Principle and definitions

2020-08-11 08:07

Capacity and energy of a battery or storage system

The capacity of a battery or accumulator is the amount of energy stored according to specific temperature, charge and discharge current value and time of charge or discharge.

Rating capacity and C-rate

C-rate is used to scale the charge and discharge current of a battery. For a given capacity, C-rate is a measure that indicate at what current a battery is charged and discharged to reach its defined capacity. 

A 1C (or C/1) charge loads a battery that is rated at, say, 1000 Ah at 1000 A during one hour, so at the end of the hour the battery reach a capacity of 1000 Ah; a 1C (or C/1) discharge drains the battery at that same rate.
A 0.5C or (C/2) charge loads a battery that is rated at, say, 1000 Ah at 500 A so it takes two hours to charge the battery at the rating capacity of 1000 Ah;
A 2C charge loads a battery that is rated at, say, 1000 Ah at 2000 A, so it takes theoretically 30 minutes to charge the battery at the rating capacity of 1000 Ah;
The Ah rating is normally marked on the battery.
Last example, a lead acid battery with a C10 (or C/10) rated capacity of 3000 Ah should be charge or discharge in 10 hours with a current charge or discharge of 300 A.

Why is it important to know the C-rate or C-rating of a battery

C-rate is an important data for a battery because for most of batteries the energy stored or available depends on the speed of the charge or discharge current. Generally, for a given capacity you will have less energy if you discharge in one hour than if you discharge in 20 hours, reversely you will store less energy in a battery with a current charge of 100 A during 1 h than with a current charge of 10 A during 10 h.

Formula to calculate Current available in output of the battery system

How to calculate output current, power and energy of a battery according to C-rate?
The simplest formula is :

I = Cr * Er
or
Cr = I / Er
Where
Er = rated energy stored in Ah (rated capacity of the battery given by the manufacturer)
I = current of charge or discharge in Amperes (A)
Cr = C-rate of the battery
Equation to get the time of charge or charge or discharge "t" according to current and rated capacity is :
t = Er / I
t = time, duration of charge or discharge (runtime) in hours
Relationship between Cr and t :
Cr = 1/t
t = 1/Cr

How Lithium-ion Batteries Work

Lithium-ion batteries are incredibly popular these days. You can find them in laptops, PDAs, cell phones and iPods. They're so common because, pound for pound, they're some of the most energetic rechargeable batteries available.

­ Lithium-ion batteries have also been in the news lately. That’s because these batteries have the ability to burst into flames occasionally. It's not very common — just two or three battery packs per million have a problem — but when it happens, it's extreme. In some situations, the failure rate can rise, and when that happens you end up with a worldwide battery recall that can cost manufacturers millions of dollars.

So the question is, what makes these batteries so energetic and so popular? How do they burst into flame? And is there anything you can do to prevent the problem or help your batteries last longer? In this article, we'll answer these questions and more.

Lithium-ion batteries are popular because they have a number of important advantages over competing technologies:

  • They're generally much lighter than other types of rechargeable batteries of the same size. The electrodes of a lithium-ion battery are made of lightweight lithium and carbon. Lithium is also a highly reactive element, meaning that a lot of energy can be stored in its atomic bonds. This translates into a very high energy density for lithium-ion batteries. Here is a way to get a perspective on the energy density. A typical lithium-ion battery can store 150 watt-hours of electricity in 1 kilogram of battery. A NiMH (nickel-metal hydride) battery pack can store perhaps 100 watt-hours per kilogram, although 60 to 70 watt-hours might be more typical. A lead-acid battery can store only 25 watt-hours per kilogram. Using lead-acid technology, it takes 6 kilograms to store the same amount of energy that a 1 kilogram lithium-ion battery can handle. That's a huge difference
  • They hold their charge. A lithium-ion battery pack loses only about 5 percent of its charge per month, compared to a 20 percent loss per month for NiMH batteries.
  • They have no memory effect, which means that you do not have to completely discharge them before recharging, as with some other battery chemistries.
  • Lithium-ion batteries can handle hundreds of charge/discharge cycles.

