By: J. David DeLeonardo, Regional Analog & Power Specialist, Future Electronics
While the Leyden Jar Capacitor (1745) predates Volta’s work (1800) by a fair bit, they are both predated by the (arguably dubious) “Baghdad Battery” (220 B.C.), so perhaps the battery should be given top billing as the oldest component. Either way, it is safe to say, whenever it occurred, the invention of the battery was closely followed by the invention of the “Tentative Tongue Volt Meter.” Although that innovation would doubtless make an interesting paper, the focus of this article will be an overview of:
- The basic parameters associated with batteries
- Present state of battery technologies
- The relative pros and cons of the most common types
- Support electronics for charging, protecting and managing rechargable cells
First, Let’s Hit the High Points
Before we dive into the pros and cons of various battery technologies, let’s first examine the parameters that are most commonly used to characterize batteries.
This term refers to the metals involved in the internal chemical reactions that drive current into/out of the battery. The most commonly used chemistries today are Lead, Nickel, and Lithium. Of course, there are many other including Zinc, Manganese, Iron, Silver, etc.
The capacity of a battery is generally given in Amp*Hours, or Ah, and is a rough approximation of the amount of energy stored in the battery when fully charged. Some care must be used with this term since the discharge of a current into a load over time integrates to an Energy term, but the amount of Energy a battery will be able to deliver will vary proportionally and substantially with the rate of discharge. Still, it is a very useful and therefore commonly used figure of merit. It should be noted that the nominal capacity of a battery is referred to as the “C” of the battery. Thus, the discharge rate out of (or charge rate INTO) a given battery is often denominated in “Cs”; a 0.1C discharge rate would be very easy for a battery to maintain with no damage, but a 10C charge rate would likely force the device under charge to spontaneously disassemble.
Type: Primary or Rechargeable
A single use battery is called a “primary cell” as opposed to a “secondary cell” which is a rechargeable cell.
State of Charge
For either primary or secondary cells, the State of Charge is a measure of how much energy is presently stored in the cell as a percent of its nominal Energy Capacity.
As with so much about batteries, this parameter is not as simple as it might seem. The voltage present at the terminals of a battery will vary substantially depending on its state of charge, its temperature, the rate at which current is being drawn from it and its state of degradation from its initial manufacture.
For secondary or rechargeable cells, the Cycle count is the number of times the cell has been charged and discharged across its non-destructive capacity range. While no clear standard is in use to strictly characterize the end-points in terms of “State of Charge,” it is usually assumed that the depth of discharge is limited to that state of minimal charge that has no substantially negative impact on the long-term cycle life of the cell. NOTE: Deep discharge cycles substantially degrade the cycle life capability of the battery. This generally holds true for all chemistries except the NiCd and NiMH types which actually benefit from periodic deep discharge.
Energy Density or Specific Energy
The Energy Density or Specific Energy of a battery is a parameter found by dividing the total energy readily accessible from a battery by the total weight. (Volumetric Energy Density is a variation on this where the energy available from the cell is divided by the total volume of the cell.) See table 1 which provides a comparison among the common rechargeable battery chemistries and their associated Energy and Volumetric Densities.
This indicates the amount of Power the cell can deliver without substantial degradation.
This is the rate at which the battery will lose stored energy even in the absence of any external load. Primary cells have substantially lower self-discharge rates than secondary or rechargeable cells. Also, as might be expected, numerous factors contribute to self-discharge. Among these are state of charge, cell temperature, cycle count and age. The following chart provides a comparison among the various rechargeable battery chemistries for self-discharge rate (see table 2).
