Types of Batteries & How They Work Part 2

Types of batteries and how they work, part 1 went into a bit of battery history followed by a general overview of how batteries work with a discussion on battery capacity and ways to connect them. Next, the post went into some details of primary battery types and chemistry.

This time around, we’re going to talk about secondary batteries, also known as rechargeable batteries. But first, I wanted to talk a bit more about battery capacity to remove any lingering confusion from the last post.

Battery Capacity Part 2

You already know the low-level definition of capacity (a.k.a. C rating or C), but since the Coulomb is not a practical unit to work with, I wanted to give you a more practical picture of what C means.

Recall that the main unit for battery capacity the Ah, and, as we’ve seen from the last post, these lower-level (Coulombs) units make up the Ah.

The C rating of a battery also represents the efficiency of the battery to store energy and transfer it to a load. A discharge rate of 1 C draws a current equal to the rated capacity that takes one hour. So, a battery with a 2000 mAh rating provides 2000 mA of current for an hour if discharged at 1 C rate.

The same battery discharged at 0.5 C would be providing 1000 mA for 2 hours. If we discharged this battery at 2 C, then it would be pumping out 4000 mA for half an hour before it died.

The internal resistance of a battery largely determines its C rate.

Battery Internal Resistance

Before we jump into the types and uses of secondary batteries, a word on battery internal resistance is in order. Note that this applies to both primary and secondary batteries.

An ideal voltage source has no internal resistance. If we connect a resistor across a 1.5V ideal voltage source, the entire 1.5V would drop across that resistor.

There is no such thing as perfect in real life though, and batteries are no exception.

All batteries, whether primary or secondary, have some internal resistance resulting from resistance on the electrodes and the electrolyte.

This resistance is usually very low, but increases as the battery discharges.

It can also cause a noticeable drop in output voltage if a low-resistance load is applied to the battery.

The picture below is a more accurate model of a typical battery.

Here, we see the battery modeled as an ideal voltage source in series with a resistor. The two resistors, R_in and R_load form a voltage divider. Voltage dividers are quite common and often useful in electronics. Looking at the formula for voltage dividers below, we can see why V_out is 1.33V and not 1.5V.

V_out = V_in (R_load/R_load+R_in))

This is not something to get totally hung up on, but maybe worth consideration depending on the project.

Types of Batteries: Cells vs Batteries

A bit of clarification is in order before we dive into the meat of this post. People often use the words cell and battery interchangeably. There is a difference though. Cells are what make up a battery. For example, a 9 V battery consists up of six 1.5 V “cells.” A typical AA, AAA, C, or D battery is made up of one cell.

Secondary Battery Cell Chemistry

Unlike primary cells, the plates in secondary batteries do not dissolve.

Once the battery is discharged, sending current through it in the opposite direction reverses the chemical reaction and recharges the battery.

Depending on the battery type, secondary batteries can charge/discharge anywhere from a few times (like rechargeable alkalines) to over 1000 times (like NiCd batteries).

Types of Rechargeable Batteries

Lead Acid Batteries

Originally invented by Gaston Plante in 1859, this type of battery was the first commercially available rechargeable. These types of batteries find their homes under the hoods of vehicles, in backup systems (like UPS systems), and some cordless equipment.

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They come in three flavors: flooded lead acid, valve-regulated lead acid (VRLA), and sealed lead acid (SLA).

Typically, lead acid batteries are used for high power applications.

For example, flooded lead acid batteries are used to power the starter in vehicles. You can likely find one under the hood of your car. Flooded lead acid batteries must also be stood upright due to the liquid inside and tend to lose electrolytes while producing gas over time.

SLA and VRLA batteries are designed to produce less gas but cannot be charged to their fullest potential due to this design. SLA batteries use a gel as the electrolyte rather than liquid acid. This allows it to be used in any position rather than just upright like flooded lead acid batteries. They also have the lowest energy density among all sealed lead acid batteries but are cheap.

On the upside, SLAs also have a very low self-discharge rate of roughly 5% per month. They also do not suffer from the memory effect and prefer shallow cycling over deep cycling. SLAs generally have a long recharge time, lasting from 8 to 16 hours depending on the battery. Store SLAs in a charged state or sulfation will occur, which can ruin the battery making it impossible to recharge.

Lead acid batteries typically come in 2, 4, 6, 8, and 12 volts with capacities ranging from one to thousands of amp-hours.

Nickel Cadmium (NiCd) Batteries

Made with nickel hydroxide as the positive electrode and cadmium hydroxide as the negative with potassium hydroxide as the electrolyte, these types of batteries were once popular but are being supplanted by nickel metal hydride batteries.

