Batteries

by on December 8, 2014

battery

Hi all
Today I want to talk about batteries. However before I begin there are a few terms I want to clarify. So here we go.

Terminology

Cell – This is where the chemical reaction takes place which produces electricity. The characteristics of the cell are determined by its chemical formulation and its construction.

Battery – A battery is comprised of one or more cells. It is a complete unit which can be used to provide power to the robot.

Power (Watt) – Power is the rate at which work is done. Often when we compare how power hungry something is we will look at how much power it uses. Power is measured in Watts. You can find power by multiplying the voltage and the current of your system (Power=Voltage x Current).

Nominal Voltage – As the energy of battery is used up, the voltage will tend to decline accordingly. The voltage will also be affected by how much current the battery is currently providing, a high load will generally cause a voltage dip. In order to rate batteries, a Nominal Voltage is used. It is approximately the midpoint between the full charge and the fully discharged voltages of the battery. The Nominal Voltage can be found on the manufacturer’s data sheet.

Energy (Watt-hour) – Energy is the rate of power usage and is measured in watt-hours. A watt hour is the power times the hours of operation. So for example if you have a 5 Watt light and want to use it for 1 hour, that is 5 watt-hours of energy. The capacity of a battery will often be specified in watt-hours. If you were paying your local electric provider you would be billed for 5 watt-hours. You can not practically measure energy however you can measure the power and the time that you were using that power.

Amp Hour (Ah) – Amp-hours are another way, albeit indirect, of expressing Energy. While watt-hours take the changing voltage of a system in to effect, amp-hours completely ignore the voltage. The amp hour rating of a battery is referred to as its ‘capacity’. When you talk about devices we typically use watt-hours. However for batteries we often talk about amp-hours since we are talking about how long a battery can maintain a current output at its rated voltage. To convert between amp-hours and watt-hours you just need to multiple or divide by the voltage depending on which way you are going. So:


Watt-hour = amp-hour * nominal voltage
and
Amp-hour = watt-hour / nominal voltage


We often talk about the density of a battery or the density of a battery technology. You can think of it as how much power/energy can I get for a given size (mass and volume). The two common ones to discuss are:
Energy Density – An energy dense battery can typically output its rated current for a long period of time. However the amount of power it can give out at a given time is often small.

Power Density – A power dense battery can put out a large amount of power at a given time, however the length of time that the battery can put that power out is often short.

Battery Types

Ok. We finally get to talk about types of batteries. There are many battery technologies out there, but there are only a few that are commonly used in robotics. When we discuss batteries it is important to remember that each battery is often built by combining multiple battery cells in series and parallel combinations to get a larger voltage or current capacity. If you have ever opened a typical 9V battery you would see the 6 “cells” that are inside the battery. Some examples of battery technologies include:
– Lead Acid
– Alkaline (Zinc/Manganese-dioxide)
– Lithium (and many flavors of lithium)
– Nickel Cadmium (Nicad)
– Fuel Cell
– Air (Zinc air and Lithium air)

The two that we will discuss are lead-acid and Lithium.
Lead Acid
Lead acid batteries are the old workhorse of robots that are reliable, easy to use, and resilient. They are great batteries that can be abused and can be operated with minimal support. The downside is they are very heavy and have a low energy density. They have a high power density and can output large amounts of current.
To charge lead acid batteries you can just apply a higher voltage to them, and they will charge. For optimal charging results you can get battery chargers that use preset charge profiles and switch between constant-current and constant-voltage modes.
A common cell is 2.1V that when used can safely drop down to 1.95V. This leads to a common battery size of 12V (12.6V down to 11.7V) which is made from connecting six cells in series. If you drain the battery past the 11.7V (or 1.95V per cell) you will probably permanently damage the battery. A common damage mode is to reduce your battery life.

Lithium Ion
Lithium ion is a relative new comer to the world of batteries. They have high energy density and as such for a similar capacity will weigh less than their lead acid counterparts. The downside is that they can be difficult to charge and can cause fires (think about the Boeing Dreamliner issues). There are also shipping restrictions on lithium ion batteries that you need to be aware of; primarily this makes shipping them by air difficult.

In order to charge a Lithium Ion battery you should have a charger (designed for your battery type) that is capable of monitoring the voltage of each cell (or string of cells) in order to properly balance the battery (more on this soon) and the temperature (so you do not create a fire). These chargers are more expensive and more complex. Lithium ion batteries can also get even trickier when putting batteries in series or parallel with each other since the battery managers and chargers can not always handle the higher voltage. One example that I have seen was with robot with a 72V nominal bus (5 batteries in series) and a solar panel that generated >80 volts. We were not able to use the desired battery since it could only be charged in 15V increments per battery and there was no easy way to charge them all with the >80V coming from the solar panels (which is easily done with lead acids). I understand in many/most cases you need to charge batteries to the max and use them till the capacity is almost depleted; but if you want to maximize the life of your batteries (for most vendors) you should charge them when they reach 40%-70% (approximately) of capacity and never let them drop all the way down or charge them all the way up.

There are many things to consider in order to safely build a safe lithium ion battery. Do not attempt to build one if you do not know how!

When you assemble a Lithium Ion battery pack you need to make sure that all of the cells are similar (“balanced”). Balanced means that voltage charge and discharge curves should be similar to one and another. If they are not, then each cell can end up with a different voltage, which reduces the capacity of the complete battery. You should also make sure that you have proper battery management circuitry to prevent such unsafe conditions as overcharge, overdischarge, overtemperature, etc. Finally, you should stick to high quality cells from reputable vendors, as low quality cells can increase the safety risk.

Communicating with the Battery

With the advent of many on-board controls for batteries, the next logical step is for the battery to be able to communicate this information to their chargers and the devices they power. This is how your laptop and cellphone knows how much battery juice is left (and why they have more than just 2 electrical contacts). The data available can go way beyond just the remaining battery life. You can query the battery for such things as the manufacturer, rated capacity, cycle count, temperature, voltage, current, etc. Batteries can even tell a capable charger exactly how they should be charged.
While there are many battery (mostly vendor specific) communication protocols out there, two common ones are SMBus and DQ. SMBus is an open protocol based on I2C and developed by an industry consortium; there are SMBus chips available from many vendors. DQ was developed by Texas Instruments and it’s chips are also widely available.

Vendors

Mathews Associates Inc. – I have dealt with them. I really like how enthusiastic they are about their product and helping with new ideas.
Bren-Tronics – I have heard good things, but I have never contacted them.
Valence – They have nice lithium batteries that seem to stand up to usage well. You can almost treat them as a lead acid drop in.
A123 Systems – Provider of lithium cells. Can be hard to get cells in low quantity.


I would like to thanks Ari for his help with creating this post and providing some of his knowledge.

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