Agreed, if a battery is completely discharged, you often need to "kickstart" it with an old charger to get some charge in it....but not for long, on SLA's and particularly lithiums, as it will shorten their life even more, as they require more regulated and smoother charge than the old wet batteries would take.
Lithium batteries in particular, will be quickly ruined if you use wrong charger, and often wont recover at all if allowed to fully discharge, having said that, they are brilliant if treated correctly, giving at least 30% more current for size and incredibly light.
I reccomended the Noco charger as, although I got it for its lithium charging ability, it recovered two SLA's that an Optimate strangely wouldn't.
Graham