|Chemistry||Gravimetric Energy (Wh/Kg)||Volumetric Energy Density (Wh/L)||Relative Advantages||Relative Disadvantages|
|Lead-Acid (Sealed)||30 - 60||70-100||- Lowest cost cell w.r.t. energy density
- High pulse rate
- Easy to charge, easy to maintain
- Relatively low self-discharge
|- Cannot be stored in discharged state
- Very low energy density
- Low (deep) cycle capability
|Nickel- Cadmium||50 - 90||150 - 210||- Fast and easy to charge
- Can be stored in any state of charge for long periods
- Lowest cost cell in terms of $/cycle
- Good cycle durability
- Extremely rugged and durable even with hard use
|- Toxic material (Cadmium) is tightly regulated/limits use
- Relatively low energy density as compared with NiMH
- Relatively high self-discharge
- Memory effect requires periodic full-discharge
|Nickel- Metal Hydride||60 - 125||200 - 400||- Medium energy density and improving
- Less prone to memory effect than NiCd
- Benign material composition makes for easy recycling
- Simple storage and transport
|- Still some memory effect
- Complex charging profile
- High self-discharge
- Limited pulse capability
- Deep discharge cycles must be avoided.
|Lithium Ion||130 - 250||330 - 550||- High energy density
- Low self-discharge
- Very low maintenance
|- Prone to thermal runaway resulting in spontaneous disassembly requires significant protection circuitry
- Subject to aging when stored
- Low pulse capability
- Subject to tight transportation regulation
- Wide discharge voltage range
|Lithium Ion (Polymer)||130 - 240||300 - 545||- Lightweight
- Improved tolerance to over-charge and higher discharge
- Simpler protection circuit
- Flexible form-factor
|- Slightly higher cost
- Decreased cycle count
- Slightly lower energy density
|Lithium Ion (Iron Phosphate)||80 - 108||150 - 220||- Improved safety
- Longer lifetime/cycle endurance
- Constant discharge voltage
- Cheap and benign material composition
- Slower decay
- Virtually no thermal runaway
- Very low self-discharge
|- Lower energy density|
|Battery Chemistry||Self-Discharge Characteristics|
|Lithium Ion (Rechargeable)||5+% in 24 hrs. after full charge; then 1.5%/month. NOTE: Protection circuit will present a steady drain on battery absent any other external load. This will add from 1% to 3% additional drain.|
|Lithium (Primary)||Loss of 9% in 5 years with Extended Life devices holding that out to 10+ years.|
|Alkaline (Primary)||3% per year. Extended Life devices can last up to 10 years at 80% capacity.|
|Lead-Acid (Secondary)||5+% per month. Highly dependent on temperature and “health” of cells.|
|Nickel (Secondary)||8% - 16% in 24 hrs. after full charge, highly dependent on “health” of cell, age and temperature. Then 8% to 16% per month thereafter.|
The charge profile of a given battery refers to the ideal means for restoring the cell to full capacity by injecting charge into the terminals. This profile may become quite complex as a function of time, depletion history and temperature. Each battery chemistry requires its own distinct charge profile and the most common cause of battery failure and pre-mature aging is improper charging.
The Proper Care and Feeding of Your Battery
Let’s now turn our attention to the elements within a Generalized Battery System. Table 3 (next page) shows a graphic representation of such a system in general terms.
Some critical points to keep in mind regarding the Power Source are:
- The nature of voltage transients that may be present.
- The impedance of the source as “viewed” from the input of the Battery Management System.
- Long lines from source to BMS will result in significant line inductance which, in turn, will produce voltage transients at connection/ disconnection.
- Protection circuits that may be active in the source such as overcurrent (OC) and over-voltage.
- If an overcurrent protection circuit is present in the Power Source, then this must be taken into account in the design of the BMS, particularly with respect to in-rush current limiting.
