A modern motorcycle battery is a marvel of compact packaging

April 24 [Wed], 2013, 11:31
You can make a simple wet-cell storage battery with only two lead plates. Submerge them in an electrolyte solution (64% water and 36% sulfuric acid by weight is standard), apply direct current and watch as the positive lead plate develops a brown coating of lead peroxide and the negative plate becomes sponge lead. Remove the voltage source and put a voltmeter across the plates and you'll find U160 laptop battery approximately 2.1 volts, regardless of the size. The larger the plates, the longer the battery can supply this voltage. Combining three such cells in series can create a 6-volt battery, and six cells can make a 12-volt battery (actually 6.3 or 12.6 volts, respectively).

When we ask the battery to produce current flow by putting a load across its terminals, the plates and acid solution undergo another chemical transformation, causing both lead plates to change into lead sulfate, consuming the acid and producing water as a by-product.

Gradually the electrolyte becomes increasingly watery and the plates more sulfated until the battery either dies or we reverse the process by returning current to the battery to restore it. The basic chemistry hasn't changed for a hundred years.

A modern motorcycle battery is a marvel of compact packaging. Since even a smooth-running motorcycle subjects the innards of a batter to much greater vibration than a car, the motorcycle battery's case will be a tighter fit to prevent the lead plates from rattling to pieces. But it's still generally true that a motorycle that vibrates a lot will have a shorter battery life, because the plates themselves are more fragile than you might expect.

They are constructed of highly active but very soft lead pastes applied to waffled supporting grids, which provide greater surface area for the chemical reaction than simple flat plates. The paste is porous to allow full penetration of the electrolyte, and the pastes on both the negative and positive plates begin as the same substance; a mixture of lead oxide, dilute sulfuric acid, water and special binders like plastic fiber that produce a material about the consistency of firm mud. Since the negative plates tend to contract in service, special expanders are added to their mix so they can't shrink to become inpenetrable and chemically inactive. Just like our simple cell, the plates are then submerged in electrolyte and given a electrical charge that forms them into positive and negative.

To make the battery more rugged, the plate grids must be made of a harder lead alloy, usually lead-antimony. Anywhere from .5%12% antimony is typical, with higher antimony mixtures making for tougher plates but shortening the sitting life of the charged battery. The big drawback to lead-antimony is that gradual corrosion of the positive grid releases the antimony, which may then form tiny hairlike bridges between the plates. These bridges are actually short circuits that gradually increase the current necessary to recharge the battery, causing increased water loss. Therefore, an older battery needs its water level checked more frequently, and a new battery that needs constant filling may have a voltage regulator problem that is overcharging the battery.

Sealed, maintenance-free batteries use calcium instead of antimony to strengthen the plate grids, since calcium does not produce internal shorts. By giving the battery box a slightly larger volume to hold an extra reserve of electrolyte, incorporating sulfation-retardants and gas-recombinant technology (GRT: a special glass mat surrounding the plates which helps the hydrogen and oxygen recombine rather than escape from the battery as a gas), the manufacturer can eliminate the filler caps. However, all these batteries are not truly sealed, but may incorporate almost invisible safety vents around the perimeter of their tops.

As advertised, the maintenance-free battery does have a natural resistance to overcharging and water loss. But unfortunately, they are also not as resilient when deeply discharged (considered to be the loss of 80% capacity) as are their lead-antimony brothers, and may be killed by such things as leaving an U330 laptop batteryelectrical load on for a long time (when a 12 volt battery might drop to just 2 volts), while a lead-antimony battery might still be saved. Some of the new high-tech "smart" battery chargers (about $60) claim to be able to restore even a deeply-discharged maintenance-free battery by increasing the initial charging voltage to as high as 20 volts to overcome the internal reisistance.

To increase current capacity, modern batteries have thin multiple plates which are connected in parallel within each cell, forming a kind of lead sandwich with a negative plate at each end and alternating positive plates within, all insulated from one another by separators. The separators are micro-porous, meaning charged ions can pass through readily, but they prevent physical contact that would constitute a short circuit, and also attempt to prevent the formation of the tiny alloy short circuits.
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