li-ion substitutes

i have just brought a whites 6000di series 2.to my horror it takes 9v and 12v battery packs .so being a tight wad i,m already trying to think of a way to save money .Has anyone successsfuly tried li-ion batteries in battery hungry old machines .i have hundreds of good laptop cells and a quality li-ion charger.would 7.4v and 11.1v do the trick? or could i add a extra cell and some gadget to lower voltage to required figure.?thanks in advance for anyone who responds to this difficult question.

It certainly sounds promising, fully-charged you're expecting 4.2 Volts per cell, so 8.4V and 12.6V for 2 & 3 cell packs. You could probably rob the protection circuitry from a laptop 3-cell battery, but you may want to buy them, I've seen them on eBay.