Lithium Ion rechargeables seem to have recently seen a 50% or so price drop.
A 4-cell, 14.8V, 8AH with charger and mounting plate can be had for $169.99
(USD). These are basically long endurance laptop batteries.

This is still too much for me given that a SLA with the same capacity is
around $25 but the downward price trend is hopeful. Someone contemplating a
transponder, electric T&B and other juice hungry gadgets in a $100,000+
glider might see Li-Ion as a possibility now.

Beyond price, my concern is voltage. 4-cell Li-Ion packs produce 14.8V and
a detail in the spec sheet admits that a new, fully charged pack might
produce 16.2V into a high resistance load. My avionics manuals say 16V max.
Is that 0.2V overvoltage likely to be a problem?

Bill Daniels