Rechargeable batteries operate under a reasonably wide temperature range. This, however, does not automatically permit charging under these same temperature extremes. While operating batteries under hot or cold conditions cannot always be avoided, the user has some control over charging. Efforts must be made to charge the batteries at moderate temperatures.

Lead acid Battery  is reasonably forgiving on temperature extremes, as we are familiar with our car batteries. Part of this tolerance is credited to the sluggishness of the lead-acid system. Some battery brands permit freezing and low level charging; others sustain damage and deliver reduced capacity and a short service life.

To improve charge performance of lead-acid batteries at colder temperatures and avoid thermal runaway during heat spells, controlling the voltage limits, to which the battery is charged, is important. Implementing such a measure can prolong battery life by up to 15%. General guidelines suggest a compensation of approximately 3mV per cell per degree Celsius. The voltage adjustment has a negative coefficient, meaning that the voltage threshold drops as the temperature increases.

Heat kills batteries. The warmer the cells, the shorter the life is. Elevated temperatures cannot always be prevented, especially during fast charging, but efforts must be made to keep this time brief. While 45°C (113°F) is acceptable if kept short, at 50°C (122°F) and above, the battery starts to suffer. Note that the cells inside the pack are always a few degrees warmer than the temperature of the housing.

Ultra-fast chargers

Some charger manufacturers claim amazingly short charge times of 30 minutes or less. With well-balanced cells and operating at moderate room temperatures, nickel-cadmium batteries designed for fast charging can indeed be charged in a very short time. This is done by simply dumping in a high charge current during the first 70% of the charge cycle.

In the second phase of the charge cycle, the charge current must be lowered. The efficiency to absorb charge is progressively reduced as the battery moves to a higher state-of-charge. If the charge current remains too high in the later part of the charge cycle, the excess energy turns into heat and high cell pressure. Eventually, venting will occur, releasing oxygen and hydrogen. Not only do the escaping gases deplete the electrolyte, they are highly flammable! A white powdery substance accumulating at the vent area indicates previous venting.

Ultra-fast charging can only be applied to batteries that are designed for fast charging. Applying a high current charge to regular cells will cause the conductive path to heat up. The contacts on portable packs also suffer if the current handling of the spring-loaded plunger contacts is underrated. These contacts may wear out prematurely. Often, a fine and almost invisible crater appears on the tip of the contact, which causes a high resistive path or forms an isolator. The heat generated by a bad contact often melts the plastic. Higher contact tensions improve the current flow.

Aged batteries with high internal resistance and mismatched cells do not lend themselves to ultra-fast charging, even if they are designed for it. Low cell conductivity turns into heat, which further deteriorates the cells. The weak cells holding less capacity are fully charged before the others and begin to heat up rapidly. Some batteries create sufficient heat to soften and distort the plastic housing. Temperature sensing is a prerequisite with fast and ultra-fast charging.

Several manufacturers offer pulse chargers. Interspersing brief discharge pulses between each charge pulse can further enhance charging. This method promotes recombination of oxygen and hydrogen gases, resulting in reduced pressure buildup and lower cell temperature. Pulse chargers are also known to reduce crystalline formation (memory) on nickel-based batteries. Most Cadex chargers for nickel-based batteries apply this feature.

Some advanced chargers regulate the charge current according to the battery's ability to accept charge. An empty battery will initially take a very high charge current. Towards the end of a charge, the current is tapered down. Aged batteries are given their due respect and are automatically charged at rates suitable to their condition.

The charge algorithm for lead-acid batteries is similar to lithium-ion but differs from nickel-based chemistries in that voltage rather than current limiting is used. The charge time of a sealed lead-acid battery is 12-16 hours (up to 36 hours for larger capacity batteries). With higher charge currents and multi-stage charge methods, the charge time can be reduced to 10 hours or less. Lead-acid cannot be fully charged as quickly as nickel or lithium-based systems.

It takes about 5 times as long to recharge a lead-acid battery to the same level as it does to discharge. On nickel-based batteries, this ratio is 1:1, and roughly 1:2 on lithium-ion.

A multi-stage charger first applies a constant current charge, raising the cell voltage to a preset voltage (Stage 1 in Figure 1). Stage 1 takes about 5 hours and the battery is charged to 70%. During the topping charge in Stage 2 that follows, the charge current is gradually reduced as the cell is being saturated. The topping charge takes another 5 hours and is essential for the well being of the battery. If omitted, the battery would eventually lose the ability to accept a full charge. Full charge is attained after the voltage has reached the threshold and the current has dropped to 3% of the rated current or has leveled off. The final Stage 3 is the float charge, which compensates for the self-discharge.

Figure 1: Charge stages of a lead-acid battery. The battery charges at a constant current to a set voltage threshold (Stage 1). As the battery saturates, the current drops (Stage 2). The float charge compensates for the self-discharge (Stage 3).

