I'm not sure that is a solution at all:
The really awkward issue is charge management - it's quite different for lithium-ion compared to NiCd chemistry. The Lithium-ion charging curve (current and voltage management) is complex and needs an IC to control it, and I think access to individual cells, to charge sequentially (one-at-a-time)*. There's also a temperature sensing function too. You can use a sharp temperature rise as a proxy for reaching full charge with NiCd's, but Lithium ion cells get hot during normal charging, so that doesn't work.
And NiCd cells don't usually explode if wrongly charged - Lithium Ion cells can do so.
E.
*You can charge NiCds as batteries, although that will knacker them fast if they're not well matched cells. Cheap chargers do this.
The voltages are almost irrelevant in comparison to the charging problem.