Even with low power techniques, our portable devices are power hungry machines. Batteries are available in a wide variety of form factors and chemistries that provide a lot of flexibility in product design. Selecting the right battery for an application will define many of the capabilities of the device and dictate how recharging is done, if at all.
Rechargeable Battery Options
While rechargeable alkalines are low cost, other rechargeable chemistries dominate the market. Rechargeable alkaline batteries have about 2/3 the power storage capability of non-rechargeable alkalines and a great better self-discharge rate than Ni-cd or NiMH batteries. However, rechargeable alkalines can only be recharged about 10 times before they cannot be recharged again which significantly reduces their usefulness as a rechargeable battery.
NiMH - Nickel-Metal Hydride
One of the most common rechargeable battery chemistries is Nickel-Metal Hydride or NiMH. NiMh batteries are similar to NiCd chemistry batteries, with a hydrogen absorbing alloy replacing the cadmium present in NiCds, giving it up to three times the power capacity of an equivalent sized NiCd battery. This has allowed NiMH batteries to replace NiCd batteries in many applications. NiMH batteries provide an output voltage of 1.2 volts, slightly lower than alkaline batteries but they can still be used as replacements for alkaline batteries since many circuits can operate at the slightly reduced voltage. NiMH batteries do have significant self-discharge rates of 30% per month for regular NiMH and 2-3% per month for Low Self-Discharge NiMH chemistries. This significantly limits the uses of NiMH batteries for low power, intermittent use applications like remotes.
NiCd - Nickel Cadmium
Nickel Cadmium (NiCd or NiCad) batteries are one of the most robust rechargeable battery types. They are capable of delivering large currents without damage or loss of capability due to their low internal resistance. The high current capability of NiCd batteries makes them very useful in high current applications such as remote controlled aircraft, boats and cars, although lithium ion and lithium polymer batteries are becoming the dominate choice due to their higher power capacity. NiCd batteries have a self-discharge rate of 15-20% per month, making them a poor choice for lower power, long life applications. NiCd batteries do contain cadmium, a heavy metal, which is toxic and an environmental hazard.
Li-Ion - Lithium Ion
Lithium-Ion (Li-Ion) chemistry batteries are a common battery chemistry in portable consumer electronics since their high energy density provide portable devices with a long battery life. They also have a low self-discharge rate of 2-8% per month and no memory effect which make them great options for products that will be recharged on a regular basis. Lithium Ion batteries typically provide 3.7 volts and with cylindrical form factor. Lithium Ion batteries use a wide range of materials that allow a range of voltages, capacities, life, and safety options.
Li-Pol - Lithium Polymer
An evolution of the lithium ion battery resulted in the lithium polymer battery by replacing the organic solvent in lithium-ion batteries with a solid polymer composite. Lithium polymer batteries are generally constructed of small pouch-like flat cells which can be flexible and molded in to a variety of shapes. This allows the lithium polymer battery to adapt to the design of a product, rather than a product being forced to use only a few standard battery form factors. The nominal voltage of lithium polymer cells is 3.7 volts and the have a self-discharge rate of 5% per month. This makes them great options for consumer electronics which are power hungry, recharged frequently, and where design is driven by style and requires batteries to work with the design.
Each battery chemistry needs to be charged in a different way, with some chemistries being very flexible and others, especially lithium chemistries, have very specific and tight charging tolerances. There are multiple ways to charge a battery including constant voltage, constant current, tapered current, pulsed charging, and trickle charging. Different charging strategies, slow and fast charging, deep charging, or recovering a battery, use a different method or combination of charging modes which also varies based on the chemistry of the battery. Two of the most important factors for charging are the input voltage and current requirements and the temperature of the battery. Lithium chemistry batteries are of the most intolerant battery chemistries which have a charging voltage tolerance of 1% to prevent damage to the battery.
Manufacturers typically supply instructions for safe handling, use, and disposal. These warn against physical damage, short-circuiting when fully charged, and overcharging.
- Alkaline - Leaking potassium hydroxide, a caustic agent that causes skin, eye, and respiratory irritation.
- NiCd - Overcharging can produce hydrogen or oxygen, both highly flamable gasses. The heavy metal cadmium is present in the battery and represents an environmental disposal and safety issue when handling ruptured or leaking cells.
- NiHM - Overcharging can cause a buildup of hydrogen which can cause the cell to rupture, burn or explode, especially if the battery vents fail.
- Lithium-Ion - Overheating can cause rupture or explosion
- Lithium Polymer - Overcharging or shorting can cause explosion or fire