That is not to say that lithium-ion batteries are flawless. They have a few disadvantages as well:

  • They start degrading as soon as they leave the factory. They will only last two or three years from the date of manufacture whether you use them or not.
  • They are extremely sensitive to high temperatures. Heat causes lithium-ion battery packs to degrade much faster than they normally would.
  • If you completely discharge a lithium-ion battery, it is ruined.
  • A lithium-ion battery pack must have an on-board computer to manage the battery. This makes them even more expensive than they already are.
  • There is a small chance that, if a lithium-ion battery pack fails, it will burst into flame.

Many of these characteristics can be understood by looking at the chemistry inside a lithium-ion cell. We'll look at this next. ­

Lithium-ion battery packs come in all shapes and sizes, but they all look about the same on the inside. If you were to take apart a laptop battery pack (something that we DO NOT recommend because of the possibility of shorting out a battery and starting a fire) you would find the following:

  • The lithium-ion cells can be either cylindrical batteries that look almost identical to AA cells, or they can be prismatic, which means they are square or rectangular The computer, which comprises:
  • One or more temperature sensors to monitor the battery temperature
  • A voltage converter and regulator circuit to maintain safe levels of voltage and current
  • A shielded notebook connector that lets power and information flow in and out of the battery pack
  • A voltage tap, which monitors the energy capacity of individual cells in the battery pack
  • A battery charge state monitor, which is a small computer that handles the whole charging process to make sure the batteries charge as quickly and fully as possible.

If the battery pack gets too hot during charging or use, the computer will shut down the flow of power to try to cool things down. If you leave your laptop in an extremely hot car and try to use the laptop, this computer may prevent you from powering up until things cool off. If the cells ever become completely discharged, the battery pack will shut down because the cells are ruined. It may also keep track of the number of charge/discharge cycles and send out information so the laptop's battery meter can tell you how much charge is left in the battery.

It's a pretty sophisticated little computer, and it draws power from the batteries. This power draw is one reason why lithium-ion batteries lose 5 percent of their power every month when sitting idle.

Lithium-ion Cells

As with most batteries you have an outer case made of metal. The use of metal is particularly important here because the battery is pressurized. This metal case has some kind of pressure-sensitive vent hole. If the battery ever gets so hot that it risks exploding from over-pressure, this vent will release the extra pressure. The battery will probably be useless afterwards, so this is something to avoid. The vent is strictly there as a safety measure. So is the Positive Temperature Coefficient (PTC) switch, a device that is supposed to keep the battery from overheating.

This metal case holds a long spiral comprising three thin sheets pressed together:

  • A Positive electrode
  • A Negative electrode
  • A separator

Inside the case these sheets are submerged in an organic solvent that acts as the electrolyte. Ether is one common solvent.

The separator is a very thin sheet of micro perforated plastic. As the name implies, it separates the positive and negative electrodes while allowing ions to pass through.

The positive electrode is made of Lithium cobalt oxide, or LiCoO2. The negative electrode is made of carbon. When the battery charges, ions of lithium move through the electrolyte from the positive electrode to the negative electrode and attach to the carbon. During discharge, the lithium ions move back to the LiCoO2 from the carbon.

The movement of these lithium ions happens at a fairly high voltage, so each cell produces 3.7 volts. This is much higher than the 1.5 volts typical of a normal AA alkaline cell that you buy at the supermarket and helps make lithium-ion batteries more compact in small devices like cell phones. See How Batteries Work for details on different battery chemistries.

We'll look at how to prolong the life of a lithium-ion battery and explore why they can explode next.

