Maintaining cordless NiMH/NiCad(NiCd) batteries

Cordless drill do not need cord because the drill use energy from the battery. The battery is a very important part of cordless drill and cordless tools in general. Exist different types of batteries for cordless tools, and each type has its own features in maintenance and use. The main battery types are nickel-cadmium (NiCad[1], NiCd) batteries, Nickel metal hydride (NiMH) batteries and Lithium-ion batteries (sometimes abbreviated Li-ion batteries). NiCd and NiMH batteries is used often in cordless tools so in this post I want to give you information about using and charging NiCd and NiMH batteries. In internet you can find a lot of information about maintaining NiMH/NiCad batteries and in this post I will try to combine information from different sources in such manner that you will can see the overall picture of how to use and how to charge NiMH/NiCd batteries.

NiCd battery


The nickel-cadmium or ‘NiCd’ battery is the most popular type of rechargeable battery in the world today, despite the development of new types offering higher energy storage density. That’s no doubt because the NiCad combines relatively low cost with fairly high storage density, the ability to deliver very high load currents on demand and the ability to be recharged very quickly.

The basic nickel-cadmium battery was invented in 1899 by Waldmar Jugner, but the modern sealed type dates from about 1947.


NiCd (NiCad) battery construction

NiCds battery are composed of an anode (positive plate of nickel oxide/hydroxide), a cathode (negative plate of cadmium metal with cadmium hydroxide) with two layers of porous separator material impregnated with electrolyte of potassium hydroxide ('‘caustic potash’'). The sandwich is then rolled up and packaged in a nickel-plated steel can.

NiCd batteries approximatively have an energy storage density of between 40 and 60 watt-hours per kilogram.

The principal advantages of NiCd over other rechargeable types is lower weight for a given quantity of stored energy, good charging efficiency, low internal resistance, thus can achieve a higher maximum discharge rate (which can be important for applications such as power tools), and non-critical charging conditions.

The well-known drawback of NiCds is the "Memory effect’."
It occurs by the formation of crystals(Cadmium crystals) inside the battery. These crystals are hard to dissolve and the ones responsible for the “memory effect”. So the trick to avoid the “memory effect” is to avoid the formation of those crystals inside the battery. This is typically accomplished by recharging the battery only when it is discharged and not when it is partially discharged. Careful recharging using the correct techniques can also be used to reverse crystal growth.
Remember, NiCd cannot be fully discharged or they will be damaged. Fully discharged usually means having a voltage below 1 V per cell (NiCd batteries are usually formed by grouping several 1.2 V cells; typical NiCd batteries are 3.6 V packs using three 1.2 V cells).
So the “trick” that is recommended by many people to solve “memory effect” by fully discharging NiCd batteries by shorting them (or any other sort of “quick discharge”) does in fact more damage than good to the batteries, even though several people claim that they can recover NiCd batteries with “memory effect” by doing this.
Also, high temperatures help the crystals to be formed. Heat is the enemy of batteries. A NiCd stored, used, or charged under high temperature conditions will die an early death. Heat causes the separator to weaken and greatly accelerates changes in the plate material.

NiCd charging & chargers


NiCd batteries need to be recharged with a reasonable amount of care, largely because they can be damaged by overcharging. The demand for rapid charging has lead to a great increase in overcharging abuse. Most all NiCd cells can be rapid charged. The trick is to stop charging when it is fully charged. The so called "rapid charge" type of cells just incorporate protection against overcharging at high currents.

NiCd batteries can charge at several different rates, depending on how the cell was manufactured. The charge rate is measured based on the percentage of the amp-hour capacity the battery is fed as a steady current over the duration of the charge. Regardless of the charge speed, more energy must be supplied to the battery than its actual capacity, to account for energy loss during charging, with faster charges being more efficient. For example, the typical "overnight" charge, called a C/10 charge, is accomplished by applying 10% of the batteries total capacity for a period of 14 hours; that is, a 100Ah battery takes 140Ah of energy to charge at this rate. At the "fast charge" rate, done at 100% of the rated capacity, the battery holds roughly 80% of the charge, so a 100Ah battery takes 120Ah of energy to charge (that is, approximately 1 hour and fifteen minutes)


How the terminal voltage of a typical NiCd cell (and
also its temperature) tends to vary during charging. Both the inflection point and the voltage peak are used for end-of-charge detection in high-end chargers.


