The 9V battery in my bass guitar tends to die rather often, and so I wanted to measure how much current the bass draws.
My bass is a Squier Jazz Bass with an active on-board preamp but passive pickups (there are only two wires going to the pickups, as shown below). Some basses have active pickups, and so their measurements may not be comparable to mine.
So here are my findings.
How much current does my active J-Bass preamp draw? Around 11.2mA.
Does the current remain constant over time? Once the battery is connected, the current quickly drops from 11.3–11.4mA to 11.24–11.25mA and then slowly drops further to 11.21mA over a couple of minutes. It never seems to go below 11.21mA.
Does the current increase when a string is plucked? No, it stays exactly the same regardless of whether one is playing.
Does the current depend on the potentiometer positions? No, it doesn’t depend on any of the 5 knobs: tone, volume, pickup pan (bridge vs neck mix), or bass/treble boost.
Does the current depend on the voltage supplied by the battery? Within a reasonable range, no. I tried a few batteries lying around (rechargeable and non-rechargeable) that happened to output 8.98V, 8.47V, and 8.31V, and the initial current consistently stayed at 11.24–11.25mA. When I plugged in a discharged battery that could only output 6.54V, the current jumped around 6-8mA, but the bass doesn’t sound good with such a battery.
Does it matter whether the amp is turned on? Nope, turning the amp off or even unplugging the cable from the amp does not affect the current drawn, as long as the other end of the cable stays plugged into the bass. Curiously, when I plugged in a bare TRS jack I had lying around (as opposed to TS jacks normally used in instrument cables), the current fell to 6.75mA.
Does the bass draw any current when it’s unplugged? Fortunately, it does not.
The only other discussion of the active bass current draw I could find is this TalkBass thread from 2012. Even though it’s about active pickups, the author also observes the same 11mA current and complains that it’s too high (compared to 1mA that their other bass draws).
If you do similar measurements for your own bass or just happen to know more on this subject, please let me know.