A Motorcycle's Battery - 10 Questions Keeping Current

Once upon a time, all you had to know about batteries was that red is positive, black is negative, and the correct way to add a little distilled water when the electrolyte level got low. These days, sealed, maintenance-free batteries free us from maintaining the electrolyte level, but they pose various new questions. So until they come up with a storage cell that tells you what it needs, when and how much, here are 10 things your battery would tell you if it could:

1. What's all that stuff sloshing around inside a motorcycle battery?
If you peeled off that plastic case-which you shouldn't-you'd find a row of lead plates, alternately charged positive and negative, immersed in a solution of 35 percent sulfuric acid and 65 percent water-aka electrolyte. A permeable, non-conductive material between the plates permits ions in the electrolyte to move back and forth

2. So how do lead and acid add up to electricity?
A chemical reaction between the lead plates and the acid turns the lead into lead sulfate, leaving free electrons on the positively charged plates. When you turn on the ignition switch, electrons move from the positively charged plates to the negatively charged ones, producing electricity. During that process, sulfur in the electrolyte moves to the lead plates. That's the discharge cycle. Your alternator or battery charger essentially reverses the process.

3. What happens if my wet cell gets dry?
Failure to add water-or adding electrolyte instead of distilled water-can harm the battery, because less water in the electrolyte means more acid. Too much acid can corrode the battery's internals, leaving sediment on the bottom of the battery that will eventually short it out.

4. Why distilled water?
As the electrolyte evaporates, there's still sulfur in the battery, so you only need to add distilled water-not mineral-laden tap water-to maintain the necessary combination of sulfuric acid and water.

5. What's going on inside the new maintenance-free kind of battery?
The same basic principles apply. But unlike serviceable batteries, maintenance-free cells use an absorbent separator material between the lead plates that looks like fine black wool. It also acts like a sponge to keep electrolyte from sloshing around. This type is called an absorbed glass mat (AGM) battery. All maintenance-free motorcycle batteries are essentially AGM batteries, though they're sometimes called GRT (gas-recombinant technology), VRLA (valve-regulated lead-acid), dry-cell or gel-cell batteries.

6. Are maintenance-free batteries really maintenance-free?
There's no fluid to check or add, but they discharge like any other battery. Despite their similar design, however, not all AGM batteries are charged the same way. If you still have an old single-rate trickle charger, replace it with a modern maintenance charger such as a Deltran Battery Tender or Yuasa SmartShot

These chargers start out with a high initial charge rate that tapers off to a float charge once the cell is back to its maximum energy capacity. Think of it in terms of opening the throttle to accelerate, then backing off to cruise. If the battery's charge drops below a certain point-all batteries self-discharge to some extent, even when not hooked up to a bike-the charger senses the drop and steps up the charge rate for as long as it's needed.

7. How can a maintenance-free battery survive if I never add water?
That's because there's no electrolyte loss. Gases created during the charging cycle are trapped inside the battery. Upon reaching a certain concentration, they recombine with the water and become part of the electrolyte again. The cases of maintenance-free batteries are built to withstand a slight amount of internal pressure, and have pop-off valves in the unlikely event the pressure build-up exceeds the bursting point of the case.

8. What's a gel-cell battery?
The gel cell is also an AGM, but with silica gel added to the electrolyte, which gives it the consistency of Vaseline. Gel-cell batteries typically have a lower self-discharge rate than other AGMs, and are used in applications where there are a lot of parasitic electrical demands on the battery-such as from a clock, or a radio with station presets-when the engine isn't running. As of this writing BMW is planning to equip some of its new models with gel-cell batteries. To charge its gel-cell batteries, BMW recommends using its own special charger-built for BMW by Deltran, which has a peak voltage slightly lower than that of typical battery chargers. But the difference in peak voltage and charge timing between it and chargers such as Deltran's Battery Tender and Yuasa's SmartShot may be so slight as to have no practical effect on the battery's life. With so few gel cells currently in the field, the jury is still out. But in the meantime, if you have a BMW with a gel-cell battery, use the BMW-approved charger, if only for your warranty's sake.

9. How quickly does a battery go dead when the bike is parked?
All batteries self-discharge at a rate of about 1 percent per day, but higher ambient temperatures accelerate the process. A battery stored at 95 degrees, for example, self-discharges twice as fast as one stored at 77 degrees.

Temperatures above 130 degrees can destroy a battery outright. Lower temperatures slow the self-discharge process, but don't put your battery in the freezer quite yet. Pure water freezes at 32 degrees, but electrolyte won't freeze until about minus 75 degrees. The electrolyte in a seriously discharged battery, however, is more water than electrolyte, because the sulfur has migrated to the lead plates. The more discharged the battery, the closer to water the electrolyte becomes, and the higher the temperatures at which it freezes. The electrolyte in a deeply discharged battery can freeze at only a few degrees below that of pure water.

10. How can I help my battery live long and prosper?
Just as regular charging can increase a battery's life, proper initial charging is essential to the continued survival of a new battery. When your shop fills that brand-new battery with electrolyte, it's only about 80 percent charged. More importantly, it will never hold more than that unless you bring it to full charge with a maintenance charger before that first ride, or have the shop put it on a charger for you.

Motorcycles in storage for the winter should be hooked up to a maintenance charger until they're put back into service. Even if you ride your bike every day, your battery might still benefit from a monthly charge, especially if most of your trips are short. It's also a good idea to regularly make sure the terminals are clean and the battery bolts are snug, that there are no cracks in the casing, and the vent hose-if it has one-isn't kinked.