through saturation, the battery should not dwell at the Stage 2 topping voltage for more than 48 hours and must be reduced to stage 3 (the float voltage level). This is especially critical for sealed systems because these systems are less able to tolerate overcharge than the flooded type. Charging beyond what the battery can take turns the redundant energy into heat, and the battery begins to gas. The recommended float voltage of most low-pressure lead acid batteries is 2.25 to 2.27V/cell (13.5 to 13.62V). Manufacturers recommend lowering the float charge at ambient temperatures above 29 C (85 F). Not all chargers feature float charge. If your charger stays on stage 2 topping charge and does not drop below 2.30V/cell (13.8V), remove the charge after 48 hours of charge to avoid overcharging, corrosion, and gassing. Long Term Storage: Lead acid batteries must always be stored in a charged state. A stage 2 topping charge should be applied every six months to prevent the voltage from dropping below 2.10V/cell (12.6V). With AGM, these requirements can be somewhat relaxed. Measuring the State of Charge and the Charge Capacity State of Charge: This is measured as a percentage of the maximum open circuit voltage OCT (rest) a battery is able to maintain. Measuring the OCV while in storage is the easiest method of measuring the state of charge (also called the charge level) of a battery. A voltage of 2.10V/cell (12.6V) at room temperature reveals a charge of at least 90 percent. Such a battery is

4 in good condition and needs only a brief topping charge prior to use. If the voltage drops below 2.10V/cell (12.6V), the battery must be charged to prevent sulfation. The table on the right shows the approximate state of charge for at-rest voltages measured for a battery in storage. Observe the storage temperature when measuring the open circuit voltage. A cool battery lowers the voltage slightly and a warm one increases it. Using OCV to estimate state-of-charge works best when the battery has rested for a few hours, because a charge or discharge agitates the battery and distorts the voltage. Specific Gravity: An alternative Measure of the State-of-Charge: A hydrometer can be used to test the specific gravity of each cell as a measure of its state of charge. Specific gravity is used as an indicator of the state of charge of a cell or battery. However, specific gravity measurements cannot determine a battery's charge capacity. During discharge, the specific gravity decreases linearly with the ampere-hours discharged as indicated in the illustration below. Approximate state-of-charge 100% 75% 50% 25% 0% Average specific gravity A "Fully Charged Battery": If a battery shows an OCV (rest) less than 12.6 V after an additional toping charge, or long term float charging, then the battery is said to be at a100% state-of-charge, but it simply won't hold as much charge as a healthy battery. The state-of-charge (measured in %) is NOT the same as the amount of charge (Ah) in a battery. A "fully charged battery" is a battery that won't take any more charge. The OCV of a fully charged, healthy battery will be 12.6 V, but less than 12.6V for a damaged battery because the damaged battery will hold a lower amount of charge. Capacity (Amp Hours) Open circuit voltage 2V 6V 8V 12V The amount of charge in a battery is the number of electrons the battery is holding ready to do work for you. It is generally measured in terms of coulombs or AmpHours (Ah). (3600 coulombs = 1 Ah, and 1 coulomb is 6.24 x electrons). The amount of charge in fully charged battery (100% charge) is called the Charge Capacity of that battery, and is typically shown as AmpHours (Ah) on the side of the battery. Although

