The rechargeable lithium is the most talked-about battery chemistry in research labs today and there are good reasons why; the energy density is the highest among commercial batteries and the self-discharge is very low. There are several types of lithium batteries of interest; the most promising chemistries are the Lithium-Ion (Li-Ion) and the Lithium Polymer (Li-Polymer).

The Lithium-Ion provides an energy density that is twice that of the NiCd at comparable load-currents. In addition, the Li-Ion deep-discharges well. In fact the Li-Ion behaves similarly to the NiCd as far as charge and discharge characteristics are concerned.

Unlike other rechargeable lithium batteries, the Li-Ion is stable and safe because no metallic lithium is used. However, certain precautions must still be used. For reasons of safety and longevity, each cell is equipped with a control circuit to limit the voltage peaks during charge and to prevent the voltage from dropping too low on discharge. In addition, the control circuit limits the maximum charge and discharge current. With these precautions in place, the danger of metallic lithium formation is eliminated.

The Li-Ion remains among the most expensive commercial batteries for the time being. Once lower priced, this chemistry may become the preferred battery for applications that are presently reserved for the NiCd.

The Lithium-Polymer provides more than three times the energy density of the NiCd and features a very low self-discharge. As far as charge and discharge characteristics are concerned, the Lithium Polymer behaves more like the SLA battery. Charge time is 8 to 16 hours. Shallow rather than deep discharge is preferred. When commercially available, the Lithium Polymer may remain fairly expensive, but the manufacturing costs will be lower than that of the Li-Ion.