Batteries are the predominant cost in the price of a cordless tool. The batteries control the tool’s runtime, and (more or less) the power that the tool outputs. There are a few main types of batteries for power tools, and each has different maintenance and usability requirements. It is important to remember though that a cordless tool is only as good as it’s battery.
Voltage is the measure of the strength of an electrical source (for a given current level). Rechargeable batteries for power tools generally contain cells with a voltage of 1.2V, 1.5V or 3.6V. Manufacturers utilize these cells to produce batteries that can have a voltage of anywhere between 3.6V and 48V. Generally speaking, the higher the voltage, the heavier the battery will be, and the more power it will be able to output.
The voltage of a battery doesn’t necessarily indicate the specific power that the tool will output. This is dictated more by the quality and design of the tool’s internal components, especially the transmission. It is therefore quite common to find a high quality 14.4V tool that is more powerful than a low quality 18V tool.
Batteries that have a voltage between 14.4V and 36V are manufactured for use on heavier duty tools such as cordless hammer drill drivers, cordless angle grinders, and cordless circular saws. The most common voltage used amongst manufacturers (due to its weight, cost, and potential power output) is 18V.
Once fully charged, the maximum runtime of a cordless tool is largely determined by the battery’s amp-hour (Ah) value. In essence, this value represents the fuel tank of the tool. The larger the fuel tank, the longer the tool will operate for. Amp-hour values will generally range from 1.3Ah to 3.3Ah, depending on the tool’s intended applications (i.e. light or heavy work). As this capacity increases, batteries will also become heavier and will take longer to charge.
Theoretically speaking, a 1.5Ah battery in a device drawing a current of 1.5 Amps will last 1 hour.
Therefore, we can assume: Operation Time (hours) = Amp-hour Value (Ah) ÷ Current Draw (A)
Unfortunately, this is not an effective way of estimating the realistic runtime of a cordless tool, as other important factors (detailed below) also come into effect.
‘Memory effect’ in some batteries can also seriously affect their runtime. This is the main reason why batteries last for very short periods of time, even right after they have been charged.
The term ‘memory effect’ is a condition found in some rechargeable batteries that results in the reduction of their capacity.
There are a few main causes of memory effect in a rechargeable battery:
- When a battery is discharged repeatedly to exactly the same level, and then recharged.
- When a battery gets old.
- When a battery is charged too slowly.
- When a battery is exposed to high temperatures.
- When a battery is completely discharged (batteries should be discharged until the point where the tool refuses to function adequately).
- The most common reason is when a battery is persistently overcharged (voltage depression).
Another factor that affects runtime, but one that isn’t related to batteries, is the efficiency of the tool itself. An efficient tool will transfer nearly all of the power from the battery to the business end of the tool. An inefficient tool (generally tools of lower quality) will waste excessive power simply transferring energy to the business end of the tool, which will severely decrease the runtime of a battery.
Some new batteries (or batteries that haven’t been used for a while) may take between 5 and 7 charge cycles to reach their peak run time.
There are three leading battery chemistries on the market today; Nickel-Cadmium, Nickel-Metal Hydride, and Lithium-ion.
Nickel-Cadmium batteries are the most common type of batteries for power tools. Their advantages and disadvantages are listed below.
- Are the cheapest batteries available for cordless tools
- Are very difficult to damage
- Perform well in colder temperatures
- Can be charged and discharged up to 800 times (i.e. 800 charge cycles)
- Are heavier than newer battery technologies of the same voltage and capacity (have a lower energy density)
- Have a relatively high self-discharge rate (approximately 20% per month)
- Slowly lose power during use
- Commonly suffer from ‘memory effect’
- Have a maximum capacity of 2.4Ah
- Contain cadmium, a very toxic heavy metal
Nickel-Metal Hydride (NiMH)
Nickel-Metal Hydride batteries were once believed to be the answer to all of Nickel-Cadmium’s shortfalls. For many reasons though, their popularity and availability has severely decreased.
- Are lighter than NiCd batteries of the same voltage and capacity (have a higher energy density)
- Are the most environmentally friendly batteries available
- Are less prone to ‘memory effect’ than NiCd batteries
- Have fewer charge cycles than NiCd batteries (up to approximately 500 charges)
- Don’t perform well in colder temperatures
- Require regular discharge
- Have a high self-discharge rate (approximately 30% per month)
- Are more expensive than NiCd batteries
- Slowly lose power during use
Lithium-ion batteries are the newest and most advanced type of battery for power tools. They have many advantages over both Nickel-Cadmium and Nickel-Metal Hydride, but still have their shortcomings. These shortcomings have been addressed by some manufacturers by introducing elements such as Phosphate, Manganese and Cobalt into their battery’s chemical composition. High quality brands will also feature electronic cell protection to stop the battery’s cells from overheating, and from being overloaded and over-discharged.
- With respect to voltage and capacity, are the lightest batteries available (have the highest energy density)
- Constantly run at their maximum power, giving them a longer runtime compared with NiCd and NiMH batteries of the same capacity
- Have very low self-discharge (approximately 5-10% per month)
- Can be taken out of the charger before the charging process is completed
- Do not need to be discharged before charging
- Require very little maintenance
- Have the most charge cycles (a single battery can be charged up to 1500 times)
- Have special protection against undercharging, overcharging, and over draining (only because they are otherwise very susceptible to damage from these elements)
- Perform well in colder temperatures
- Are environmentally friendly (relatively)
- Are the most expensive battery available
- Are very prone to heating up if used on high drain cordless tools
- Do not perform well in very hot temperatures
There are only a few main suppliers of Li-ion cells that all the main power tool manufacturers use in their batteries. The best quality cells are made by Panasonic and are usually found in the more expensive power tool brands. Sony cells are good quality and are found in lots of brands’ cordless tools. Samsung cells are also recommended. Most other brands source their battery cells from obscure third-party companies.
Always ensure the cordless tool’s switch is off before disconnecting the battery, and never let any metal come into contact with the battery’s terminals.
If an old battery becomes redundant, please take it to one of the many waste management companies in your local area devoted to safe battery disposal. Please don’t put them straight in the bin, and never expose them to fire as they can explode.
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