Freedom from the cord
It's difficult to imagine that not so long ago it would have been exceedingly hard to drill a hole in a concrete breeze block in the wall down the garden, let alone a tile or a piece of metal. There simply wasn't enough battery power. Today, thanks to the electrical energy of a hammer drill, all of that has changed. A generation of power tool users obtained their freedom when electricity made its way into the home and DIY (do it yourself) stores provided them with all the mod cons to build, repair and maintain their homes. Then along came new performance and power with cordless devices running on fast recharging lithium-ion batteries. Today, there are affordable professional tools with fast-charging, extended user time available for all. These include drills, hedge and grass trimmers and lawnmowers.
Freedom provided by the lithium-ion battery
A lithium-ion battery, known also as Li-ion or LIB, is a secondary cell. In other words, it is a battery that is designed to be recharged – as opposed to a primary cell which has to be disposed of when the charge has been drained. The charging process involves moving lithium ions from the cathode (positive) to the anode (negative electrode) of the battery. In the IEC's Electropedia, the electrotechnical vocabulary entry describes a lithium-ion battery as a "secondary battery with an organic solvent electrolyte and positive and negative electrodes which utilize an intercalation compound in which lithium is stored."
There are many different types of Li-ion batteries, essentially differentiated by their cathode oxide materials which can be cobalt, manganese or phosphate. One type that is ideal for power tools is the LiNiMnCoO2 (Lithium Nickel Manganese Cobalt Oxide), or NMC. The NMC is also used widely for EVs (electric vehicles) since it has the lowest self-heating rate. Others are LMO (Lithium Manganese Oxide) and LFP (Lithium Iron Phosphate).
More work with less weight
Lithium-ion batteries first became available in the 1970s but really only developed extensively during the 1990s when they started being used in laptop computers. Today, they are among the most popular rechargeable batteries for portable power tools. Li-ion batteries have good energy density. They are fast-charging and have no memory effect which means you don't have to discharge them completely before recharging them again, and they have a slow loss of charge when they are not being used. For example, in comparison with a NiMH (nickel-metal hydride) battery, which will lose around 20 % of its charge every month, a Li-ion battery will only lose roughly 5 %. Additionally, due to their electrodes, Li-ion batteries are much lighter than NiMH or a lead-acid battery that, in comparison, would take six kilogrammes to store the same amount of energy as a one-kilogramme Li-ion battery. Their minimal weight makes them ideal for handheld power tools.
However, there are also a certain number of negative aspects to Li-ion batteries. They do not last for many years, degrade fast if they are subjected to high temperatures, lose their charging properties if allowed to discharge completely, get stressed if subjected to high voltage charging and, because of their chemistry, have been known to burst into flames when overheated because of a short between the two electrodes.
Adding to its overall cost, a Li-ion battery pack has to have a protection circuit to be able to control all the safety aspects. It uses a sensor to monitor the battery temperature, a voltage converter and regulator circuit to maintain safe levels of voltage and current, and a voltage tap to monitor the energy capacity of individual cells in the battery pack. Then it monitors the battery charge state to make sure it charges as quickly and fully as possible.
Today's top-of-the-range DIY tools come with practical charging stations that automatically switch off when the battery is fully charged. In addition, batteries are protected against overloading, overheating and total discharge. This helps extend battery lifetime. Often devices are supplied with two batteries which means that the first can be removed from the tool and charged on its own in the charging station while continuing work with the second battery in place.
IEC protects the user and the electronics
A cordless tool has the advantage of not needing to be connected to the mains, except for recharging. Over and above the obvious advantage of total freedom of use in virtually any surroundings, it removes the additional safety risk of cutting through or damaging the power cable. If earlier power tools used 7.2V battery packs, today battery voltages have increased and it is common to find an 18V battery or even 24V, 28V or 36V battery in DIY professional-grade tools such as hammer drills.
Resistance to dust and water is equally important for power tools. One entire range of tools announced this month by a huge Japanese multinational consumer electronics corporation features dust- and water-resistant technology.
