Although most of the market’s development focus is on advanced battery technologies such as NiMH or Lithium-ion, some companies are still working to improve on the venerable lead-acid platform. Accordingly, there are hybrid and electric vehicle manufacturers who are opting to use or at least to test some of these lead-acid variants in their vehicle applications.
Two of the more recent examples are e-max’s use of a lead-acid battery with a low-sodium silicate electrolyte (“silicon power battery”) in its electric scooter, and the recently announced testing of Power Technology’s advanced glass matt lead-acid battery prototype by an unnamed “major hybrid automobile manufacturer.”
Invented in 1859 by French physicist Gaston Planté, lead-acid batteries are more than 145 years old. The lead-acid battery has evolved significantly into different types over the years, although the basic electrochemistry of them all is the same.
In a lead-acid battery, the negative electrode is lead (Pb), the positive electrode is lead dioxide (PbO2) and the electrolyte is a dilute solution of sulfuric acid (H2SO4).
As the battery discharges, the active materials in the electrodes (lead dioxide in the positive electrode and lead in the negative electrode) react with sulfuric acid in the electrolyte to form lead sulfate and water. On recharge, the lead sulfate on both electrodes converts back to lead dioxide (positive) and sponge lead (negative), and the sulfate ions (SO42- ) are driven back into the electrolyte solution to form sulfuric acid.
Lead-acid batteries have one of the worst energy-to-weight ratios (lead is heavy), although the energy-to-volume ratio is good. They are inexpensive and can supply the high surge currents needed in starter motors—one of their most pervasive applications.
There are three basic types of lead-acid batteries on the market today: flooded (or wet), gel cell and absorbed glass mat (AGM).
Flooded (or wet) lead-acid batteries have their electrodes immersed in liquid electrolyte. Gases created during charging are vented to the atmosphere and distilled water must be added occasionally to bring the electrolyte back to its required level. The most familiar example of this type is the conventional 12V automobile battery.
Gel cell batteries use fumed silica in the electrolyte, causing it to harden into a gel. On subsequent charges some water is lost, drying the gel until a network of cracks and fissures develops between the positive and negative electrodes, decreasing performance.
Absorbed glass mat (AGM) batteries immobilize the electrolyte with a highly porous and absorbent microfiber glass mat.
The “silicon power battery” used in the e-Max electric scooter is a variant of the lead-acid gel cell. Developed and manufactured by Guangdong Jiangmen Yuyang Special Batteries in China, the silcon power battery uses a low-sodium silicate compound and a new method for forming the gel that, according to the company, decreases hazardous emissions during manufacturing, reduces cracking in the battery gel under operation, reduces the self-discharge rate and increases specific energy and cycle life.
E-max had been using Nickel Zinc batteries in other scooter models.
|Battery type||Energy density|
|Cycle life||Charge time|
|“Silicon Power Battery”||45-52||>500||2–3||85||0.36–0.42|
The advanced glass mat battery developed by Power Technology that is undergoing testing by the unnamed hybrid manufacturer replaces the lead plates or grids of conventional lead-acid batteries with lead-tin alloys deposited on lightweight, open pore substrates such as carbon or aluminum to enhance the cyclability of the resulting high-surface area electrode for use as an anode and/or cathode in lead-acid batteries.
Power Technology claims that the resulting battery provides four times greater surface area, is 30–50% smaller and lighter, and is 60 to 68% efficient than conventional lead-acid batteries.
Other companies such as Firefly Energy, a spin-off from Caterpillar, are also looking at the application of a lightweight, porous composite to replace the lead plates in conventional lead-acid batteries.
Firefly claims that its Advanced Battery technology can eventually remove approximately 70% percent of the lead from a traditional lead-acid battery design but still execute lead-acid chemistry for energy storage.
If that proves out, Firefly anticipates that it can deliver a jump in specific power, energy and cycle life approaching, but at about one-fifth the cost, of the present state of advanced material batteries such as NiMH and Lithium-ion.
(A hat-tip to Aaron Harvey!)