I had a problem with a replacement battery that lasted only 6 months. It was a Yuasa brand, I thought it might be the alternator but I tested the system and it was providing plenty of juice. But the battery should not be going bad that quickly.
To test the system here is what you want to do, get a multimeter and then do these two tests.
1. First, see if there is a draw on the battery when the bike is off, remove the negative terminal and put one lead on the cable and one on the negative battery post. I think you want the multimeter on the ohms setting. Should be be less than 3 mA(milliamps)
2. If there is no leakage then put the negative battery cable back on and then start the bike. Make sure the high beam is on as well. Rev the bike to 5000 RPM and hold it there, now measure the voltage across the two battery terminals. Should be between 13.4V and 15.1V.
Here are the numbers I got at varying rpms.
1000 - 12.5
2000 - 14.2
3000 - 14.3
4000 - 14.2
5000 - 14.2
If those measure up then it is a bad battery, if these two tests fail you have a problem in the alternator or the regulator.
Hope this helps, its not a bad idea to get a battery tender especially for use in the winter when you cant ride as often. Get a battery tender that has quick release connections, should come with two ends, one with clips and one with circle terminal connectors. Hook the circle terminator connectors to the bolts on the battery and run the other end up into the underseat storage and secure it with a zip tie. This way you can quickly hook and unhook the battery tender without having to remove the front seat.
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