Attribution: Joe Piccirilli
The growth in home automation is requiring ever more microprocessors, which require clean power to operate optimally and in a number of instances, redundant backup power supplies. Modern residential system design also requires taking into consideration battery powered cars, battery backup for renewables and whole-home battery systems.
Batteries have become a hot topic. Lead Acid batteries. Lithium Ion batteries. How do they differ and which ones are better? What new battery technologies are being tested in laboratories? Batteries, batteries, batteries. For system integrators, having a basic understanding of batteries is rapidly becoming a critical part of your skill set.
This will be the first in a series of articles to help all of you get a handle on this big and complicated subject. Before we get to the strengths and weaknesses of different battery chemistries, let’s begin with basic battery terminology.
First, what is a battery?
A battery is a container of negatively charged particles, called electrons. Since like-charged particles repel each other, the closer they are packed (density) the more the repelling force is. A chemical reaction inside the battery produces free electrons which discharge as electricity. The electricity that is discharged from any battery is called direct current (DC).
Homes typically use alternating current (AC). In order for batteries to power a home, you have to use a device called an inverter, which converts DC to AC.
Battery Chemistries – the two main categories
Lithium Ion. There are a variety of different Lithium chemistries available, all with their own set of strengths 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. Another plus for a lead acid battery is that it is >94% recyclable. Making it one of the most recycled products on the planet.
Amp Hours – Batteries are rated in Amp Hours, sometimes expressed as just amps. A 100 amp 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 this information)
Battery Strings – A group of batteries wired together to produce a certain amount of voltage.
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.
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.
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.
Cut Off Voltage – The minimum allowable voltage. If is this voltage that generally defines the “empty” state of the battery.
Cycle Life (number at a specific DOD) – How many times a battery can 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.
Discharge Rate (expressed as E) – How quickly you can discharge a battery, relative to its capacity, without compromising safety or cycle life. The Tesla 10 kWh power wall is rated at E/5. That means it can be discharged at no more than 2 kW and maintain its cycle life.
Energy Density – How many watts can be stored per unit of volume.
Name Plate Energy – Total amount of energy stored in a battery.
PSOC – Partial State of Charge (less than 100%, greater than 0%).
Specific Power – Watts per unit of mass.
State of Charge – Available percentage of total capacity.
Usable Energy – Amount of energy that can be used while maintaining the stated cycle life.
Batteries can be designed:
- 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
In conclusion, no one battery exists that incorporates all of these design elements. 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.
(A word 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.)