It can work. I used to do the same thing using a high current 24 Vdc power supply....but don't expect it to double the life of the battery....with luck you might get a few more weeks use out of it. Bottom line, the battery is on it's last leg.
Just so you understand what is happening....
The battery inside actually consists of a thin coated metal plate and insulator coiled up and shoved inside the batery case. What happens is that either a mechanical chemical short develops inside. What the power supply is doing is melting the short. In most cases, it won't be long before the short comes back.
The other failure is oxidation (for lack of a better word). With lack of use, a coating develops on the coiled up plate reducing the usable surface area of the battery, and thus the amp capacity. Some people refer to this as the 'memory'. Using NiCads until they are almost fully discharged and then fully recharged will help reduce the surface buildup.
A few details about NiCad batteries.
The fully charged voltage is around 1.4 Vdc vs 1.5 for an alkaline battery.
When the voltage hits about 0.9, it's fully discharged.
If the voltage is around 1.2 volts without a load....it is most likely close to discharge.
When a NiCad battery starts to get warm while it is charging (standard charger), it is most likely close to full charge. The heat is caused by the excess energy going being disapated. Un other words...you don't have to regulate the voltage (unlike a car battery)....just the current....the battery keeps soaking up the juice until the electrolyte is saturated....then it starts to get warmer.
A NiCad will self discharge about 10% per month. Hence, they are not a good choice for a backup flashlight. Leaving them on the charger all the time will also cause the oxcide to buid up thus reducing the life of the battery.
NiCads have about a 10 year life and/or 300 to 1000 cycles.
I have not worked with the Lithiums so I can't give any details on those.