5 measuring the state-of-charge (%) is easy to do with a volt meter (vidi supra), the Charge Capacity (Ah) requires a special measuring device called a "charge counter". The charge counted is used to discharge the battery, and it has a circuit that counts the electric charge that flows out of the battery. This is not current (amps) nor is it voltage. It is charge, measured as coulombs, AmpHours, or number of electrons. If you measure the AmpHours of a fully charged battery, you should see the same Ah value that is printed on the battery. A decrease in Ah of a fully charged battery is the most sensitive way to detect the decline of a battery's health. (I) Sulfation: Battery Problems This is the most common battery problem. Any time a battery is used, the charge level drops and sulfur/lead/oxides (sulfates) are formed. This is a natural part of the electrochemistry of the process. The sulfurfates stick to the cell plate and, in theory, is returned to sulfuric acid in the electrolyte solution during re-charging. However, over time the sulfates harden (crystalize) and stick very hard to the cell plates. This reduces the charge capacity of the cell. Preventing Sulfation: The best way to prevent sulfation is to keep a lead-acid battery fully charged because the sulfate does not form. This can be accomplished three ways. The best solution is to use a charger that is capable of delivering a continuous "float" charge at the battery manufacturer's recommended float or maintenance voltage for a fully charged battery. A second and less desirable method is to periodically recharge the battery when the State-of- Charge drops to 80% or below. Maintaining a high State-of-Charge tends to prevent irreversible sulfation. Temperature also matters! Lower temperatures slow down electro chemical reactions and higher temperatures speed them up. A battery stored at 95 F (35 C) will self-discharge twice as fast than one stored at 75 F (23.9 C). Recovering a Sulfated Battery: Here are three methods to try to recover a badly sulfated battery: 1. Light Sulfation: Check the electrolyte levels and apply a constant current at 2% of the battery's RC or 1% of the AH capacity rating for 48 to 120 hours at 14.4 VDC or more, depending on the electrolyte temperature and capacity of the battery. Cycle (discharge to 50% and recharge) the battery a couple of times and test its capacity. You might have to increase the voltage in order to break down the hard lead sulfate crystals. If the battery gets above 125 F (51.7 C) then stop charging and allow the battery to cool down before continuing. 2 Heavy Sulfation: Replace the old electrolyte with distilled, deionized or demineralized water, let stand for one hour, apply a constant current at four amps at 13.8 VDC until there is no additional rise in specific gravity, remove the electrolyte, wash the sediment out, replace with fresh electrolyte (battery acid), and recharge. If the specific gravity exceeds 1.300, then remove the new electrolyte, wash the sediment out, and start over from the beginning with distilled

6 water. You might have to increase the voltage in order to break down the hard lead sulfate crystals. If the battery gets above 125 F (51.7 C) then stop charging and allow the battery to cool down before continuing. Cycle (discharge to 50% and recharge) the battery a couple of times and test capacity. The sulfate crystals are more soluble in water than in electrolyte. As these crystals are dissolved, the sulfate is converted back into sulfuric acid and the specific gravity rises. This procedure will only work with some batteries. 3. Desulfators: Use a desulfator also known as a pulse charger. A list of some of the desulfator or pulse charger manufacturers is available at Much has been said about pulse charging of lead acid batteries. There are apparent advantages in reducing sulfation. However, manufacturers and service technicians are divided on the benefits, and the results are inconclusive. (II) Over Charged Cells Over charging occurs when a battery, or a cell within a battery, sees a voltage over the "gassing voltage" 2.4 V/cell (14.4V). When this happens, chemical reactions occur that were never meant to occur in a lead acid system. Hydrogen and oxygen gas bubbles are formed (an extremely explosive mixture). In addition, the electrode plates undergo corrosion. The resulting battery will not accept a complete charge. The charge capacity declines and the OCV (rest) may be less than 12.6V. The damage that occurs is not recoverable. (III) Unbalanced Cells Aging batteries pose a challenge when setting the optimal float charge voltage. In a healthy 12V battery, there are 6 cells. The float voltage, V f, is equally distributed between each cell (V f /6 per cell). However, a battery with unbalanced cells will not distribute the float voltage equally, and some cells will be in an over-charging situation while others will be in an undercharging situation. As a result, some will be gassing and some will be sulfating. A fully charged, unbalanced 12V battery will show an open circuit voltage (at rest) less than the 12.6 V of a healthy battery. Equalizing the Charge in the Cells of a Battery: An equalization charge can bring all cells back to a similar level. This is done by increasing the charging voltage to 2.50V/cell (15V) for 2 to 16 hours. For flooded lead acid batteries, equalization should be done if the specific gravity differences among the cells is greater than The specific gravity should be measured periodically until the difference at or below For Closed lead acid cells, equalization is more difficult to monitor. Some recommendations suggest doing an equalization every six months, or after 20 cycles. Others suggest trying equalization when the OCV (rest) is significantly less than 12.6V. Then, equalization is continued until OVC (rest) returns to 12.6V or no longer shows an improvement. The battery temperature should be monitored during equalization, and the process should be stopped if the battery becomes to hot.