Lithium-ion Battery Life and Death

Lithium-ion battery packs are expensive, so if you want to make yours to last longer, here are some things to keep in mind:

  • Lithium ion chemistry prefers partial discharge to deep discharge, so it's best to avoid taking the battery all the way down to zero. Since lithium-ion chemistry does not have a "memory", you do not harm the battery pack with a partial discharge. If the voltage of a lithium-ion cell drops below a certain level, it's ruined.
  • Lithium-ion batteries age. They only last two to three years, even if they are sitting on a shelf unused. So do not "avoid using" the battery with the thought that the battery pack will last five years. It won't. Also, if you are buying a new battery pack, you want to make sure it really is new. If it has been sitting on a shelf in the store for a year, it won't last very long. Manufacturing dates are important.
  • Avoid heat, which degrades the batteries.

Exploding Batteries

Now that we know how to keep lithium-ion batteries working longer, let's look at why they can explode.

If the battery gets hot enough to ignite the electrolyte, you are going to get a fire. There are video clips and photos on the Web that show just how serious these fires can be. The CBC article,"Summer of the Exploding Laptop," rounds up several of these incidents.

When a fire like this happens, it is usually caused by an internal short in the battery. Recall from the previous section that lithium-ion cells contain a separator sheet that keeps the positive and negative electrodes apart. If that sheet gets punctured and the electrodes touch, the battery heats up very quickly. You may have experienced the kind of heat a battery can produce if you have ever put a normal 9-volt battery in your pocket. If a coin shorts across the two terminals, the battery gets quite hot.

In a separator failure, that same kind of short happens inside the lithium-ion battery. Since lithium-ion batteries are so energetic, they get very hot. The heat causes the battery to vent the organic solvent used as an electrolyte, and the heat (or a nearby spark) can light it. Once that happens inside one of the cells, the heat of the fire cascades to the other cells and the whole pack goes up in flames.

It is important to note that fires are very rare. Still, it only takes a couple of fires and a little media coverage to prompt a recall.

Different Lithium Technologies

Firstly, it is important to note that there are many types of “Lithium Ion” batteries. The point to note in this definition refers to a “family of batteries”.
There are several different “Lithium Ion” batteries within this family which utilize different materials for their cathode and anode. As a result, they exhibit very different characteristics and therefore are suitable for different applications.

Lithium Iron Phosphate (LiFePO4)

Lithium Iron Phosphate (LiFePO4) is a well-known lithium technology in Australia due to its wide use and suitability to a wide range of applications.
Characteristics of low price, high safety and good specific energy, make this a strong option for many applications.
LiFePO4 cell voltage of 3.2V/cell also makes it the lithium technology of choice for sealed lead acid replacement in a number of key applications.

LiPO Battery

Of all the lithium options available, there are several reasons why LiFePO4 has been selected as the ideal lithium technology for replacement of SLA. The main reasons come down to its favourable characteristics when looking at the main applications where SLA currently exist. These include:

  • Similar voltage to SLA (3.2V per cell x 4 = 12.8V) making them ideal for SLA replacement.
  • Safest form of the lithium technologies.
  • Environmentally friendly –phosphate is not hazardous and so is friendly both to the environment and not a health risk.
  • Wide temperature range.

Features and benefits of LiFePO4 when compared to SLA

Below are some key features a Lithium Iron Phosphate battery which give some significant advantages of SLA in a range of applications. This is not a complete list by all means, however it does cover the key items. A 100AH AGM battery has been selected as the SLA, as this is one of the most commonly used sizes in deep cycle applications. This 100AH AGM has been compared to a 100AH LiFePO4 in order to compare a like for like as close as possible.

Feature – Weight:

Comparison

  • LifePO4 is less than half the weight of SLA
  • AGM Deep cycle – 27.5Kg
  • LiFePO4 – 12.2Kg

Benefits

  • Increases fuel efficiency
    • In caravan and boat applications, towing weight is reduced.
  • Increases speed
    • In boat applications water speed can be increased
  • Reduction in overall weight
  • Longer runtime

Weight has a large bearing on many applications, especially where towing or speed in involved, such and caravan and boating. Other applications including portable lighting and camera applications where the batteries need to be carried.

