Cordless Tool Batteries – A Primer
Just Looking for Power Tool Batteries?
We sell them. Get to the battery you need. (I only put this here, because I noticed a lot of people were finding this article, when they were simply looking for a battery.)
Power tool batteries are made up by taking several lower voltage cells and hooking them together in series. Each cell has the same rating in Amp hours (a measure of how much electricity flows through a circuit in one hour with a flow rate of 1 ampere). So an 18v tool might have 12 cells at rated at 1.5 volts and 2.4 Amp hours, which would produce an 2.4Ah 18 volt battery.
Almost all of the cordless tools on the market today use one of the following three battery technologies:
- NiCad / NiCd (Nickel Cadmium) – Nickel Cadmium batteries are commonly found in tools from Dewalt, Bosch, Porter Cable, Milwaukee, Skil and Hitachi. Advantages and disadvantages of NiCad are:
- Least expensive cell technology
- Has the least temperature sensitivity of the three battery types
- A large number of charge cycles is possible
- Stored batteries quickly lose charge
- Can develop a sort of “memory effect” (see the below section on memory) if not regularly drained and fully charged (sometimes referred to as conditioning)
- Cadmium is a heavy metal and is considered an environmental hazard
- NiMH (Nickel Metal Hydride) – Nickel Metal Hydride batteries are used extensively by Makita and Panasonic. The advantages and disadvantages of NiMH batteries are:
- Much higher potential capacity than NiCad batteries
- Requires less conditioning than NiCad
- While you still shouldn’t just toss a NiMH battery, it is considerably less toxic than a NiCad
- Much more sensitive to temperature than NiCad
- Some studies show that the battery performs less reliably after repeated empty to full charges
- Loses a charge more quickly than NiCad
- Li-ion (Lithium Ion) – Lithium Ion are relatively new to the power tool industry. Lines using them have been launched by Milwaukee (the 28 Volt V28 Tools) and Makita (The 18 Volt LXT Line). In addition, Dremel makes a lithium ion rotary tool, which was the first lithium ion tool we carried. We expect to see more tools in this category soon, as Bosch has announced 10.8 volt and 36 volt tools and Dewalt is also jumping into the 36 volt waters. Others have been dropping hints as well. Lithium ion is relatively new in power tools, but some general advantages and disadvantages of lithium ion cells are:
- Much higher capacity than even NiMH
- Charge loss during storage is less than half that of NiCad
- No memory at all, no periodic conditioning is needed
- From the early reports we are hearing on the Makita and Milwaukee units, lithium ion seem at least as temperature sensitive as NiMH
- The charging process is much more complicated from a technology standpoint, meaning more expensive chargers or batteries (depending on the manufacturing choices)
- The battery manufacturing cost is substantially higher
So Why are Power Tool Batteries so Expensive?
From time to time I hear someone complain about the price of tool batteries, although thanks to a few of our power tool manufacturers coming out with battery 2-packs, I’ve been hearing less of that particular complaint. But the idea behind the complaint is that I can go out and buy a 12 pack of AA batteries for under $8. That is 12 cells x 1.5 volts, or 18 volts total.
Of course, this point of view requires you to forget that your standard alkaline batteries are not rechargeable. So if you get 400+ charge cycles out of a cordless tool battery, then that $8 18 volt battery suddenly costs you $3,200. That is not even taking into account current draw and other issues that would make a AA cell less than ideal for the purpose.
The costs in rechargeable batteries also are influenced by the makeup of the battery. NiCad, NiMH and Li-Ion are all much more expensive per cell than a lead-acid or alkaline cell. In addition, there are costs in designing the packaging, wiring the cells and otherwise making it work safely and effectively in a power tool.
Voltage depression is the technical name for what people commonly refer to as memory in Nickel-Cadmium or Nickel Metal-Hydride batteries. While NiCad and NiMH are both capable of developing voltage depression, it is less likely with power tools than with low amp draw items like laptops or cordless phones. The effect is caused when the cells in a battery are drained to the same level at the same speed frequently. To quote a Makita training pamphlet on batteries:
This does not happen with tools in industrial applications due to the fact that tools are used in various applications. For instance, sometimes a battery will be used in a high current draw application such as installing large lag screws and other times in a low current draw application such as installing deck screws. The changes in draw and the rate of discharge prevent memory from occurring.
The voltage depression is actually caused by crystalline structures forming on the nickel plate (on both NiMH and NiCad) and the Cadmium plate (on NiCad). The larger the crystals become, the less energy your battery can produce.
The Amp hour (Ah) Rating
I gave the technical definition of an Amp hour above. Now, I’m going to actually explain what that means to you. If you had a 9.6 volt battery rated at 1.3Ah and another 9.6 volt battery rated at 2.6Ah, the 2.6Ah could power the same tool doing the same job two times as long. The best metaphor for the Ah rating is the gas tank on a car. Depending on how you drive a car, if you maintain it and the engine that is in it, your mileage will vary widely, but you’ll always go farther if you have a bigger gas tank if all the other factors are the same.
A Great Resource to Learn About Batteries
I stumbled across a site a couple weeks ago, and using my existing knowledge and some information I picked up there, I wrote this little primer. However, I recommend everyone should go to Battery University to learn from the real experts. Their information is not specific to power tool batteries, but much of the material applies equally to all types of batteries.
If anyone out there has any questions, I’ll do what I can to answer them, or in other words, yes I take requests.