Servo - September 2007

• Nickel-metal hydride (NiMH) rechargeable batteries not only offer better performance than NiCds, they aren’t considered quite as poisonous to the environment as NiCd cells. Today, they are the premier choice in rechargeable batteries. They are a bit more expensive than most other battery types; prices have come down recently. As with rechargeable alkalines, NiMH cells require a recharger made for them. As luck would have it, most of the latest rechargers will work with rechargeable alkalines, NiCds, and NiMHs, so a single charger may be all you need if you want to experiment with different battery types. It is important that you do not use a standard NiCd recharger with NiMH, or vice versa.

column, we’ll lump Li-ion and polymer Li-ion cells together, but just know there are differences between the two, and if plan on using either one, that you use the absolute correct recharger or recharging circuit, and that you observe all operating specifications.

Common Battery Cell Sizes

• Lithium-ion (Li-ion) cells are frequently used in the rechargeable battery packs for laptop computers and high-end camcorders, and other electronic devices. These batteries are expensive and require specialized rechargers — in fact, if you don’t use the right recharger, you could end up so severely overheating the battery that it violently explodes, so be careful! On the plus side, Li-ion cells are surprisingly lightweight for the high level of current output they provide.

• Sealed lead-acid (SLA) batteries are much like the battery in your car, except that the electrolyte is in gel form, rather than a sloshy liquid of water and acid. SLAs are “sealed” to prevent most leaks, but in reality, the battery contains pores to allow oxygen into the cells. SLA batteries are rechargeable using simple circuits, and are a good choice for very high current demands.

Not long ago, all household electronics used the same small assortment of battery cells. These came in standard sizes, most commonly AA, C, D, and rectangular nine-volt (in the old days, we called these transistor batteries, because they were commonly used in small pocket transistor radios).

Table 1 shows the most standard cell sizes, from N to D. Of these, only N, AAA, AA, C, D, and nine-volt are commonly available at the local retail store. You can locate the other fractional sizes online at various battery specialty outlets. They are typically used to build up battery packs of different sizes and shapes of NiCd or NiMH batteries.

Note that while the dimensions of common battery sizes are largely standardized, the weight can vary, depending on the construction and contents of the battery. The weight provided in the table is representative only for a cross-section of battery chemistry types.

Cell voltage is especially critical when used to create a battery pack. Depending on the battery technology, the combined output of a standard four-cell AA holder may produce 4. 8, 6, or 14. 8 volts. Obviously, this can have a noticeable effect on anything you connect to the batteries. With higher voltages, many kinds of motors run faster, but also hotter. An electronic circuit may burn out if it gets too much voltage, or may not operate if the voltage is too low.

Forgetting about Li-ion cells for the moment, the most common issue with battery cell voltage is using NiCd or NiMH rechargeables in a circuit that was meant for alkaline batteries. Rather than the usual six volts obtained when using alkaline or other non-rechargeable cells, the circuit will get only 4. 8 volts when using rechargeable batteries. In some instances, the 1.2 volt decrease is not critical. However, in other circuits — especially those incorporating a five-volt linear regulator — the 4. 8 volts is not sufficient to power the device. Either the electronics fail to operate completely or work only sporadically.

The decrease in voltage also affects motors. The higher voltage means more current to the motor, so the motor exhibits additional torque. For non-stepping motors — such as standard permanent magnet motors used in gear motors and R/C servos — the higher voltage makes the

And Now a Word About Battery Voltage

• Polymer batteries are among the latest in rechargeable technology. They are used for medium- to high-current electronics applications like cellular phones. These batteries use lithium as a component, but they are not quite the same as the Lithium and Li-ion cells mentioned above. Polymer batteries can be manufactured with thicknesses as small as 1 mm wafers. In this

You already know that different kinds of batteries produce different voltages. Even more, different battery technologies produce different voltages for any given standard size. For example, an AA alkaline cell produces a nominal (normal, average) 1.5 volts; for NiCd or NiMH, the same size cell produces 1.2 volts. To confuse matters even more, most AA-size Li-ion cells produce 3. 7 volts.