Choosing a Rechargeable Battery

Reference Data Sheet

Introduction

Advances in battery technology have resulted in a wide variety of rechargeable (secondary) batteries, making selection challenging for various applications. This data sheet aims to provide a basic understanding of available types, their operation, and key considerations for choosing the most suitable battery. The focus is on batteries for domestic and light industrial electronic/electrical equipment, excluding heavy-duty automotive, traction, large uninterruptible power supplies, or specialized applications like solar power.

Five main types of rechargeable batteries commonly used are: Sealed Lead-Acid (SLA), Rechargeable Alkaline-Manganese (RAM), Nickel-Cadmium (NiCad), Nickel-Metal Hydride (NiMH), and Lithium-ion (Li-ion).

Key Battery Concepts

Battery Capacity

Secondary batteries store electrical energy in chemical form. Their capacity, indicating the amount of energy they can store and deliver, is typically specified in ampere-hours (Ah) or milliamp-hours (mAh). 1Ah is equivalent to 3600 coulombs of charge. This stored charge can be discharged and recharged at various rates.

Energy Density

Energy density, measured in watt-hours per kilogram (Wh/kg), quantifies how much electrical energy a battery can store relative to its weight. This is influenced by the battery's chemistry. SLA batteries have the lowest energy density (around 30Wh/kg), NiCads offer up to double this, NiMH up to 80Wh/kg, and Li-ion batteries can reach up to 140Wh/kg.

The C Rate

The 'C Rate' specifies battery charge and discharge current levels relative to its nominal capacity (C). A 1C discharge rate means the battery discharges at its nominal capacity value, theoretically lasting one hour. Lower rates (e.g., 0.5C) extend discharge time, while higher rates (e.g., 2C) shorten it. Charging rates are also categorized: >0.5C is 'fast', 0.2C-0.5C is 'normal', 0.05C-0.2C is 'slow'/'trickle', and lower rates are 'float'. Fast charging/discharging can generate heat, reducing efficiency and effective capacity. Very slow rates can also reduce capacity due to internal self-discharge.

Cycle Depth and Memory Effect

A 'cycle' is a single discharge-and-charge sequence. The 'depth' of a cycle refers to the proportion of the battery's capacity drawn. 'Deep cycling' involves near-complete discharge, while 'shallow cycling' involves partial discharge. Some batteries, like NiCads, are prone to 'memory effect', where repeated shallow cycling reduces their useful capacity. NiCads perform best with deep cycling and should not be float charged.

Cell Voltage

Nominal cell voltages are approximations. Actual cell voltage varies during charging and discharging. For example, an SLA cell might reach 2.5V during charging and drop to 1.33V when fully discharged. This variation is important to consider for applications requiring stable voltage.

Rechargeable Battery Types Explained

SLA Batteries

Sealed Lead-Acid (SLA) batteries, a development of flooded lead-acid types, use a gel-type electrolyte, allowing them to be sealed and used in various positions with minimal maintenance. They have a low self-discharge rate and do not suffer from memory effect, performing well with shallow cycling. SLAs are the cheapest option and best suited for applications where low-cost power storage is key and weight/bulk are not primary concerns, such as UPSs and emergency lighting.

RAM Batteries

Rechargeable Alkaline-Manganese (RAM) batteries are derived from primary alkaline batteries. They use a porous zinc gel negative electrode and offer a low self-discharge rate (approx. 10% per year), are free from memory effect, and handle shallow cycling well. They can be rapidly charged using pulsed techniques. RAM batteries have a shorter cycle life than NiCads, especially when deeply cycled, making them suitable for low-cost consumer applications with shallow cycling.

NiCad Batteries

Nickel-Cadmium (NiCad) batteries offer higher energy density than SLAs at a relatively low cost, making them popular for portable equipment like cordless tools and instruments. They can be used in any position but suffer from memory effect, making them unsuitable for shallow cycling or float charging. NiCads perform best with deep cycling and fast, pulsed charging. They have a higher self-discharge rate than SLAs. Despite newer types, NiCads remain popular due to their cost-effectiveness and long working life when properly treated.

