My 2012 Kawasaki Ninja 250 has a weird problem: as soon as it gets cold, the battery drains in two weeks even if I'm not riding. Do you think it's the battery dying or maybe the alternator not charging properly? How can I check that without dropping 200 bucks at the mechanic right off the bat?
Don't just replace the battery straight away without checking the alternator first, or you'll just be throwing money down the drain 😅 The easiest way is to test it with a multimeter: you measure the battery voltage when it's at rest (should be around 12V), then measure it again with the engine running (should go up to at least 13-14V). If it doesn't go up, your alternator isn't charging properly anymore, especially with the cold making batteries sluggish. A battery that drains in two weeks over winter is usually a combo of both: the battery's getting old and loses performance in the cold, and at the same time the alternator isn't putting out enough juice, so it never fully recharges between trips.
Cold reduces a motorcycle battery's capacity by 30-40%, so it's normal for it to be weaker in winter. But if it's completely dead after two weeks without riding, there's a current leak somewhere - either the battery itself is shot, or something's drawing power even when the engine is off (alarm, GPS, stuck relay, etc).
Before you test the alternator, first check if there's a parasitic drain. Disconnect the negative cable from the battery and put your multimeter on amps between the cable and the terminal. If there's current flowing (more than a few milliamps), you've got a leak. It could be something silly like a light staying on or faulty electronics. That's the thing a lot of people forget to check before blaming the alternator.
If there's no parasitic drain, then you can test the alternator: engine running, measure the voltage at the battery terminals with your multimeter. It should read 13-14V. If it's lower (like 12V or less), the alternator isn't charging enough. You can also do a simple test: if the battery lasts several weeks normally but only dies in winter, it's clearly a weak battery plus cold problem, not the alternator. Buy a new battery for 50-60 bucks and test it - it's cheaper than an alternator anyway.
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