The best method of charging is the method so called delta-V method. If one plots the terminal voltage of the cell during a charge with a constant voltage, it will continue to rise slowly as charging progresses. At the point of full charge, the cell voltage will drop in a fairly short time. The amount of drop is small, about 10 mV/cell, but is distinctive. There are circuits out there built specifically to look for this. This method is expensive and tedious, but gives good reproducible results. There is a danger in this though. In a battery with a bad cell this delta - V method may not work, and one may end up destroying all the cells, so one needs to be careful. If one ends up putting in more than double the charge capacity of the cell, then something is wrong.

Another cheap way is to measure the cell temperature. The cell temperature will rise steeply as full charge is reached. When the cell temperature rises to 10 degrees C or so above ambient, stop charging, or go into trickle mode.

Whatever method one chooses, a failsafe timer is a requirement with high charge currents. Don't let more than double the cell capacity of charge current flow, just in case. (i.e. for a 800 mAh cell, no more than 1600 mAh of charge).

A lot of work done by battery researchers in recent years, has shown that NiCds
respond better to a pulsed charging waveform than to a steady DC current. By applying the charge current in one second pulses with brief ‘rest’ periods between them, the cells are better able to absorb the charge efficiently. This is particularly true at the higher charge rates used in fast chargers.

‘NiMH’ battery


The first consumer grade NiMH batteries began to appear at the end of the 1980s. Positive electrode development was done by Dr. Masahiko Oshitani from Yuasa Company, who was the first to develop high-energy paste electrode technology. The association of this high-energy electrode with high-energy hydrid alloys for the negative electrode, discovered by Philips Laboratories and French CNRS labs in the 1970s, led to the new environmentally friendly high energy NiMH battery.

The nickel-metal hydride or ‘NiMH’ battery is in many ways a development from the NiCd. The construction of most NiMH batteries is almost identical to that of NiCds.
Like NiCds the positive plate is of nickel with nickel oxide/hydroxide, and the electrolyte is potassium hydroxide. However in the NiMH battery the negative electrode is made from a hydrogen storage alloy such as lanthanium-nickel or zirconium-nickel. NiMH batteries have up to 30% higher energy storage density compared with NiCds, but still display some memory effect. They’re not as happy with deep discharge cycles, though, and tend to have a shorter working life. The self-discharge rate is also about 50% higher than NiCds.
Compared to NiCd, NiMH batteries have a higher capacity and are less toxic, but are still slightly more expensive. In addition, a NiCd battery has a lower self-discharge rate (for example, 20% per month for a NiCd, versus 30% per month for a NiMH under identical conditions).

Charging NiMH batteries


The charging voltage for NiMH batteries is in the range of 1.4-1.6 V/cell. A fully charged cell measures 1.35-1.4 V (unloaded), and supplies a nominal average 1.2 V/cell during discharge, down to about 1.0-1.1 V/cell (further discharge may cause permanent damage).

Unlike NiCds, NiMH batteries tend to dissipate heat during all of the charging process — not just following the full charge point, as with NiCds. This tends to mean that NiMH batteries can only be charged at about half the rate of NiCds, unless temperature sensing is used to limit charging current.

Broadly speaking, though, most of the charging techniques which is used for charging NiCd batteries can be used with NiMH batteries. The main difference is that although the charging voltage characteristic of a NiMH cell has the same basic
shape, the actual voltage levels are different. So chargers for NiMH cells must usually be arranged to sense the terminal voltage rate of change, and use the positive delta voltage (+DV) method of end-of-charge detection.

Many modern pulse-type chargers are designed to charge either NiCd or NiMH cells, and can automatically sense which type is present — adjusting their charging characteristic to suit.

Summary


  • Do let the cells discharge to 1.0V/cell on occasion through normal use.

  • Don't leave the cells on trickle charge for long times, unless voltage depression can be tolerated.

  • Do protect the cells from high temperature both in charging and storage.

  • Don't overcharge the cells. Use a good charging technique.

  • Use correct charger type for NiCd or NiMH battery.


Remember, you CAN NOT USE charger which charge NiCd batteries for charging NiMH battery and you CAN NOT USE charger which charge NiMH batteries for charging NiCd battery. If you want to charge NiCd and NiMH batteries by one charger, use charger that supports both types.


For more information you can read following:



Footnotes

  1. ^ The abbreviation NiCad is a registered trademark of SAFT Corporation and should not be used to refer generically to nickel-cadmium batteries, although this brand-name is commonly used to describe all nickel-cadmium batteries. On the other hand, the abbreviation NiCd is derived from the chemical symbols of nickel (Ni) and cadmium (Cd), though it is not to be confused with a chemical formula.



 


1 comments:

  1. Nice article about cordless drills batteries. Thank you very much for sharing.

    on February 23, 2008 at 2:58 PM