Correct settings of the voltage limits are critical and range from 2.30V to 2.45V. Setting the voltage limit is a compromise. On one end, the battery wants to be fully charged to get maximum capacity and avoid sulfation on the negative plate. A continually over-saturated condition at the other end, however, would cause grid corrosion on the positive plate. It also promotes gassing, which results in venting and loss of electrolyte.

The voltage limit shifts with temperature. A higher temperature requires slightly lower voltages and vice versa. Chargers that are exposed to large temperature fluctuations should be equipped with temperature sensors to to adjust the charge voltage for optimum charge. Figure 2 compares the advantages and limitations of various peak voltage settings.

Figure 2: Effects of charge voltage on a sealed lead-acid battery (SLA). Cylindrical lead-acid cells have higher voltage settings but are lower for VRLA and car batteries.

The battery cannot remain at the peak voltage for too long; the maximum allowable time is 48 hours. When reaching full charge, the voltage must be lowered to maintain the battery at between 2.25 and 2.27V/cell. Manufacturers of large lead-acid batteries recommend a float charge of 2.25V at 25°C.

Car batteries and valve-regulated-lead-acid batteries (VRLA) are typically charged to between 2.26 and 2.36V/cell. At 2.37V, most lead-acid batteries start to gas, causing loss of electrolyte and possible temperature increases. The exceptions are small sealed lead acid batteries (SLA), which can be charged to 2.50V/cell without adverse side effect.

The cylindrical Cyclone by Hawker requires a very high peak voltage of 2.60V/cell. Failing to apply the recommended voltage threshold causes a gradual decrease in capacity due to sulfation. Follow manufacturer's recommended settings on these lead-acid variations.

Large VRLA batteries are often charged with a float-charge current to 2.25V/cell. A full charge may take several days. It is interesting to observe that the current in float charge mode gradually increases as the battery ages in standby mode. The reasons may be electrical cell leakages and a reduction in chemical efficiency.

Aging affects each cell differently. Since the cells are connected in series, controlling the individual cell voltages during charge is virtually impossible. Even if the correct overall voltage is applied, a weak cell will generate its own voltage level and intensify the condition further.

Much has been said about pulse charging lead-acid batteries. Some experts believe there is a benefit in reduced cell corrosion but manufacturers and service technicians are not in full agreement on the effectiveness. There are also disagreements on the 'equalizing charge'. An equalizing charge raises the battery voltage for several hours above that specified by the manufacturer. Although beneficial in reversing sulfation, the side effects are elevated temperature, gassing and loss of electrolyte if the service is not administered correctly. A periodic discharge of about 10% is said to benefit the battery but little conclusive evidence is available.

Lead-acid batteries must always be stored in a charged state. A topping charge should be applied every six months to avoid the voltage from dropping below 2.10V/cell on an SLA. Prolonged storage below the critical voltage causes sulfation, a condition that is difficult to reverse. (See also: "How to restore and prolong lead-acid batteries")


Charging lead-acid batteries with a power supply

Lead-acid batteries can be charged manually with a commercial power supply featuring voltage regulation and current limiting. Calculate the charge voltage according to the number of cells and desired voltage limit. Charging a 12-volt battery (6 cells) at a cell voltage limit of 2.40V, for example, would require a voltage setting of 14.40V.

The charge current for small lead-acid batteries should be set between 10% and 30% of the rated capacity (30% of a 2Ah battery would be 600mA). Larger batteries, such as those used in the automotive industry, are generally charged at lower current ratings. Cells constructed of a non-antimonial lead grid material allow higher charge currents but have a lower capacity. The cylindrical Cyclone is sealed and can sustain a pressure of up to 3.5 Bar (50 psi). A pressurized cell assists in the recombination of gases.

Observe the battery temperature, voltage and current during charge. Charge only at ambient temperatures and in a ventilated room. Once the battery is fully charged and the current has dropped to 3% of the rated current, the charge is completed. A good car battery will drop to about 40mA when fully charged; a bad battery may not fall below 100mA.

After full charge, remove the battery from the charger. If float charge is needed for operational readiness, lower the charge voltage to about 13.50V (2.25V/cell). Most chargers perform this function automatically. The float charge can be applied for an unlimited time.

State-of-charge reading based on terminal voltage

The state-of-charge of a lead-acid battery can, to a certain extent, be estimated by measuring the open terminal voltage. Prior to measuring, the battery must have rested for 4-8 hours after charge or discharge and resided at a steady room temperature. A cold battery would show slightly higher voltages and a hot battery would be lower. Plate additions of calcium and antimony will also vary the open terminal voltage with calcium being a little higher than antimony. Furthermore, AGM has a higher voltage plateau than the flooded lead acid and the readings on Figure 3 may not apply for AGM systems. Due to surface charge, a brief charge will raise the terminal voltage and provide inflated state-of-charge reading. For example, a 30 minute charge could wrongly indicate 100% SoC if no rest is applied.

