My C310K has a 28 volt system, and uses two Gill G-25 batteries in
series. Seems to be less expensive to use a pair of commonly available
12 volt instead of a single 24 volt battery. I replaced them in 2002,
the same time I replaced the very old voltage regulators/load balancer
with newer solid state regulators. I also had both generators
overhauled at Aerotech of Louisville and replaced the drive belts. I
went back through the logbooks and found that batteries had lasted an
average of 6 years (almost like clockwork), and my batteries at that
time were 6 years old.

Brian
N3827X