NiMH Batteries

Nickel-Metal Hydride (NiMH) batteries offer up to 30% higher energy density than NiCads but still exhibit some memory effect. They are less suited for deep discharge cycles and have a shorter working life than NiCads. NiMH batteries dissipate more heat during charging and require slower charging rates (about half that of NiCads), often needing temperature sensing. Their self-discharge rate is also higher than NiCads. NiMH batteries are ideal for applications requiring compact power sources without deep cycling, such as mobile phones and laptops.

Li-ion Batteries

Lithium-ion (Li-ion) batteries offer extremely high energy density, making them very compact and lightweight. They are not subject to memory effect and have a low self-discharge rate. However, Li-ion batteries are significantly more costly and cannot be charged as rapidly or trickle/float charged. They require built-in protection against over-discharge and overcharging due to the reactivity of lithium. Their primary applications are where maximum energy storage in minimal space and weight is critical, such as in laptop computers, PDAs, and camcorders.

Battery Comparison Chart

TYPE (CHEMISTRY)	NOMINAL VOLTS	ENERGY DENSITY (Wh/kg)	CYCLE LIFE	CHARGING TIME	MAXIMUM DISCHARGE RATE	COST	PROS & CONS	TYPICAL APPLICATIONS
SLA	2.0 (30)	LONG (Shallow cycles)	8-16h	MEDIUM (0.2C)	LOW	Low cost, low self-discharge; happy float charging; but prefer shallow cycling	Emergency lighting, UPS's, solar power systems, wheelchairs etc.
RAM	1.1 (75 initial)	SHORT TO MEDIUM (pulsed)	2-6h	MEDIUM (0.3C)	LOW	Low cost, low self-discharge; prefer shallow cycling; no memory effect but short cycle life	Portable emergency lighting, toys, portable radios, cassette & CD players, test instruments, etc.
NiCad	1.2 (40-60)	LONG (Deep cycles)	14-16h (0.1C) OR <2h (with care, 1C)	HIGH (>20)	HIGH	Prefer deep cycling, good pulse capacity; but have memory effect; fairly high self-discharge rate	Portable tools & appliances, model cars & boats, data loggers, camcorders, portable transceivers & test equipment.
NiMH	1.2 (60-80)	MEDIUM	2-4h	MEDIUM (0.2 - 0.5C)	HIGHER	Very compact energy source; but have some memory effect; high self-discharge rate	Cellphones & cordless phones, compact camcorders, laptop computers, PDAs, personal DVD & CD players.
Li-ion	3.6	VERY HIGH (>100)	LONG	3-4h (1C +0.03C)	MED/HIGH (<10)	VERY HIGH	Very compact, low maintenance; low self-discharge; but need great care with charging	Compact cellphones & notebook PCs, digital cameras and similar very small portable devices.

Discharge Curves

The diagram illustrates typical discharge curves for one cell of each main rechargeable battery type at specified discharge rates. The X-axis represents the percentage of capacity discharged (0-100%), and the Y-axis shows the terminal voltage.

The Li-ion (graphite anode) curve starts around 4.0V and maintains a relatively flat voltage until a sharp drop near the end of discharge.

The SLA battery curve (at a 0.2C rate) starts around 2.2V and shows a steady decline in voltage.

NiCad (at 1C rate) and NiMH (at 0.2C rate) batteries show similar discharge patterns, starting around 1.4V and decreasing steadily, with NiMH slightly higher initially.

The RAM battery curve (at 0.25C rate) begins around 1.5V and also exhibits a steady voltage decrease.

Making a Choice

Selecting the right battery involves considering application demands, cost, and weight. For low cost, NiMH and Li-ion might be ruled out initially. For maximum capacity in a small, lightweight package with less concern for cost, SLA, RAM, and NiCad might not be ideal. The final choice depends on factors like discharge-recharge cycling patterns, required voltage regulation, available recharging time, and battery maintenance. The comparison chart and discharge curves provided offer further assistance in this decision-making process.

Jaycar Electronics Product Range

Jaycar Electronics offers a wide range of rechargeable batteries and charging units. This includes various SLA, NiCad, and NiMH batteries in different capacities and sizes, as well as charging units designed with smart charging functions. For more information, refer to the Jaycar Electronics Engineering Catalogue 2000, pages 146-153, or visit their website at www.jaycar.com.au.