They don’t last long before needing a recharge and suffer from the notorious memory effect (technically called voltage depression) caused by the formation of cadmium crystals on the plates. High temperatures can also contribute to crystal formation. Due to this, they are not good for applications that involve shallow cycling or spending time on a float charger. They are good for deep cycle applications. To minimize crystal formation, recharge the battery only when it is completely discharged. NiCds will usually have a high self-discharge rate of about 20% to 40% per month.

The upshot is that they can charge/discharge around 1000-1500 times or more before going bad. They have a nominal cell voltage of 1.2 V per cell, so are not always directly interchangeable with alkaline batteries, which have a nominal voltage of 1.5 V.

Nickel Metal Hydride (NiMH) Batteries

These types of batteries have taken the place of NiCd batteries on many applications, but as we’ll see, do have some short-comings.

They use a nickel/nickel hydroxide on the positive electrode and either lanthanum-nickel or zirconium-nickel as the negative electrode with potassium hydroxide as the electrolyte.

NiMH batteries are more environmentally friendly than NiCds and have an energy density 30%-40% higher. They do have a slight memory effect, but not anywhere near as bad as nickel cadmium batteries. The self-discharge rate is high at about 30% per month.

The downside is that they do not like deep discharging cycles like NiCds do. Also, they cannot recharge as many times (usually 150-500 times), limiting their useful work life. They also should fully discharge before recharging to prevent crystalline formation.

Although some may tell you otherwise, it is not a good idea to use a NiCd charger on NiMH batteries and vice versa. To get the full life out of a given secondary battery, it should be recharged by a charger that is designed specifically for its type.

Lithium Ion (Li-Ion) and Lithium Polymer (Li-polymer) Batteries

Chances are that if you own a smartphone or a laptop you’re using a lithium based battery. And for good reason.

Lithium is the lightest of all metals and the batteries have an extremely high energy density.

Lithium, like sodium and potassium, is very reactive and can be dangerous. Because of this, these types of batteries use lithium ions from other chemicals such as lithium-cobalt dioxide rather than lithium metal itself.

Li-ion batteries have a negative electrode made up of aluminum coated lithium-cobalt dioxide, lithium-nickel dioxide, or lithium-manganese dioxide. Copper coated with carbon usually constitutes the positive electrode. The electrolyte is made of a lithium salt such as lithium-phosphorous hexafluoride, dissolved in a solvent like a mixture of ethylene carbonate and dimethyl carbonate.

These batteries sport the twice the energy density of NiCds, have no memory effect, and low self-discharge rate (roughly 6% per month). They can stand up to somewhat of a deep discharge (but not as deep as NiCds). Li-ion and Li-polymer batteries have a nominal cell voltage of 3.6 V.

Some cons are that they are expensive and do not charge as quickly as NiCds. Nor should you trickle charge or float the battery. Many of us have seen videos or heard of cell phones (and other things) exploding or bursting into flames. Because of this, lithium battery packs must be made with built in protection against overcharging or excessive discharge.

They also have a limited lifespan of 2-3 years, even if they sit on the shelf never used. Beware when you see lithium batteries online at discounted prices. They likely have been sitting around for a while and may have little useful life left.

The lithium polymer battery is a slightly lower-cost version. It sports a similar energy density to that of Li-ion, but uses a dry solid polymer electrolyte. The electrolyte resembles a plastic film and does not conduct electricity, rather it allows an exchange of ions.

It’s possible to make these batteries very thin (as little as 1mm) and they are more rugged.

On the downside, they cannot deliver higher current bursts due to high internal resistance. To get around this, many Li-polymer batteries in cell phones and other mobile devices also contain a gel electrolyte. This reduces the internal resistance.

Below is a chart from the fourth edition of Practical Electronics for Inventors which may be useful when choosing a battery for a particular application or project (I know the text is a bit tough to read, sorry!).

Other Types of Rechargeable Batteries

This post went into detail about the secondary batteries you’re most likely to see, but there are others.

I did not mention rechargeable alkaline batteries. They have no memory effect, and do not self-discharge, so they must be great, right?

Unfortunately, these types of batteries have a big shortcoming. They can only take a small number of charge/discharged cycles. For the cost, it’s not worth it.

Some of the other, more esoteric types of secondary batteries include nickel-zinc (NiZn), nickel-iron (NiFe; invented by Thomas Edison), sodium sulfur (NaS) and there are others. Perhaps we’ll talk about some of these in a future post.

Meanwhile comment and tell us what types of batteries you use most often!

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