Advancements in Lead Acid

Invented by the French physician Gaston Planté in 1859, lead acid was the first rechargeable battery for commercial use. Early models were flooded, and during the mid-1970s the sealed or maintenance-free versions emerged in which the liquid electrolyte is transformed into moistened separators and the assembly is placed in a sealed enclosure. Safety valves control the venting of the gases generated during charge and discharge. The sealed enclosure simplifies transportation and allows installing the battery sideways without spillage.

There are two basic types of lead acids: the starter battery and the deep-cycle battery. The starter battery is designed to crank the vehicle engine, and the deep-cycle battery is made to provide continuous power in golf cars, wheelchairs, forklifts and standby systems. From the outside, these batteries look alike but there are fundamental differences in design. While the starter battery is made for a high peak power of several hundred amperes and cannot be deep cycled, the deep-cycle battery has moderate power output but permits cycling. Let us now look at the mechanical differences between these two battery systems.

Starter batteries have a very low internal resistance and this is achieved by adding extra plates for maximum surface area (Figure 1). The plates are thin and the lead is applied in a sponge-like form that has the appearance of fine foam. This extends the surface area on the plates to achieve low resistance and maximum power density (current handling). Plate thickness isless important because the discharge is short and the battery recharges while driving;the emphasis is on power rather than capacity.

Starter batteries are marked with CCA (cold cranking amps), which represents the amount of current a battery can deliver at cold temperature. To test CCA under SAE J537, the battery is cooled to –18°C (0°F) and discharged for 30 seconds at the rated CCA. A battery marked 600 CCA would be discharged at 600A for 30 seconds at –18°C (0°F). During discharge, the voltage cannot drop below 7.2 volts. If it does, the battery fails and the CCA test must be repeated to find the true CCA value. [see BU-902a: How to Measure CCA (Cold Cranking Amp)] and [see BU-904: How to Measure Capacity] Most starter batteries are also marked with RC (reserve capacity) or Ah (ampere hours).

Deep-cycle lead acid batteries are built for maximum capacity and high cycle count, and this is achieved by making the lead plates thick (Figure 2). Although the battery is designed for cycling, full discharges still induce stress. It is advised to keep the cycles moderate by preventing a full discharge and allowing the battery to charge more often. Deep-cycle batteries are marked in Ah, indicating the current the battery can deliver over time. At 1C, for example, a 60Ah battery should deliver 60A for one hour. [see BU-402: What is the C-rate?]

A starter battery cannot be swapped with a deep-cycle battery and vice versa. While a senior may be tempted to install a low-cost starter battery instead of the more expensive deep-cycle in his wheelchair, the starter battery won’t last because the thin sponge-like plates would quickly dissolve with repeated deep cycling. Trucks, buses, public safety and military vehicles use a combination of starter/deep-cycle battery, but these are big and heavy. As a simple guideline: the heavier the battery, the more lead it contains, and the longer it will last. Table 3 compares the typical life of starter and deep-cycle batteries when deep-cycled.

Ever since Cadillac introduced the electric starter motor in 1912, lead acid remains the natural battery choice for engine cranking. Lead is toxic and environmentalists are trying to find an alternative. Europe succeeded in keeping nickel-cadmium batteries out of consumer products, and authorities want to do the same with the starter battery. The alternative is lithium-ion, but the cold-start performance is not as good as lead acid and the price is too high.

Start-stop in micro-hybrid cars stresses a regular lead acid battery and shortens the life. To get a long lifespan on continuous start/stop and allow fast charging to utilize the energy from regenerative braking, battery manufacturers are refining existing technologies and experimenting with new systems. Below is a list of existing and new lead acid batteries.

Absorbent Glass Mat (AGM)

AGM is an improved lead acid battery with higher performance than the regular flooded type. Instead of submerging the plates into liquid electrolyte, the electrolyte is absorbed in a mat of fine glass fibers. This makes the battery spill-proof, allowing shipment without hazardous material restrictions. The plates can be made flat like the standard flooded lead acid and placed in a rectangular case, or wound into a conventional cylindrical cell.

AGM has very low internal resistance, is capable of delivering high currents and offers long service even if occasionally deep-cycled. AGM has a lower weight and provides better electrical reliability than the flooded lead acid type. It also stands up well to high and low temperatures and has a low self-discharge. Other advantages over regular lead acid are a better specific power rating (high load current) and faster charge times (up to five times faster). The negatives are slightly lower specific energy (capacity) and higher manufacturing costs.

