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Cordless tool battery evolution

Tom Brown

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There are a few tool items that I want to document. We'll start with lithium based tool batteries.

Lithium batteries are built with a chemistry that has an active lifespan. Typical cells are good for about five years, if not abused. Even a cell that has hardly been used will rapidly lose it's ability to store power after about five years.

Lithium cells are also limited by a finite number of charge/deplete cycles. The number varies widely based on the peak charge voltage.

Let's look at Milwaukee cordless tools.

M12 = 3 x 18650 Lithium Ion cells charged to about 4 volts.

M18 = 5 x 18650 Lithium Ion cells charged to about 3.6 volts.

Since we know cycle life varies inversely with the charge voltage, we can assume M18 batteries will be capable of substantially more charge cycles than M12 batteries. Interestingly, M18 batteries come with a 3 year warranty these days while M12 batteries come with a 2 year warranty. Makes sense.

Lithium Ion cells have two typical current capacities. That is 1.5Ah and 2.0Ah.

M12 with 3 x series wired 18650 @ 1.5Ah = 1.5Ah pack

M12 with 3 x series wired 18650 @ 2.0Ah = 2.0Ah pack

M12 with 2 parallel sets of 3 x series wired 18650 @ 1.5Ah = 3.0Ah pack

M12 with 2 parallel sets of 3 x series wired 18650 @ 2.0Ah = 4.0Ah pack


M18 with 5 x series wired 18650 @ 1.5Ah = 1.5Ah pack

M18 with 5 x series wired 18650 @ 2.0Ah = 2.0Ah pack

M18 with 2 parallel sets of 5 x series wired 18650 @ 1.5Ah = 3.0Ah pack

M18 with 2 parallel sets of 5 x series wired 18650 @ 2.0Ah = 4.0Ah pack


The next generation of 18650 cells have a typicall current capacity of 2.8Ah. That means we can expect 2.8 and 5.6Ah batteries from tool makers at some point in the future. That's over 100 Watt hours of power from an 18 volt system. I assume they will delay as long as possible. Those batteries could have been on the market quite a while ago. As soon as someone brings LiPo packs to market, the industry will all be there within the year.

Not so long ago, we were using NiCd based tools with huge 18 volt packs that had just over 20 Watt hours of power capacity. It's a whole new world for those without a cord.
 

OutCole'd

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Tom, you can kiss that beer sign goodbye. I fell asleap half way through this post. :D
 

Tom Brown

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Further, the cells will last longer if stored depleted. Since that isn't practical, a reasonable practice for rarely used tools is to try to store them with about 1/2 charge. It will still have a reasonable capability when you go to use it and it will be a lot easier on the battery pack.

For people who use their tools frequently, forget the half charge thing. You're limited by charge cycles, not time related chemistry fatigue.

That goes for anything with a lithium based battery, including laptops. Having a laptop plugged in and fully charged is the worst thing you can do to a lithium battery and the reason laptop batteries take a pretty good capacity hit, even after the first year and even if they are rarely used without being connected to mains power.
 

Crazyhippy

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Tom's vibrator batteries should last longer than anyone else... but they don't.
 

NdaWind

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Tom
 

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