Power

Batteries

In general, each battery chemistry has a nominal voltage (actually a curve of voltage over discharge). For example, alkaline batteries are 1.5V, NiMH is 1.2V, lithium ion is 3.7V, etc.

Capacity is usually indicated in amp-hours. A 1 Ah battery can source 1A for one hour. Smaller capacities are indicated in mAh.

Capacity is sometimes indicated in watt-hours, especially for chemistries with dramatic voltage drops during discharge.

Cells can usually be made larger, for more capacity at a given voltage. For example, an alkaline D cell is simply a larger version of an AAA cell, with very similar electrical characteristics apart from capacity.

Cells can be paralleled to get more capacity, or placed in series for greater voltage.

Due to the internal characteristics of some battery chemistries, putting cells in parallel or series should be done with great care to avoid overloading individual cells. This is done either using specialized ICs or with careful binning and balancing of cells.

A primary battery cannot be recharged. A secondary battery can be recharged. This terminology is rarely seen. Just check if it says rechargeable.

Lithium primary

Coin cells

CR2032 coin cells (20mm diameter, 3.2mm thickness) are common in blinkies and small battery powered transceivers. They are very bad for most of the purposes we put them to because of a very high internal resistance (up to 30 ohm). The internal resistance is high enough that most LEDs can be connected to them without a current limiting resistor.

They are only rated for about 0.2mA sustained current draw, and cannot reliably source more than 10-15 mA.

Bluetooth Low Energy has apparently been engineered to work well with CR2032. The receivers typically draw 10-15mA and transmitters a bit less, but BLE has very low duty cycles and parts such as nRF51822 can get away with being powered with a CR2032.

CR2032 cells are rated for 210mAh, but only at less than a mA current.

CR2016 are half the height and half the capacity.

Lithium thionyl chloride (Li-SOCl₂)

Li-SOCl₂ have very low self discharge and very high energy density. They also have very low voltage drops, staying at 3.6V through almost all of the discharge cycle. A D cell typically has a capacity of 20 Ah.

Li-SOCl₂ are used in FBI tracking devices.

Beware that these batteries are considered a class 3 hazmat - they don't leak or explode often, but if they do the contents are extremely toxic.

Also, they have relatively high ESR. This makes them unsuitable for applications that require large current draw unlike other lithium chemistries. However there are a few papers that discuss pairing this chemistry with a supercapacitor to increase there efficiency for designs that have bursty loads.

Lithium ion/lithium polymer (secondary)

These are the most common kind of lithium cells for electronics. They charge at 4.2V and discharge all the way to 2.75V. That makes them a challenge to use with most ICs that can't go above 3.6V. One easy and cheap strategy is to operate the circuit at 2.5V, supplied by a buck converter. 1.8V is also viable for some circuits: most digital and RF ICs can operate at 1.8V, but many sensors can't.

1)While lithium cells can go to 2.75V, you should not let them fall below 3 Volts per cell. Some applications even use a cutoff of 3.3v. 2)Individual cells can be scavenged from many different things like laptop batteries or R/C battery packs. If you have a bad lithium battery pack, it is usually the fault of one of the cells. Re-purposing the good cells from a bad pack is possible. 3)Another easy way to use them easily is by having a LDO 3.3v regulator and powering your microcontrollers at 3.3V. 4)The chemistry will ignite if brought to 60 Celsius. This will cause the rest of the pack to ignite in something referred to as thermal-runaway.

Charging lithium secondary cells should be done with a specialized battery charging IC. They are charged with constant current up to a certain voltage, then constant voltage. This is known as Constant Current Constant Voltage or CCCV charging. Charging at 0.5C is usually safe; higher charge rates require monitoring battery temperature through a thermistor built into the lithium cell.

Note that if you use a battery charger that has auto termination, you should be very careful about connecting the load directly to the battery while the charger is powered: the load will appear as the battery charging, which means that the charge will never terminate. There are several solutions to this problem. Some battery charging ICs have a separate pin for the load. There is also a simple circuit with a P-channel MOSFET and a Zener diode that can turn off current from battery to load while an external power supply is connected.

Lithium ion/polymer batteries are among the most dangerous electronic components a hobbyist will encounter, especially individual soft cells without hard packaging. Treat them with great care. Do not put cells in parallel or series yourself. Do not expose to excessive forces.

Lithium iron phosphate (LiFePO₄)

LiFePO₄ are much safer than lithium ion/polymer, with roughly 2/3 the energy density. They are charged in a similar way. LiFePO₄ cells discharge at 3V, through almost the full discharge cycle. Because of their voltage, they can directly power most ICs without a regulator.

In New York, you will often see LiFePO₄ battery packs on delivery people's electric bicycles.

Regulators

A regulator maintains a constant voltage (or in the case of a current regulator, a current voltage).

You will sometimes encounter unregulated power. For example, an unregulated 12VDC regulator fed with 120V AC power might consist of a 10:1 transformer followed by a diode bridge rectifier and decoupling capacitors. The output voltage will be approximately 12V, but will vary with the input voltage and other factors. It is a cheaper way of supplying power if you only need approximately, but not exactly, 12V, for example because you know that the voltage will be further regulated to lower voltages.

A 5V DC regulator will attempt to maintain its output voltage at 5V and a properly functioning one will do so as long as conditions remain within its specification. Most regulators specify a maximum current. For example, the popular LM7805 linear regulator will only work well up to 1.5A. If the load is well above the specified maximum load, the regulator may no longer be able to maintain regulation, and its behavior is unspecified. Typically it will be able to supply higher currents at a lower, unregulated, voltage. In rarer circumstances, it will fail completely.

Buck, boost and buck/boost ICs

Robert will volunteer to teach an advanced class in buck, boost, buck/boost, and sepic theory, especially if Guan will cohost.

The theory for all is the same: utilize reactive elements and rapid changes in current or voltage to exchange current for voltage at fairly high efficiency. This works because these components output current or voltage proportional to the rate of change of voltage or current in, so modifying the duty cycle of a PWM input will control the output.

Around that core are rectifiers and filters to turn the pulsed signal to smooth DC, and drivers to increase efficiency.

Flyback

These are similar to the switching regulators above, with the addition of a flyback transformer. The flyback transformer adds isolation, which is crucial for safety when using mains-powered circuits. Because the transformers can have different numbers of windings on each side, a greater range of voltages can be realized.