The best way to get the longest battery life is to ride your bike often or, at the very least, keep the battery on a maintenance charge. We recommend an automatic charger of two amps or less; large automotive units can do serious harm to the smaller motorcycle batteries. Regular charging serves two purposes: A fully charged battery won't sulfate as quickly as a depleted one, and a charged battery will not freeze. Simply running your bike at idle won't do the job. Most charging systems put out too little current at idle to properly recharge the battery. Go ride.
So how do you know when your battery is ready for the recycling yard? (Aside from getting nothing more than a "click-click" from the starter button.) Flooded-cell batteries can be tested with a hydrometer. This instrument measures the specific gravity of the electrolyte and is an indicator of battery health. AGM types are tested with a voltmeter in conjunction with a load tester. Simply measuring the unloaded voltage won't tell you anything useful. A battery may show 13 volts and still not be able to turn the starter-that's called a surface charge.
Sometimes it's hard to know if the problem is in the battery or the bike's charging system. There's an easy way to check. With the engine running, a voltmeter placed across the battery terminals should show between 13 and 14 volts. If the bike's charging system is weak, you might not see these figures until the engine is revved up. Take into account any extra electrical loads such as brighter headlights or aftermarket accessories. Finally, for long-term storage, consider disconnecting or removing the battery altogether. Keep-alive circuits on fuel-injected bikes and clocks may only take a few milliamps, but over a long winter, that's enough to drain your battery flat.-John Driscoll
Seven Ways To Better Battery Life1. Ride often. As with the rest of your bike, frequent use is good for the battery. Although lead-acid batteries used in motorcycles don't have problems with "memory" as do the fussy Ni-cads in your computer or cell phone, they do benefit from being kept at full charge. Riding frequently keeps the battery charged and helps prevent plate sulfation. Incidentally, lead-acid batteries self-discharge at a rate of 1/100-volt per day.
2. Keep it clean. A battery covered in muck with corroded terminals won't live as long as one that's squeaky clean. Apply a thin layer of lithium grease to the terminals. If you spill acid, neutralize it with a solution of baking soda and water (one pound of baking soda per gallon of water).
3. Always charge a new battery. Even though a new battery will have a partial charge, if you just throw it in and use it, it'll never be capable of the full rated output. Charge it fully before using.
4. Buy the right battery. You might get by with a smaller (physically, or with a lower amp/hour rating) battery for a while, but the demands of your bike's starter and electrical system will trash it in short order. Motorcycle makers specify batteries just big enough to do the job.
5. Keep an eye on the electrolyte level for flooded-cell batteries. If the battery goes dry, it'll sulfate or "moss up" the plates quickly; once you've damaged the plates, even replenishing the electrolyte and recharging the battery won't bring it back.
6. Be sure your charging system is up to snuff. Your bike's service manual will have specifics on how much voltage you should see at the battery terminals at a certain rpm. If the charging system isn't doing the deed, start troubleshooting right away before you're faced with throwing money at replacement batteries.
7. Put the battery on a low-current maintenance charger during the off season or if you expect it'll be more than a couple of weeks between rides.
Thanks to Lewis Lakey of Roy's Repair in Minneapolis, MN, for the technical advice. Used prices are from the AMA Official Motorcycle Value Guide, and reflect full retail value for original or reconditioned, well-maintained motorcycles.