BU-202: New Lead Acid Systems

Most battery systems allow reasonably fast charging of one hour or so. The energy can also be withdrawn in about the same time, meaning that the charge and discharge times can be made similar. Lead acid is unique in that the battery can be discharged at a very high rate but requires more than 14 hours to fully charge. Lead acid also needs periodic equalization to de-sulfate the plates and correct other ills.

The answer to the inherent low charge acceptance relates to the formation and dissolution of lead sulfate on the negative electrode, which is pure lead. On discharge, lead sulfate adheres to the surface and dissolves again on charge. The process is sluggish and when trying to hasten the charge, excess electrons have nowhere to go; this leads to hydrogen generation and water loss. With age, the lead sulfate crystals engrain, which reduces the charge acceptance even further.

The positive electrode also contains lead sulfate, but it supports a high charge rate. It is clear that the negative electrode is the problem with lead acid batteries. New lead acid systems try to solve this problem by adding carbon to this electrode with promising results.

Advanced Lead-carbon

Scientists have known for years that sulfate accumulation prevents the classic lead acid from delivering sustained performance; partial charge and aging are the main culprits because the negative lead plate is not sufficiently scrubbed. The advanced lead-carbon (ALC) solves this by adding carbon to the negative plate (cathode). This turns the battery into a quasi-asymmetric supercapacitor to improve charge and discharge performance.

Figure 1 illustrates the classic lead acid cell with the lead negative plate being replaced with a carbon electrode to benefit from the qualities of a supercapacitor.

The ALC is being tested as a replacement for the classic starter battery in start-stop applications and in 48V micro and mild hybrid systems. Rapid charging on regenerative breaking is a decisive advantage with these batteries, a task that is difficult to achieve with regular lead acid. Although larger and heavier than Li-ion, the ALC is low-cost, operates at subfreezing temperatures and does not need active cooling — advantages Li-ion cannot claim. Unlike regular lead acid, lead carbon can operate between 30 and 70 percent state-of-charge without fear of becoming sulfated. The ALC is said to outlive the regular lead acid battery, but the negative is a rapid voltage drop on discharge, resembling that of a supercapacitor.

Firefly Energy

The composite plate material of the Firefly Energy battery is based on a lead-acid variant, and the maker claims that the battery is lighter, longer living and offers a higher active material utilization than current lead acid systems. It is also one of the few lead acid batteries that can operate for extended time in partial-states-of-charge. The battery includes carbon-foam electrodes for the negative plates, which gives it a performance that is comparable to NiMH but at lower manufacturing costs. Firefly Energy was a spin-off of Caterpillar, and in 2010 it went into bankruptcy. The company was later revived under separate ownership but folded again. Since 2014, the battery is manufactured in India under Firefly Batteries Pvt. Ltd.

Altraverda Bipolar

Similar to the Firefly Energy battery, the Altraverda battery is based on lead. It uses a proprietary titanium sub-oxide ceramic structure called Ebonex® for the grid and an AGM separator. The un-pasted plate contains Ebonex® particles in a polymer matrix that holds a thin lead alloy foil on the external surfaces. At a specific energy of 50–60Wh/kg, the battery is comparable with NiCd and is said to be well suited for high voltage applications. Based in the UK, Altraverda works with East Penn in the USA.

Axion Power

The Axion Power e3 Supercell is a hybrid battery/supercapacitor in which the positive electrode is made of standard lead dioxide and the negative electrode is activated carbon. The assembly process is similar to lead acid. The Axion Power battery offers faster recharge times and longer cycle life on repeated deep discharges than what is possible with regular lead acid systems, opening the door for the start-stop application in micro-hybrid cars. The lead-carbon combination lowers the lead content on the negative plate, which results in a weight reduction of 30 percent compared to a regular lead acid. This, however, also decreases the specific energy to 15–25Wh/kg instead of the 30–50Wh/kg with a regular lead acid. Another negative is a steep voltage decline on discharge that shares similarities with the supercapacitor.

CSIRO Ultrabattery

The Ultrabattery by Commonwealth Scientific and Industrial Research Organisation (CSIRO) of Australia combines the asymmetric ultracapacitor with the lead acid battery, sharing similarities with the advanced lead-carbon described above. The capacitor enhances the power and lifetime of the battery by acting as a buffer during charging and discharging. This is said to prolong battery lifetime by a factor of four over regular lead acid systems while boosting the power by 50 percent. The manufacturer further claims a 70 percent cost reduction over current batteries in hybrid electric vehicles. CSIRO batteries were tested in a Honda Insight HEV, and the results were said to be positive. The battery is also being tested for start-stop applications in micro-hybrid cars. Unlike other advanced lead acid, the ability to rapid-charge is a decisive advantage over the regular lead acid. Furukawa Battery in Japan licensed the technology and also makes the battery.

EEStor

This is the mystery battery/supercapacitor combination that has received much media attention. The battery is based on a modified barium titanate ceramic powder and claims a specific energy of up to 280Wh/kg, higher than lithium-ion. The company is very secretive about their invention and releases only limited information. Some of their astonishing claims include: One-tenth of the weight of a NiMH battery in a hybrid application; no deep-cycle wear-down, 3–6 minute charge time; no hazardous material; similar manufacturing costs to lead acid; and a self-discharge of only 0.02 percent per month, a fraction of that of lead acid and Li-ion. Tests conducted in 2013 did not find meaningful levels of energy because of high resistance between the layers. Research is continuing.

Enhanced Flooded Battery (EFB)

Car manufacturers are aware of the added stress when a regular starter battery is in start-stop mode. AGM (absorbent glass mat) batteries can withstand the repeat start function, but car manufacturers looking for a lower cost solution came up with the enhanced flooded battery (EFB). Tests reveal that the EFB performs better than the regular flooded version, but it is not as good as AGM. Performance appears to be directly related with battery cost.

Summary

Battery experts believe that the core limitation of the lead acid battery is the utilization of lead. Lead-based technology has significant unused performance potential. Improving the active material is said to unlock such prospect by attaining a deeper understanding and getting access the analytical tools to investigate the phenomenon. Adding carbon-based material to the negative electrode lowers sulfation, improves conductivity and increases charge acceptance.

References

[1] Courtesy of Advanced Lead-Acid Battery Consortium (ALABC)