AGM batteries are commonly built to size and are found in high-end vehicles to run power-hungry accessories such as heated seats, steering wheels, mirrors and windshield wipers. Starter batteries also power navigation systems, traction and stability control, as well as premium stereos. NASCAR and other auto racing leagues choose AGM products because they are vibration resistant. Start-stop batteries are almost exclusively AGM because the classic flooded type is not robust enough; repeated micro cycling would induce capacity fade. [see BU-806a: How Heat and Loading affect Battery Life]

AGM is the preferred battery for upscale motorcycles. It reduces acid spilling in an accident, lowers weight for the same performance and can be installed odd angles. Because of good performance at cold temperatures, AGM batteries are also used for marine, motor home and robotic applications.

As with all gelled and sealed units, AGM batteries are sensitive to overcharging. These batteries can be charged to 2.40V/cell (and higher) without problem; however, the float charge should be reduced to between 2.25 and 2.30V/cell (summer temperatures may require lower voltages). Automotive charging systems for flooded lead acid often have a fixed float voltage setting of 14.40V (2.40V/cell), and a direct replacement with a sealed unit could spell trouble by exposing the battery to undue overcharge on a long drive. [see BU-403: Charging Lead Acid]

AGM and other gelled electrolyte batteries do not like heat and should be installed away from the engine compartment. Manufacturers recommend halting charge if the battery core reaches 49°C (120°F). While regular lead acid batteries need a topping charge every six months to prevent the buildup of sulfation, AGM batteries are less prone and can sit in storage for longer before a charge becomes necessary.

The following are important lead acid systems in limited use or under field test.

Axion Power

The Axion Power e3 Supercell is a hybrid battery/ultracapacitor in which the positive electrode consists of standard lead dioxide and the negative electrode is activated carbon. The assembly process is similar to lead acid. The battery offers faster recharge times and longer cycle life on repeated deep discharges than with regular lead acid systems, and this opens the door for the start-stop application. The lead-carbon combination of the Axion Power battery lowers the lead content on the negative plate, which results in a 30 percent weight reduction compared to a regular lead acid. This also lowers the specific energy to 15–25Wh/kg instead of 30–50Wh/kg of a regular lead acid battery.

Altraverda Bipolar

The Altraverda battery is based on lead and 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. With 50–60Wh/kg, the specific energy is about one-third larger than regular lead acid and is comparable with NiCd. Based in the UK, Altraverda works with East Penn in the USA, and the battery is well suited for higher voltage applications.

Firefly Energy

The composite plate material of the Firefly Energy battery is based on a lead-acid variant that is lighter, longer living, and has 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 went into bankruptcy. The company was revived under separate ownership. Today, Firefly International Energy manufactures the Oasis line of batteries in limited quantities in the US.

CSIRO Ultrabattery

The CSIRO Ultrabattery combines an asymmetric ultracapacitor and a lead acid battery in each cell. The capacitor enhances the specific power by acting as a buffer during charge and discharge. This produces 50 percent more power than a regular lead acid. Furthermore, the ultracapacitor/lead acid combination is said to prolong the battery service life by a factor of four. The manufacturer also claims that the battery is 70 percent cheaper to produce than current hybrid electric vehicle (HEV) batteries. CSIRO batteries are able to take fast charge to capture the energy from regenerative breaking. The battery is being tested for start-stop applications and is undergoing road trials in a Honda Insight HEV with good results. Furukawa Battery licensed the technology.

EEStor

EEStor is also based on the battery/ultracapacitor combination but goes further by using a modified barium titanate ceramic powder. The battery claims to have a specific energy of up to 280Wh/kg, higher than lithium-ion. The company only releases limited information and their claims are: One-tenth of the weight of NiMH in a hybrid application, no deep-cycle wear-down, 2–6 minute charge time, no hazardous material, similar manufacturing costs to lead acid, and a self-discharge that is only 0.02 percent per month, a fraction of lead acid and Li-ion. Some claims may be simply too good to be true and the real test will be everyday field use.

Summary

Batteries seem to advance slowly and this is especially apparent when comparing batteries to the rapid developments of other technologies. This is not idleness on part of the research engineers but overcoming insurmountable technical hurdles to meet the requirements of long life, high specific energy (capacity), safe operation, minimal maintenance and low price. In addition, the battery must work at hot and cold temperatures, deliver high power on demand, charge quickly and be environmentally friendly. No battery meets all criteria and manufacturers optimize the characteristics to meet user demands. As long as the battery is based on the electrochemical process, limitations will remain.