Cordless tool battery stability

Cordless tools batteries are simple, right? Well, Matt didn't think so when he was researching this article on cordless tool batteries. There is a lot of information there, but one thing he didn't really address was stability. His article was already pretty long, so that got left out in the interest of getting it published in the first place.

Basics of battery ratings

Cordless tool batteries are measured in Volts and Amp Hours. Volts is the amount of juice at any given moment, while Amp Hours gives reference to how long the battery can last when doing a job. That's about a simple as I can make it. Ok, one more try using a water pipe as a metaphor. Volts is the speed of the flow of the water, Amps is the diameter of the pipe that can be filled, and Amp Hours is the length of time that this flow can be maintained.

What is battery stability?

Since batteries end up having chemistry tied to them, and nobody that isn't wearing a funny white coat really wants to read about chemical reactions, I'll try to keep this more of a high level overview instead of really in-depth.

What it essentially boils down to is that stability is a measure of how many charges the battery can take before a cell goes bad. Based on the chemical makeup of the battery, this can vary quite a bit. This is also where the newer technologies really shine.

Ni-Cad (Nickel-Cadmium)

This is really one of the oldest battery technologies. The chemical makeup is Nickel and Cadmium, which ends up having the worst symptoms from voltage depression. I won't go into the details about voltage depression - you can read it at the provided link if you are interested. What I will say is that the chemical makeup of Ni-Cad means you see the least amount of time between failures in most cases.

Ni-MH (Nickel Metal Hydride)

This is a slightly newer technology, but it still has voltage depression issues. Although the symptoms aren't typically as quick to develop as Ni-Cad, they do exist and still have to be dealt with. But here's the big secret - The higher the Amp Hour rating of the battery, the less stable the chemistry of the battery. Let me repeat that in different language - Longer life per charge (higher Ah rating) means fewer charge cycles before a cell goes bad.

Li-Ion (Lithium Ion)

For power tools, this is the newest technology. This is similar to what cell phones have been using for years, but with much larger cells to give a cordless tool some actual run-time. For example, my cell phone has a 3.7V 1000 mAh battery. The smaller ones on cordless tools right now are 18V with 3000 mAh (3 Ah). That's quite a difference. But what about stability?

When talking about stability in a lithium battery, we're talking about the different chemical makeups. This is different than the concerns with Ni-Cad or Ni-MH batteries. Basically, when the manufacturers were originally attempting to make Li-Ion for power tools, there were many explosions because of chemical instability before anyone got it right.

That's not a big problem any more, but we still see people searching for V28 explosions at times. I don't know if they want video or are just doing research to make sure that they're safe, but either way it sounds like someone heard about some old problems.

Chem Is Try

The fun thing I remember about chemistry from my days at UNL is that Chem Is Try. Nothing ever comes out perfect the first time, and conditions can contribute to unseen problems. This is not only true in skydiving (as the bumper sticker to the right points out), but also in chemistry.

Milwaukee spent over 7 years on R&D for their V28 series of tools. Makita didn't want to spend that long, so they decided to use Sony Li-Ion cells which were being used in other high-demand applications (digital cameras, camcorders, etc.) already. But the point is, the battery chemistry used by these manufacturers has been tested for quite some time now to make sure that the batteries don't go boom. A few of those could cripple their reputation, and in this industry reputation is a powerful ally or an even stronger adversary.