An advantage of Lead-Acid, NiCd, Lithium Ion and some NiMH cell types is that they have quite low internal resistance compared Alkaline cells: Even an aging lead acid battery that is near the end of its useful life may seem to be “OK” based on a load test as its internal resistance can remain comparatively low even though it may have lost most of its storage capacity!
One could ask, then, why now simply parallel Alkaline cells, with their ready availability, long life and high storage capacity with one of these other cell types and get the best of both worlds? In theory you could – if you had some sort of charge control circuitry that was capable of efficiently meting out the energy from the alkaline pack and using it to supplement the “other” storage medium (e.g. lead-acid, lithium-ion, etc.) but you cannot simply connect the two types in parallel and expect to efficiently utilize the available power capacity of both types of storage cell – this, due to the wildly different voltage and charge requirements.
Even if you do use a fairly small-ish (e.g. 7-10 amp-hour) lead-acid or lithium-ion battery pack, even though its internal resistance may be low compared to that of alkaline packs, it likely cannot source the 15-20 amp current peaks of, say, a 100 watt SSB transceiver without excess voltage drop, particularly if it isn’t brand new.
This is where the use of “Ultracapacitors” come in.
Details at KA7OEI’s blog.