All of the new power tools have been tested against IEC 60529, Degrees of protection provided by enclosures (Ingress Protection Rating Code) [IP codes were featured in the July 2010 e-tech] and given the highest rating of IP561, which classifies them as "Dust-protected" and "Protected against powerful water jets".
The two digits and an optional letter or number classify the degrees of protection provided against the intrusion of solid objects (including body parts like hands and fingers), dust, accidental contact, and water in electrical enclosures. The first digit 5 signifies that ingress of dust is not entirely prevented, since the tool cannot be entirely air and water tight but that dust will not interfere with the operation of the equipment. The second digit which deals with liquids, indicates that powerful jets of water will have no harmless effects. The final digit deals with drop tests and mechanical impact resistance and is often given an IK number, as specified in IEC 62262, Degrees of protection provided by enclosures for electrical equipment against external mechanical impacts (IK code).
Safety measures for hedge cutting
In the old days, one used to cut the garden hedge with a pair of pruning shears. The blades quickly became blunt and the task was slow and arduous. That laborious effort was thankfully superseded when the electric hedge trimmer came along. Safety measures incorporated into the design essentially consist of an RCD (residual current device) sensor that reacts to changes in the flow of electricity.
RCDs are managed by IEC SC (Subcommittee) 23E: Circuit-breakers and similar equipment for household use. Its publications deal with the possibility of a person coming into contact both indirectly due to a fault and directly through contact with an electric current that causes an electric shock.
If there is the slightest detection of current leaking through the neutral wire from the live wire, perhaps in the form of the user acting as a ground, the circuit trips and the machine stops. To use the trimmer it is also compulsory to keep contact with both hands on the handles. If one is released, the trimmer blade(s) stops. Using a conventional electric hedge trimmer, it isn't easy to manage the long cable trailing down the garden, and even when one end is kept knowingly tucked over a shoulder, there is a high risk of cutting through the power cable and blowing all the fuses in the house.
Now hedge trimming has become a lot easier with a light-weight cordless electrical device that runs on a fast-charging Li-ion battery. The first models had a battery that only provided half an hour of cutting time before needing to be put to charge for several hours. Now that working time has more than doubled and charging time has been cut down considerably so that it's easily possible to run a hedge trimmer on two batteries, simply charging one while using the other.
Mowing the lawn silently and powerfully
Electric lawnmowers have been around for decades but were only really practical for straight stretches of uninterrupted grass, and even then there was still the risk of running over the cable, cutting off the electricity and receiving an electric shock. Any obstacle such as a flower bed or tree added to the complexity. It's only in the last couple of years, with the advent of new Li-ion battery technology, that the cordless electric model has really arrived on the market.
Greater scope for use means setting up recycling strategies
A recent rotary model from a global German manufacturer, is incredibly light-weight at 12.8Kg. Its 36V lithium-ion battery can be fully recharged in 90 minutes. Other Li-ion models are also now coming into the market.
But, if energy storage is a lot more efficient and less expensive, the recycling industry also needs to produce strategies to deal with the many Li-ion batteries that need to be disposed of. The Japanese company Nippon Mining & Metals aimed to be the first company in the world to extract lithium and manganese from used lithium-ion batteries on a commercial scale, along with cobalt and nickel and opened a test plant in 2009. Meanwhile, in the US (United States) ecogeek reports that Toxco has just won a USD 9.5 million grant from the Department of Energy to develop a more efficient recycling process that would be applicable to car-sized batteries as well as smaller batteries.
Debate for the future
What else does Li-ion have in store? Today's technology is only at the outset. As the charging times reduce and the capacity increases, so more and more models using Li-ion batteries are being introduced to the market. One would hope that future debate might include a standardized approach with common charging devices that would enable batteries to be exchanged freely between household tools. That would cut down on production costs, shipping prices, storage needs and benefit the world in terms of energy use and efficiency.