the main factor might be the current capacity of those batteries - I recall something like 5 amps may be required? check Hans clear instructions on voltage and current.
note the amplifier is a huge factor if you only have 13.8 volts available just now - 3 watts to 25 watts is a much huger jump in dB than 25 to 50 watts. the former is around 2 s-units, the later is about half an s-unit, assuming 6 dB per s-unit.
my amplifier is nearing completion .... I plan to initially use it at the lower voltage, until I am confident all is well with it. and that may be enough of a boost.
73 curt wb8yyy