Motorcycle Battery Unwrapped | Starts & Sparks

The Oft-Cussed Battery, Unwrapped

Think the engine is the heart of your motorcycle? Guess again. A better candidate is the battery. Why? Because it gets your engineand everything elsestarted. Without it, youre not going anywhere.

The inner workings of a motorcycle battery arent complexlead plates, alternately charged positive and negative, immersed in electrolyte, a solution of sulfuric acid and water. The plates are separated by a non-conductive permeable material that lets ions in the electrolyte move freely between the plates. A reaction between the plates and the electrolyte produces electricity. Turn on the ignition, hit the starter button, and electrical power flows out of the battery to the starter motor in whats called the discharge cycle.

As soon as the engine starts, the alternator begins replacing the charge the battery just gave up in what’s called the charge cycle. Hydrogen and oxygen gas are produced as a by-product of the charge cycle. Since these two gases are the components of water, the concentration of acid in the electrolyte goes up. If it gets too high, the electrolyte can damage the lead plates and the battery case. That’s why it’s necessary to check the electrolyte level in wet-cell batteries, and add water when it falls below a preset level.

Wet-cell batteries are usually identified by their translucent cases and screw-on caps. You dont see them much in modern motorcycles any more, because around 1985 battery technology made a leap that relegated wet-cells primarily to vintage bikes. This was the AGM (absorbed glass mat) battery, which is the standard motorcycleand pretty much every other kind of vehiclebattery design today. AGM batteries work the same way as wet-cells, except between the lead plates are mats, like cotton gauze, that hold the electrolyte like sponges. The mats keep the electrolyte from sloshing around inside the battery and act as shock absorbers to protect the lead plates (and plate-to-plate connectors) from breakage due to vibration and impact.

The most significant aspect of the AGM design is the sealed case. You not only dont need to add water to the electrolyte, you cant. The hydrogen and oxygen given off during the charge cycle collect inside the case and, when they reach a certain concentration, recombine into water and drop back into the electrolyte. Because the interior of the battery is no longer open to outside air, theres less corrosion, too. The case itself, though sealed, has a pop-off valve in the unlikely event internal pressure gets too high.

Some AGM batteries come from the factory filled with electrolyte and charged, while others come with the electrolyte separate for the dealer to put in when the battery is sold. The first way is easier for everyone involved at the point of sale, but not necessarily better in the long run. Batteries self-discharge over time, and the longer a battery sits on the shelf, the lower its charge will be when you buy it. If it stays too low for too long, lead sulfate crystals form on the lead plates, a process called sulfation. At this point even a conventional recharge might not bring the battery back to health. The charge will come up initially, but after a couple of days it will drop back down to where it was. The better bet is to have the dealer add the electrolyte and put the battery on a charger for a while before you take it home so you start out with a fresh, full charge.

To keep that charge up, put your battery on a maintenance charger whenever youre not going to ride for a week or so, especially if your bike has electrical accessories that draw current even when the ignition isnt on. Clocks and radio presets use power all the time, and can eventually draw your battery down enough so that the next time you hit the starter button, all youll hear is a click.

Quick Facts

Lithium-ion batteries are making headlines for their light weight and smaller size, but before you ditch your AGM for a Li-ion make sure your bike is a suitable candidate. Li-ion batteries dont supply strong current in the cold, and can be permanently damaged if the state of charge drops too low. Putting a Li-ion battery on a bike that draws current with the key offdue to an alarm, stereo, or other accessoryis not a good idea.

Super-compact Lithium-ion (Li-ion) batteries can save a lot of weight. This one, by MOTY, weighs just 1.52 pounds. They’re not the best option for all bikes, though.
Super-compact Lithium-ion (Li-ion) batteries can save a lot of weight. This one, by MOTY, weighs just 1.52 pounds. They’re not the best option for all bikes, though.