Do a hydrometer test and see if your cells are all the way up. My guess is they are not. My memory isn't as sharp as it once was. Something says a fully charged cell is around 2.2 volts. So, you have a total of 8 cells x 2.2 v = 19.6 volts.
FYI, my 18 v dewalt batteries measure around 20 volts. Wonder if we could modify an 18 v cordless charger to trickle charge your 2 8 volt batteries?
I found an old 18 v ni-cd trickle charger. I rigged it up tonight to charge my dead 18v Li-ion. Unlike my dewalt charger, this charger doesn't shut off, it's mechanical, no brain box. It's charging the ion battery at .25 amps and a tick over 20 volts. Can't see how this will hurt an already dead battery.