not really, there is a discharge curve on both li-ion, ni-mh, and lithium batteries.
some are flatter than others, but NO battery maintains constant voltage, it just doesnt work like that
I had a discussion with the Lectrosonic engineers about this issue. They claim the issue is all rechargeables don't present accurate enough readings, so what many do is look at the voltages. The problem (and I can attest to this with the Deva too!) is the voltage will read 14v for quite some time, then drop into the 13v range for a very short time, followed quickly by 12, 11, and then 10v (the voltage cutoff on the Deva). Lectro's recommendation is to use a time-based system, so you know exactly how long a certain battery last in the unit. The only problem I see with this, is as the batteries charge and discharge, they tend not to last quite as long, so you could end up with a dead unit sooner than you thought.
While I'm no expert in this area, my findings seem to mirror what Lectro's engineers told me would happen.
Wayne