Input Protection Circuitry
As mentioned earlier, voltage transients from the power source and or the cable can require the use of protection devices such as TVS diodes and MOVs. Current limiting can be accomplished with Resettable Electronic Fuses, Standard Fuses and PTC Thermistors.
|Chemistry||Relative Tolerance to Over-Charge/Over-Voltage||Discharge Characteristics|
|Lithium Ion||Very low, about 0.1 volt tolerance above 4.2V nominal||Wide variation in terminal voltage over discharge range of 4.2 down to 2.8. Harmed by over discharge. High pulse capability.|
|Lithium Polymer||Low; about 0.3 volts above nominal 4.3V cell voltage||Somewhat narrower range of discharge: 4.2V down to 3.0V. A bit more resilient against over discharge, but still vulnerable. Moderate pulse capability.|
|Lithium Iron Phosphate||Moderate; about 0.7V above 3.5V nominal cell voltage||Narrowest discharge range of the lithium types: 3.3V to 2.6V; less vulnerable to over discharge. Moderate pulse capability.|
|Lead-Acid||Moderate; about 0.4V above 2.3 nominal cell voltage||Moderate discharge range: 2.2V to 1.9V per cell. Harmed by over discharge. Very high pulse capability.|
|Nickel - Cd/MH||Moderate; but appropriate charge termination is difficult to achieve and over-charge reduces cycle life||Moderate discharge range: 1.3V to 0.9V. Actually benefits from periodic full discharge.|
Battery Management System
For some battery chemistries in low cost systems, the BMS can be as simple as an appropriately rated power resistor of a chosen value to limit the current at maximum charge and then allow the charge voltage to climb to a max “trickle charge” value as the charge current decreases. However, as BMS ICs have become more economical and battery performance demands have increased, in most cases, the BMS is far more complex than that. Options and features found in a BMS include:
- Linear Charger
- Charge from the power source is allowed through a pass element transistor (internal or external to the IC) in a controlled manner with the cell voltage being monitored and compared to the programmed limits.
- Switching Charger
- The switching charger IC acts as a controlled current/controlled voltage switching power supply. Charge is passed from the source to the cell via a switch mode power supply resulting in substantially lower losses.
- Fixed (or resistor programmed) set points, protections, indicators and alarms.
- Here, the user sets the performance of the charge system by either installing appropriate resistors or choosing an appropriately pre-configured IC. Here, there is no “intelligence” to the system and there is a limited amount of flexibility in terms of modifying the charge algorithm in response to cell aging. However, nearly all modern charge ICs have the ability to take the cell temperature into account and can modify the charge routine appropriately.
- Power Path Control
- This refers to the ability to regulate the flow of power among the various elements in the battery system under various conditions. For example, if the battery is completely discharged or even removed, the BMS could allow power to flow from the source out to the load directly rather than from the battery to the load. Power path control can also include the ability of the BMS to prioritize which of two input power sources to use to power the load and or charge the battery.
- Gas Gage/Coulomb Counting
- Here, an attempt is made on the part of the BMS to monitor the charge into and out of the battery. This ongoing measurement is used to arrive at a refined estimate of the “State of Charge” and overall health of the battery cell.
- Front End Protection
- While this can be a separate IC or set of discretes, most BMS include circuitry to protect the cell(s) under charge from excessive current or voltage.
- Over-Discharge Lockout
- This is to protect the cell from being excessively discharged below a preprogrammed voltage. While some chemistries are more vulnerable to this than others, all batteries, other than the NiCd and NiMH, suffer degradation to some degree when discharged below a given threshold for an extended period of time. The Ni chemistries actually require periodic deep discharge to maintain full capacity.
- Over-Temperature Protection
- All battery chemistries are vulnerable to over-temperature effects, so nearly all BMS devices are able to accept temperature data in order to modify the ongoing charge. Further, many batteries now include temperature measuring devices, as well as other protection circuits.
• These include P and N channel FETs used for power conversion, cell protection and power path control. They are increasingly integrated into the BMS.
• Increasingly, batteries of particularly vulnerable chemistries are being equipped with a range of built-in circuitry for:
- Protection from overcurrent charge or discharge.
- Protection from over-discharge (under-voltage lockout).
- Means to monitor cell temperature.
- Means to communicate to BMS via one or two wire (SMBus) communication bus.