Using Multiple Batteries with EMG Active Electronics
The majority of bassists run their active electronics with a single 9-volt battery but there may be additional options available. Most EMG active electronics are capable of handling voltages ranging from 9 to 27 volts. Adding additional batteries will give differing results depending upon how they are wired.
Here is how the standard 9-volt active circuitry is wired. The black wire from the battery is connected to the ring of the stereo output jack. When a cord is plugged into the jack the circuit is closed and the electronics get turned on. Removing the cord turns off the electronics, thus preserving battery life. So while 12's like the Hamer Chaparral come standard with a stereo jack, the output is mono.
Quote from EMG Website: "If you've got room for multiple batteries in your guitar, you can use two batteries wired in series to power your onboard circuitry at 18 volts. The output level will not appreciably increase, but you'll have increased headroom and crisper transients. This is especially useful for percussive / slap bass styles where you can generate enormous instantaneous power levels across the entire frequency spectrum."
Wiring batteries in series means to solder the positive lead of one battery to the negative lead of the other. Wiring them in series increases the amount of voltage while keeping the current capacity the same. Some players claim to have attained significantly better sound quality and increased headroom after switching to 18 volts, others attest to only moderate improvements in both areas.
From EMG Website: "Although most of our products are rated for 27 volts, we recommend a maximum of 18 volts. The additional benefits of 27 volts versus 18 volts are negligible."
Another factor to consider is the amount of space that is available in the body cavity for additional batteries. Many basses will barely have enough room to accommodate the second battery, three batteries may simply not fit. Extra batteries also increase the weight of the bass.
From EMG Website: "You can also wire two batteries in parallel to provide a regular 9-volt supply but with much longer lifespan between battery changes."
When wiring batteries in parallel the positive leads are soldered to other positives, the negatives with the other negatives. Wiring in parallel increases the current capacity while keeping the voltage the same.
The modifications shown can be easily done with a soldering iron and some electrical tape. They are also easily reversible. Always use fresh batteries to get the best results.