Issue 143 June 2002
Invisible Components

---

Battery Basics

Rechargeable batteries come in a bewildering array of configurations and chemistries. Lithium ion batteries seem to be the hands-down favorite in the hand-held arena due to their high energy density. Nickel cadmium batteries, the old standby, have given way to nickel metal hydride in an effort to keep cadmium out of the waste stream. Lead-acid batteries haven’t gone away yet, either. Choices, choices!

Years ago, a bicycle headlight might have consisted of a flashlight bulb and a pair of D cells. In round numbers, that was a 1-W system: less than half an amp from a 3-V supply. If you’ve ever ridden with such a setup, you know why 1 W isn’t enough light and why they were called "glow worms."

Automotive headlights dissipate 50 to 75 W, so a 20-W spotlight sounds about right for a bike. Simple division, though, tells you that delivering 20 W from a 12-V supply requires 1.7 A. In terms of the usual hand-held electronic gadget, that is a lot of current.

Most portable devices draw a few tens to perhaps a few hundreds of milliamps and incorporate power-saving techniques to reduce the current whenever possible. Lighting systems, on the other hand, have a high and constant current drain. Lower power dissipation means less light and if I wanted less light, I’d use a smaller bulb.

Batteries have four key specifications: capacity in ampere-hours (Ah), energy density in watt-hours per kilogram (Wh/kg), lifetime in recharge cycles, and, last but not least, price. A battery’s capacity tells you how long it will supply a given load, its density sets the overall size, and its lifetime indicates how soon you must worry about the cost of a replacement. Obviously, you want the highest capacity in the smallest container with the longest life at the lowest cost, a rat race that has driven battery development in some truly weird directions.

Sealed lead-acid (SLA) batteries have an energy density around 30 Wh/kg and a life of a few hundred cycles. NiCd batteries offer triple the lifetime and double the density at twice the price. NiMH batteries have a slightly lower lifetime and higher energy density than NiCd batteries at 150% of their price (if cadmium weren’t toxic, you’d never see a NiMH battery). Lithium ion batteries can hit 100 Wh/kg with best-case lifetimes around 1000 cycles, but at four times the price of an equivalent-capacity SLA battery.

Within each battery family physically larger batteries provide greater capacity, so you can choose a size to suit your application. Each battery also has a maximum current rating that is limited by its internal chemistry and plate arrangement. Ignoring the current specification can lead to serious disappointment, because it directly affects the battery’s lifetime.

In battery-speak, "C" represents the nominal capacity in ampere-hours at a specific discharge current, typically 1/20 of the capacity (0.05 C) for SLA batteries. For example, a 5-Ah SLA battery would last for 20 h while supplying 250 mA. Other battery chemistries are rated at the nominal 1-C discharge rate, so a 5-Ah NiCd battery could supply 5 A for 1 h.

Capacity and efficiency are inversely related to discharge current, so you cannot expect a 5-Ah battery to supply, say, 50 A for 0.1 h. SLA batteries can provide an average discharge rate up to about 0.2 C (1 A for a 5-Ah battery). Maximum average currents can reach 1 C, but the capacity drops off with high-current, high-duty cycle loads.

NiCd batteries can supply much higher average currents, up to 2 C, with NiMH batteries at 0.5 C and lithium ion batteries at 1 C under ideal conditions. All of these values depend on the battery’s construction, with physically smaller batteries having lower ratings.

Battery voltages are not strongly temperature-sensitive, but their current capacity declines about 10% per 10°C below 25°C. Around here, March temperatures near 0°C are not unusual, which means adding another 25% to the initial capacity (work it out: a 20% reduction equals a 25% increase).

You cannot, however, extract all of a battery’s rated energy capacity during a discharge cycle. SLA batteries and lithium ion batteries react badly to deep cycling, while NiCd and NiMH can tolerate nearly complete discharges. Over-discharging a battery will cause irreparable damage to its weakest cell, so you must ensure that the load cuts out before that point.

Obviously, you must know all of the values for the particular batteries under consideration for a project. I can only supply a rough outline here, based on average numbers and estimates. You get to fill in your own details!