With sufficient rest and stable temperature, voltage measurements provide an amazingly accurate SoC estimation for lead acid batteries. It is important that the battery is free of polarization. If connected in a system, such as in a car, there are steady auxiliary loads, not to mention frequent starting and driving.

Note: The BCI readings apply to flooded batteries with antimony doping. Calcium will raise the voltage by 5 - 8%. Calcium is commonly used for maintenance-free lead acid batteries.
After charge or discharge, allow the battery to rest for a minimum of eight hours before assessing the state-of-charge by measuring the terminal voltage.


Battery as a buffer

While dwelling on float-charge, an external load can be connected to a lead-acid battery. In such a case, the battery acts as a buffer. Micro-towers on cell sites work this way. During off-peak periods, the batteries get fully charged. On peak traffic times, the load exceeds the net supply provided by the rectifier (charger) and the battery supplies the extra energy. A car battery works in a similar way.

When configuring a battery as a buffer, make certain that the battery has the opportunity to fully charge between loads. The net charge must be greater than what is drawn from the battery. Some chargers switch to fast charge after a deep discharge, others simply use the float charge to recharge. Allow up to 48 hours to fully recharge on float charge. Deep discharges should be avoided if possible. Assure that the float charge voltage is set correctly.

With the efforts of going green, there is renewed interest in finding new battery systems for wheeled mobility and automotive applications to lower our dependency of hydrocarbon. This page summarizes the work done with lead-based chemistries.

When compiling data of new battery system, the battery inventors are leaning towards publishing the positive attributes and the negatives are kept under wraps. This is why much hyped systems that show great potential on paper do not always make it to commercial applications but quietly die.

Firefly Energy

The composite plate material of the Firefly Energy battery is based on a lead acid system that is lighter, longer living and has higher active material utilization than current lead acid systems. The first battery reaching the market will include foam electrodes for the negative plates. The performance is comparable to NiMH with possible lower manufacturing costs. A design concern is microtubule blockage caused by PbSO4 crystal growth that will be dominant during low discharge conditions. In addition, crystal expansion could cause surface area reduction. Firefly Energy is a spin-off of Caterpillar.

Axion Power

The Axion Power e3 Supercell is a hybrid battery/ultracapacitor in which the positive electrode is standard lead dioxide and the negative electrode is activated carbon. The assembly process is similar to lead acid. The Axion Power battery is aimed at the hybrid car market and the company claims faster recharge times and longer cycle life on deep discharge than with customary lead acid systems. Specifications show a low power density of 12Wh/kg.

Altraverda

Like Firefly Energy, the Altraverda battery is based on lead acid, using a proprietary titanium sub-oxide ceramic structure [Ebonex] for the grid, and AGM separators. The un-pasted plate contains Ebonex particles in a polymer matrix with a thin lead alloy foil on the external surfaces. The energy density is 50-60Wh/kg. Based in the UK, Altraverda is working with East Penn in the USA, Exide in India and Vladar Enterprise in the Ukraine.

CSIRO Ultrabattery

The CSIRO Ultrabattery combines an asymmetric ultracapacitor and a lead acid battery in each cell. The capacitor enhances the power and lifetime of the battery by acting as a buffer during charging and discharging. The technology has been licensed to Furukawa Battery. The lifetime is 4 times longer than customary lead acid systems and produces 50% more power. It is also 70% cheaper to make than current hybrid electric vehicle (HEV) batteries. CSIRO batteries are undergoing road trials in a Honda Insight HEV and show good results.

EEStor

This is the mystery battery/ultracapacitor that receives much media attention. The battery is based on modified barium titanate ceramic powder. EEStor claims that the battery has a specific energy of up to 280Wh/kg. The company is very secretive about their invention and only limited information is available. Financial Post, 26 June 2008 compares EEStor with specifications of NiMH and customary lead acid systems.

Source: ESTOR

The answer is YES. Lead-acid is the oldest rechargeable battery in existence. Invented by the French physician Gaston Planté in 1859, lead-acid was the first rechargeable battery for commercial use. 150 years later, we still have no cost-effective alternatives for cars, wheelchairs, scooters, golf carts and UPS systems. The lead-acid battery has retained a market share in applications where newer battery chemistries would either be too expensive.
Lead-acid does not lend itself to fast charging. Typical charge time is 8 to 16 hours. A periodic fully saturated charge is essential to prevent sulfation and the battery must always be stored in a charged state. Leaving the battery in a discharged condition causes sulfation and a recharge may not be possible.

