The battery keeps going flat

On recent big BMW boxers (R 1200/1250/1300 GS and RT), the combination of the original battery + starter can show a weakness when hot: the high compression ratio makes hot-engine cranking laborious (a struggling starter, a 'clunk' then slow rotation), particularly if the battery is a few years old or no longer quite at its best. It isn't necessarily a dead battery: terminals, engine earth and state of charge should be checked, and a good-capacity AGM battery plus a healthy starter cure the issue. If the battery goes flat as well, also look for a current drain or a charging fault.

This is the most common cause of a battery that won't hold a charge. A motorcycle battery lasts on average 3 to 4 years under normal use, sometimes less if it's suffered repeated deep discharges, winters without maintenance, or heat. Over time, the internal plates sulfate and lose capacity: the battery charges but drains quickly, no longer delivers enough current for the starter, and eventually can't make it through a single night. On classic serviceable lead-acid batteries, a low electrolyte level speeds up the end; maintenance-free batteries (AGM, gel) and newer lithium batteries have their own characteristics. Signs: starting that gets harder and harder after a few days of sitting, needing to recharge often, resting voltage that stays low even after a full charge (below 12.4V), or that collapses below 9.5V during starting (a dead cell). Diagnosis is basic: measure resting voltage with a multimeter (12.6-12.8V expected when full), ideally backed up with a charge/discharge test at a shop or with a battery tester. Replacing it with a new battery of the correct spec (capacity, dimensions, type, a recognized brand like Yuasa, Varta, Bosch) is a basic job. Remember to charge a new battery before first use if needed.

The charging system recharges the battery while you ride: an alternator (a fixed stator and a magnet-carrying rotor driven by the engine) produces current, and a regulator/rectifier converts it to DC and limits the voltage so it doesn't overcharge the battery. If this system fails, the battery no longer gets recharged while riding and gradually drains, even if it's new and in good condition: you end up stranded after a few dozen kilometers, with all accessories on. Conversely, a regulator that no longer limits voltage (overcharging, above roughly 15V) destroys the battery through excess. The usual culprits: a failed regulator/rectifier (the most common failure, and it runs quite hot), a shorted or open stator, or burnt/corroded connectors in the charging circuit (a common and inexpensive cause). The key diagnostic step uses a multimeter: at the battery terminals, with the engine running and revved up, voltage should climb above resting voltage, typically to around 13.5 to 14.8V (check your manual). If it doesn't climb, the charging system is at fault. This is an intermediate-level job for the multimeter diagnosis; replacing the stator can be more involved (opening the alternator cover). Strong clue: a battery that drains even while riding points directly to the charging system.

A current leak is something that stays powered with the ignition off and drains the battery while parked, especially over several days. A motorcycle has a very small normal resting draw (ECU memory, clock, factory alarm: often just a few milliamps), but beyond that, the battery drains abnormally fast. Typical causes: a poorly wired accessory left permanently connected (USB charger, GPS, heated grips, electronic alarm), a power-hungry or faulty alarm, a relay that sticks closed, or damaged wiring shorting to ground. The telltale sign: the battery drains mainly while parked (overnight, after a few days without riding), while it charges normally while riding — which sets it apart from a charging fault. Diagnosis is intermediate: measure the leakage current with a multimeter in ammeter mode, in series on the negative terminal, ignition off, then pull the fuses and unplug accessories one by one until the current drops, which points to the faulty circuit. Caution: never start the engine or switch on a heavy load with the ammeter in series (you'd blow its fuse). Once you've found the culprit, rewire the accessory properly, replace the sticking component, or repair the wiring.

This is the simplest, most common, and cheapest cause, and should be ruled out first. The battery terminals and their connectors need to make a perfect electrical contact, because the starter draws very high current (roughly 100 to 200 A for the split second it engages) and the charging current also passes through these same contacts. If a terminal is loose, or covered with a film of oxidation or a whitish sulfate buildup, contact resistance appears: it causes a voltage drop that keeps the starter from spinning properly and disrupts recharging while riding, giving the impression of a battery that's draining. Terminals corrode from humidity, time, or slight electrolyte leaks. Signs: temperamental, intermittent starting, sometimes nothing at all until you wiggle a wire and it starts again, greenish or white deposits on the terminals. Diagnosis and the fix are basic and nearly free: disconnect (negative terminal first), clean the terminals and connectors down to bare metal, tighten firmly, and protect with dielectric grease. It's so simple and quick that you should always start here before suspecting the battery, the alternator, or a current leak.