Feature – Greater Cycle Life:

Comparison

  • Up to 6 time the cycle life
  • AGM Deep cycle – 300 cycles @ 100% DoD
  • LiFePO4 – 2000 cycles @ 100% DoD

Benefits

  • Lower total cost of ownership (cost per kWh much lower over life of battery for LiFePO4)
  • Reduction in replacement costs – replace the AGM up to 6 times before the LiFePO4 needs replacing

The greater cycle life means that the extra upfront cost of a LiFePO4 battery is more than made up for over the life use of the battery. If being used daily, an AGM will need to be replaced approx. 6 times before the LiFePO4 needs replacing

Feature – Flat Discharge Curve:

Comparison

  • At 0.2C (20A) discharge
  • AGM – drops below 12V after
  • 1.5 hrs of runtime
  • LiFePO4 – drops below 12V after approximately 4 hrs of runtime

Benefits

  • More efficient use of battery capacity
  • Power = Volts x Amps
  • Once voltage starts to drop off, battery will need to supply higher amps to provide same amount of power.
  • Higher voltage is better for electronics
  • Longer runtime for equipment
  • Full use of capacity even at high discharge rate
  • AGM @ 1C discharge = 50% Capacity
  • LiFePO4 @ 1C discharge = 100% capacity

This feature is little known but is a strong advantage and it gives multiple benefits. With the flat discharge curve of LiFePO4, the terminal voltage holds above 12V for up to 85-90% capacity usage. Because of this, less amps are required in order to supply the same amount of power (P=VxA) and therefore the more efficient use of the capacity leads to longer runtime. The user will also not notice the slowing down of the device (golf cart for example) earlier.

Along with this the effect of Peukert’s law is much less significant with lithium than that of AGM. This results in having available a large percentage of the capacity of the battery no matter what the discharge rate. At 1C (or 100A discharge for 100AH battery) the LiFePO4 option will still give you 100AH vs only 50AH for AGM.

Feature – Increased Use Of Capacity:

Comparison

  • AGM recommended DoD = 50%
  • LiFePO4 recommended DoD = 80%
  • AGM Deep cycle – 100AH x 50% = 50Ah usable
  • LiFePO4 – 100Ah x 80% = 80Ah
  • Difference = 30Ah or 60% more capacity usage

Benefits

  • Increased runtime or smaller capacity battery for replacement

The increased use of the available capacity means the user can either obtain up to 60% more runtime from the same capacity option in LiFePO4, or alternatively opt for a smaller capacity LiFePO4 battery while still achieving the same runtime as the larger capacity AGM.

Feature – Greater Charge Efficiency:

Comparison

  • AGM – Full charge takes approx. 8 hours
  • LiFePO4 – Full charge can be as low as 2 hrs

Benefits

  • Battery charged and ready to be used again more quickly

Another strong benefit in many applications. Due to the lower internal resistance among other factors, LiFePO4 can accept charge at a much great rate than AGM. This allows them to be charged and ready to use much faster, leading to many benefits.

Feature – Low Self Discharge Rate:

Comparison

  • AGM – Discharge to 80% SOC after 4 months
  • LiFePO4 – Discharge to 80% after 8 months

Benefits

  • Can be left in storage for a longer period

This feature is a big one for the recreational vehicles which may only be used for a couple of months a year before going into storage for the rest of the year such as caravans, boats, motorcycles and Jet Skis etc. Along with this point, LiFePO4 doesn’t calcify and so even after being left for extended periods of time, the battery is less likely to be permanently damaged. A LiFePO4 battery is not harmed by not being left in storage in a fully charged state.

So, if your applications warrant any of the above features then you will be sure to get your monies worth for the extra spent on a LiFePO4 battery. Follow up article will follow in the coming weeks which will include the safety aspects on LiFePO4 and different Lithium chemistries.

 

 

 

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