But we’re not going to leave you without an answer. First, see if your bike has an accessory circuit. Many do, usually fuse-protected at 5, 10, or 15 amps. Almost certainly, you can add that much and still be under the total charging system’s limit. Second, you can run a simple test. Connect a digital voltmeter to the battery terminals. Start the bike and rev the engine to an rpm equivalent to a slow highway speed. Begin turning on the accessories. The battery voltage should remain steady until the charging-system capacity is reached, and then it will decline. You’re looking for no less than 13.8 volts across the battery because this is what it takes to charge it. If at a cruise rpm you still have 13.8 volts (or close to that), you’re good. Now let the engine idle. Watch the voltage. You might have to turn a few of those accessories off to keep the voltage where you want it. Remember that under this load the battery is charging on the highway but discharging (if the voltage is at or below 12.6 volts) at idle.