The most commonly used battery chemistries are lithium ion (Li-ion), Nickel-Cadmium (NiCad), and Nickel Metal Hydride (Ni-MH). All of the above have their own particular merits and flaws, some of which we will discuss below. Alti-tech has opted to use Li-ion batteries in all our products, believing it to be the best technology available for our applications.
One must consider three major factors when selecting a battery chemistry for an application: physical size, runtime (how long a charge will last), and longevity (how many charge/discharge cycles the battery will provide). In portable power applications, lithium-ion (Li-ion) is the most desirable due to its compact size and light weight - or in other words, its high energy density. Energy density is defined as the amount of energy stored in a given system or region of space per unit volume. Not only does Li-ion enjoy great run times, but the life of the battery often reaches over 800 cycles, which typically translates to a lifetime of about 2 years.
Compared to Ni-MH and NiCad, Li-ion batteries have lower self-discharge rates, meaning that the cells can be stored with a full charge and then used immediately after being taken out of storage without needing to be recharged. An added bonus is that unlike Ni-MH and NiCad batteries, Li-ion batteries have no liquid electrolyte and therefore cannot leak. Li-ion batteries have much lower toxicity and small quantities can be disposed of in the regular trash, unlike Ni-MH and NiCad which need special handling for disposal.
Both Ni-MH and NiCad suffer from a phenomenon known as the "memory effect"; a build-up of crystalline metal deposits on the electrodes. This reduces the active surface area of the electrodes and can result in self-discharge and electrical short in some cases. Regular maintenance is required to keep these battery chemistries at high capacities. Maintenance consists of completely discharging the battery and then recharging in order to restore the electrodes. Li-ion cells do not need to be maintained in this manner and can also be charged at a partially discharged state - "topped off", if you will - without suffering any ill effects.
Li-ion cells can be configured in both serial and parallel when making a battery pack. If you want to make a 10Ah battery, you need only continue adding cells in parallel until you achieve the target capacity. Other chemistries require complete re-tooling to produce larger capacity battery packs. This simple fact has allowed Li-ion manufacturers to perfect the design of a more versatile (almost modular) single-size cell while Ni-MH and NiCad manufacturers must design separate cells for higher capacities.
A Li-ion cell has an average voltage of 3.7V, while Ni-MH and NiCad are about the same at around 1.2V. Simply put, this means that a 1000mAh Li-ion cell holds approximately three times the energy of a 1000mAh Ni-MH or NiCad cell. Take into account the lighter weight and the higher energy density and this ratio increases to 5 or 6 times the energy per cell. A higher voltage per cell means fewer cells are required to provide an equal amount of energy. This is the reason that almost every small electronic device now uses Li-ion cells.
Li-ion is not without its drawbacks, however. Unlike Ni-MH or NiCad, every Li-ion battery pack needs "pack protection" which is a dedicated circuit board that provides over-charge, over-discharge, and short circuit protection. The cells must also be manufactured in very low humidity environments, and often cells have to have matching (or very close) impedances in order for them to be put into parallel. The manufacturing cost of Li-ion cells is therefore much higher than that of Ni-MH or NiCad cells. The shipment and transportation of Li-ion batteries and cells is also regulated by the UN and US Department of Transport, both of which require special handling procedures for large Li-ion packs. You can see our documents about shipping Li-ion here.
Alti-tech feels that of the three chemistries, Li-ion is the best overall. We use pre-approved (CE, UL, FCC) battery packs from E-One Moli Energy and so avoid most of the shipping regulations and the complications of designing pack protection. Battery charging is of course something that needs to be carefully regulated with Li-ion batteries, but Alti-tech's experience and charger technology takes care of that for you!