Battery Basics – Letter from the CEO
February 26, 2016
In what I hope will be an ongoing series of posts, we want to inform our followers about what we believe is the next great opportunity for the CI professional within intelligent energy management and security channels.
Batteries can be an extensive and complicated subject. For my latest article, I want to provide readers with a basic primer to better prepare readers to further understand more in depth articles to be presented in the future.
The following definitions will be added to the "Glossary of Technology," found at http://bit.ly/1Ro9rTV.
Amp Hours – Batteries are rated in amp Hours sometimes expressed as just amps. A 100 amp hour battery rating means it can discharge 100 amps for one hour. If it is a 12 volt battery it can also be referred to as a 1.2 kilo watt battery (Watts = Volts x Amps). Not all 100 amp hour batteries are the same. Some batteries can only discharge at a 10 amp rate, but can do so for 10 hours. Other batteries can only discharge at higher rates. The spec is only useful if you know the discharge rate. In most cases, you have to really comb the spec sheets to find that information.
Energy Density – Number of watts stored per unit of volume.
Specific Power – Watts per unit of mass.
Cycle Life (number at a specific DOD) – How many times can a battery be charged and discharged before it is no longer useful.
Depth of Discharge (DOD) – How much of the total available energy has been extracted from the battery. Usually measured as a percentage.
Name Plate Energy – Total amount of energy stored in a battery.
Usable Energy – Amount of energy that can be used while maintaining the stated cycle life.
Charge rate (expressed as C) – How quickly you can charge a battery without compromising its stated cycle life. It is expressed as a multiple or a divider of full capacity (C). A 1C rating means you can charge a 100 amp hour battery with 100 amps.
Discharge Rate (expressed as E) – The rate at which you can discharge a battery, relative to its capacity, without compromising safety or cycle life. The Tesla 10 kWh Powerwall is rated at E/5. That means it can be discharged at no more than 2 kW and maintain its cycle life. Whereas, the RoseWater Hub is rated E/1.25 meaning it can discharge more than 16 kW/h and still maintain its cycle life.
State of Charge – Available percentage of total capacity.
PSOC – Partial State of Charge (less than 100%, greater than 0%).
BMS – Battery Management System. A device that manages the charge and discharge rates within a string of batteries or cells. It maintains a consistent state of charge from battery to battery or cell to cell.
Battery Strings – A group of batteries wired together to produce a certain amount of voltage.
Cells – The smallest component of a battery. Cells are grouped together to make a battery of a certain voltage.
Charge Controller – A device that manages the charge voltage and current so that a battery is not overcharged.
Cut Off Voltage – The minimum allowable voltage. This is the voltage that generally defines the "empty" state of the battery.
What is a Battery?
A battery is a container of electrons, which are negatively charged particles. Since like-charged particles repel each other, the more negatively charged particles you pack into a container, the more they want to get away from each other. The closer the particles are, the stronger the repelling force is, relative to magnets.
In general, all batteries operate in the same fashion. A chemical reaction inside the battery produces free electrons. Always keep in mind that batteries produce direct current (DC). A typical home uses alternating current (AC). In order for batteries to power a home, you must use a device called an inverter. An inverter converts direct current into alternating current.
Battery Chemistries – the Two Main Categories
Lithium Ion - There has been a lot printed, both good and bad, about lithium ion batteries. You should know that there are a variety of different lithium chemistries available, all with their own set of strength and weaknesses. Some cycle better than others, some are smaller and lighter than others, and some are safer than others.
Lead Acid – The oldest and least expensive battery chemistry. It is heavier than lithium chemistries and does not have the same cycle life, particularly in deep discharge applications. However, it is significantly less expensive and, in general, far safer than any other battery chemistry.
There are many other chemistries that are currently in use or are being developed, but that discussion will have to wait for a different article. If anyone is interested in a particular chemistry, I would be happy to address it as a post.
Batteries can be designed with varied specs, such as:
- To be deeply discharged.
- To operate best in partial states of charge.
- To be charged or discharged very rapidly.
- To be charged or discharged very slowly.
- To be light weight.
- To be very small.
- To work in extreme temperatures.
The question you should be asking is "Why can't a battery be designed that does all of these things?" The answer is science, safety and money. In today's world a system designer has to consider the application, the power requirement, the environment and the budget in order to make the appropriate battery choice.
Lastly, a brief note about battery specification sheets – in my experience they are written more as marketing pieces than technical specs. In many cases, you really have to dig to find useful comparative information. I believe the battery choice is best left up to the team that designed the device.