Finding the ideal charge voltage limit is critical. A high voltage (above 2.40V/cell) produces good battery performance but shortens the service life due to grid corrosion on the positive plate. A low voltage limit is subject to sulfation on the negative plate. Leaving the battery on float charge for a prolonged time does not cause damage.

Lead-acid does not like deep cycling. A full discharge causes extra strain and each cycle robs the battery of some service life. This wear-down characteristic also applies to other battery chemistries in varying degrees. To prevent the battery from being stressed through repetitive deep discharge, a larger battery is recommended. Lead-acid is inexpensive but the operational costs can be higher than a nickel-based system if repetitive full cycles are required.

Depending on the depth of discharge and operating temperature, the sealed lead-acid provides 200 to 300 discharge/charge cycles. The primary reason for its relatively short cycle life is grid corrosion of the positive electrode, depletion of the active material and expansion of the positive plates. These changes are most prevalent at higher operating temperatures. Cycling does not prevent or reverse the trend.

The lead-acid battery has one of the lowest energy densities, making it unsuitable for portable devices. In addition, the performance at low temperatures is marginal. The self-discharge is about 40% per year, one of the best on rechargeable batteries. In comparison, nickel-cadmium self-discharges this amount in three months. The high lead content makes the lead-acid environmentally unfriendly.

Plate thickness

The service life of a lead-acid battery can, in part, be measured by the thickness of the positive plates. The thicker the plates, the longer the life will be. During charging and discharging, the lead on the plates gets gradually eaten away and the sediment falls to the bottom. The weight of a battery is a good indication of the lead content and the life expectancy.

The plates of automotive starter batteries are about 0.040" (1mm) thick, while the typical golf cart battery will have plates that are between 0.07-0.11" (1.8- 2.8mm) thick. Forklift batteries may have plates that exceed 0.250" (6mm). Most industrial flooded deep-cycle batteries use lead-antimony plates. This improves the plate life but increases gassing and water loss.

Sealed lead-acid

During the mid 1970s, researchers developed a maintenance-free lead-acid battery that can operate in any position. The liquid electrolyte is gelled into moistened separators and the enclosure is sealed. Safety valves allow venting during charge, discharge and atmospheric pressure changes.

Driven by different market needs, two lead-acid systems emerged: The small sealed lead-acid (SLA), also known under the brand name of Gelcell, and the larger Valve-regulated-lead-acid (VRLA). Both batteries are similar. Engineers may argue that the word 'sealed lead-acid' is a misnomer because no rechargeable battery can be totally sealed.

Unlike the flooded lead-acid battery, both SLA and VRLA are designed with a low over-voltage potential to prohibit the battery from reaching its gas-generating potential during charge because excess charging would cause gassing and water depletion. Consequently, these batteries can never be charged to their full potential. To reduce dry-out, sealed lead-acid batteries use lead-calcium instead of the lead-antimony. The optimum operating temperature for the lead-acid battery is 25*C (77*F). Elevated temperature reduces longevity. As a guideline, every 8?C (15*F) rise in temperature will cut the battery life in half. A VRLA, which would last for 10 years at 25*C (77*F), will only be good for 5 years if operated at 33*C (95*F). Theoretically the same battery would last a little more than one year at a desert temperature of 42*C (107*F).

Figure 1: Sealed lead-acid battery

The sealed lead-acid battery is rated at a 5-hour (0.2) and 20-hour (0.05C) discharge. Longer discharge times produce higher capacity readings because of lower losses. The lead-acid performs well on high load currents.

Absorbed Glass Mat Batteries (AGM)

The AGM is a newer type sealed lead-acid that uses absorbed glass mats between the plates. It is sealed, maintenance-free and the plates are rigidly mounted to withstand extensive shock and vibration. Nearly all AGM batteries are recombinant, meaning they can recombine 99% of the oxygen and hydrogen. There is almost no water is loss.

The charging voltages are the same as for other lead-acid batteries. Even under severe overcharge conditions, hydrogen emission is below the 4% specified for aircraft and enclosed spaces. The low self-discharge of 1-3% per month allows long storage before recharging. The AGM costs twice that of the flooded version of the same capacity. Because of durability, German high performance cars use AGM batteries in favor of the flooded type.

Advantages

• Inexpensive and simple to manufacture.
• Mature, reliable and well-understood technology - when used correctly, lead-acid is durable and provides dependable service.
• The self-discharge is among the lowest of rechargeable battery systems.
• Capable of high discharge rates.

Limitations

• Low energy density - poor weight-to-energy ratio limits use to stationary and wheeled applications.
• Cannot be stored in a discharged condition - the cell voltage should never drop below 2.10V.
• Allows only a limited number of full discharge cycles - well suited for standby applications that require only occasional deep discharges.
• lead content and electrolyte make the battery environmentally unfriendly.
• Transportation restrictions on flooded lead acid - there are environmental concerns regarding spillage.
• Thermal runaway can occur if improperly charged.