waaaaay back with the FR3, we quickly selected a more powerful 12 volt battery pack
that could be slipped under the backpad and simply plugged in instead of the power cable,
thus increasing reliability on gigs dramatically
today there are even better options, as some Lithium Ion packs are available with a
rated constant 12 volt (and other voltages as well) output
this is achieved by using one more cell than typical packs, then adding a voltage
regulator output circuit to drop it down to the target voltage.
Since the native internal voltage is higher than the output, you get zero fatigue
from any weaker cell and usable power typically lasts until depletion
the FR 3 and 4 shut down when a "brownout" lower voltage is reached
such packs (from Talent as one example) usually come with a simple
wall wart recharger, and are reasonably priced with MaH ratings reflected in the prices